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[Illustration: THE AUTHOR’S OBSERVATORY.]




  THE

  PRACTICAL ASTRONOMER,

  COMPRISING

  ILLUSTRATIONS OF LIGHT AND COLOURS--PRACTICAL
  DESCRIPTIONS OF ALL KINDS OF TELESCOPES--THE
  USE OF THE EQUATORIAL-TRANSIT--CIRCULAR,
  AND OTHER ASTRONOMICAL
  INSTRUMENTS,

  A PARTICULAR ACCOUNT OF THE
  EARL OF ROSSE’S LARGE TELESCOPES,
  AND OTHER TOPICS CONNECTED WITH ASTRONOMY.


  BY THOMAS DICK, LL.D.

  AUTHOR OF THE “CHRISTIAN PHILOSOPHER,” “CELESTIAL SCENERY,”
  “THE SIDEREAL HEAVENS,” &C. &C.


  Illustrated with One hundred Engravings.


  SEELEY, BURNSIDE, AND SEELEY,
  FLEET-STREET, LONDON.
  MDCCCXLV.




PRINTED BY L. SEELEY.




PREFACE.


The following work was announced several years ago in the preface
to the volume on “The Sidereal Heavens;” since which time numerous
enquiries have been made after it by correspondents in England, the
West Indies, and America. It was nearly ready for publication three
years ago; but circumstances over which the Author had no controul,
prevented its appearance at that period. This delay, however, has
enabled him to introduce descriptions of certain instruments and
inventions which were partly unknown at the time to which he refers.

The title “Practical Astronomer” has been fixed upon, as the shortest
that could be selected, although the volume does not comprise a variety
of topics and discussions generally comprehended in this department of
astronomy. The work is intended for the information of general readers,
especially for those who have acquired a relish for astronomical
pursuits, and who wish to become acquainted with the instruments by
which celestial observations are made, and to apply their mechanical
skill to the construction of some of those which they may wish to
possess. With this view the Author has entered into a variety of minute
details, in reference to the construction and practical application
of all kinds of telescopes, &c. which are not to be found in general
treatises on Optics and Astronomy.

As _Light_ is the foundation of astronomical science, and of all the
instruments used for celestial observation, a brief description is
given of the general properties of light--of the laws by which it is
refracted and reflected when passing through different mediums--and of
the effects it produces in the system of nature--in order to prepare
the way for a clear understanding of the principles on which optical
instruments are constructed, and the effects they produce.

As this, as well as every other physical subject, forms a part of the
arrangements of the Creator throughout the material system--the Author
has occasionally taken an opportunity of directing the attention of
the reader to the Wisdom and Beneficence of the Great First Cause, and
of introducing those moral reflections which naturally flow from the
subject.

The present is the ninth volume which the Author has presented to
the public, and he indulges the hope that it will meet with the same
favourable reception which his former publications have uniformly
experienced. It was originally intended to conclude the volume with a
few remarks on the _utility_ of astronomical studies, and their _moral_
and _religious_ tendency, but this has been prevented, for the present,
in consequence of the work having swelled to a greater size than was
anticipated. Should he again appear before the public as an author, the
subject of discussion and illustration will have a more direct bearing
than the present on the great objects of religion and a future world.

_Broughty Ferry, near Dundee, August, 1845._




CONTENTS.


  PART I.

  ON LIGHT.


  INTRODUCTION.

  Necessity of light to the knowledge and happiness of all sentient
  beings--Its beautiful and enlivening effects--An emblem of the
  Deity--Provision made for its universal diffusion

  _page_ 1-7.


CHAPTER I.

GENERAL PROPERTIES OF LIGHT.

Interesting nature of this study--Different hypotheses which have
been formed respecting the nature of light--It radiates in straight
lines--Moves with amazing velocity--Flows in all directions from
luminous bodies--Duration of its impressions on the eye--Supposed
to have a certain degree of force or momentum--Experiments in
relation to this point--Its intensity diminished in proportion to
the square of the distance--Its reflection from opake bodies renders
objects visible--Intensity of reflected light--Subject to the law
of attraction--Forms a constituent part of certain bodies--_Solar
phosphori_, and the phenomena they exhibit--Produces certain effects on
planets and flowers, exemplified in a variety of instances--Supposed to
have an influence on the _propagation of sound_

  _page_ 8-37

Reflections on the nature of light, and the multifarious effects
it produces throughout the universe--A representation of the
Divinity--Wisdom and Goodness of God displayed in its formation

  _page_ 37-40.


CHAPTER II.

ON THE REFRACTION OF LIGHT.

Nature of refraction--Illustrated by experiments--Angle of
refraction--Familiar experiments illustrative of refraction--Refraction
explains the causes of many curious and interesting phenomena--Its
effect on the heavenly bodies--On the twilight--Illustrated by figures

  _page_ 41-53.


EXTRAORDINARY CASES OF REFRACTION IN RELATION TO TERRESTRIAL OBJECTS.

Extraordinary appearance of the coast of France from
Hastings--Appearance of a ship seen by Captain Colby, beyond the
coast of Caithness--Scoresby’s view of his father’s ship when
beyond the horizon--Phenomenon near the Himalaya mountains--Bell
Rock light-house--Summary statement of the diversified effects of
refraction--Reflections on the beneficent and diversified effects
produced by the law of refraction--It increases the length of the day,
particularly in the polar regions--Is the cause of that splendour which
appears in the objects around us--Quantity of refraction in respect
to terrestrial objects, and its utility--Its effects may be more
diversified in other worlds

  _page_ 53-63.


CHAPTER III.

ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT
SUBSTANCES, OR LENSES.

Refraction the foundation of optical instruments--Various forms of
lenses--_Parallel_, _converging_, and _diverging_ rays--Illustrated by
diagrams--_Concave_ lenses, their effects, and how to find their focal
distances--IMAGES formed by convex lenses--Illustrated by
experiments--Principles in relation to images formed by lenses--Their
magnifying powers, &c.

  _page_ 63-75.


REFLECTIONS DEDUCED FROM THE PRECEDING SUBJECT.

Property of the rays of light in forming _images_ of objects--Wonderful
results and discoveries which have flowed from this property--in
relation to our knowledge of the scenery of the heavens and the minute
parts of nature--and of our views of the attributes of Deity

  _page_ 75-80.


CHAPTER IV.

ON THE REFLECTION OF LIGHT.

Nature of reflection--Plane, convex, and concave speculums--Angle of
reflection--Reflection of objects from plane mirrors, illustrated
by figures--_Reflection by Convex and Concave mirrors_--Properties
of _convex_ mirrors, and the purposes to which they are applied.
Properties of concave speculums, and their utility--Of the _images_
formed by concave speculums--Illustrated by a variety of figures and
experiments--Their power of magnifying and burning--Amusing deceptions
produced by--Resemblance between the properties of convex lenses, and
concave mirrors--Quantity of light reflected by polished surfaces

  _page_ 81-106.


UNCOMMON APPEARANCES OF NATURE PRODUCED BY THE COMBINED INFLUENCE OF
REFLECTION AND REFRACTION.

_Fata Morgana_--The Mirage--Inverted images of ships seen in the
horizon--Appearance of Dover castle at Ramsgate--Spectre of the
Brocken--Scenes in the Highlands of Scotland--Large cross seen at Migné
in France--Dr. Wollaston’s illustrations of such phenomena--Utility of
science in dissipating superstitious fears

  _page_ 106-118.


REMARKS AND REFLECTIONS IN REFERENCE TO THE PHENOMENA DESCRIBED ABOVE.

Light, the beauty of the universe, and a symbol of the Divinity--In
other worlds it may produce an infinite variety of sublime scenery

  _page_ 118-122.


CHAPTER V.

SECT. 1.--ON THE COLOURS OF LIGHT.

Colours, the beauty of nature--Opinions which were formerly entertained
respecting their cause--Sir I. Newton’s experiments with the
Prism--Colours and phenomena produced by the prism--Imperfection of
optic lenses--Various illustrations--Differently  rays have
not the same illuminating power--Heating and chemical properties of
some of the rays of the solar spectrum--property of communicating the
Magnetic power--Fraunhofer, and his discoveries in reference to the
spectrum--Experiments on white and  light

  _page_ 123-137.


SECT. 2.--ON THE COLOURS OF NATURAL OBJECTS.

Colours not in the objects themselves, but in the light which falls
upon them--Illustrations of this position--Atmosphere the source of a
variety of colours--Various natural phenomena, in relation to colour,
explained

  _page_ 137-143.


SECT. 3.--PHENOMENA OF THE RAINBOW.

Rainbow described--Experiments to illustrate its cause--Descriptions
of its various phenomena, and optical explanations of their
causes--Rainbows exhibiting _complete circles_--Their appearance in
different countries--Summary view of the principal facts respecting the
rainbow--_Lunar_ rainbows--Scriptural allusions to the rainbow--Whether
there was any rainbow before the deluge

  _page_ 144-157.


SECT. 4.--REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS.

Beauty and variety derived from colours in the scenery of
nature--Colours produced by the atmosphere in different countries--What
would be the aspect of nature, in heaven and on earth, were there only
one colour--How it would affect the common intercourse and employments
of society--Wisdom and Beneficence of the Creator displayed in the
diversity of colours--Throughout all the systems of the universe, a
diversity of colours prevails--This subject has a tendency to inspire
us with gratitude

  _page_ 158-168.


PART II.

ON TELESCOPES.


CHAPTER I.

HISTORY OF THE INVENTION OF TELESCOPES.

The telescope a noble instrument--Effects it produces--Whether
known to the ancients--Friar Bacon’s ideas respecting
telescopes--First constructed in Holland--The invention claimed
by different persons--Galileo’s account of the construction of his
telescope--Discoveries which he made with this instrument--How his
discoveries were received by the learned--Specimens of learned nonsense
brought forward by pretended philosophers--Supposed length of Galileo’s
telescope--Various claimants to the invention of this instrument

  _page_ 169-183.


CHAPTER II.

OF THE CAMERA OBSCURA.

Appearance of objects in a camera obscura--The dark chamber--This
instrument serves to explain the nature of a refracting
telescope--Particulars to be attended to, in exhibiting objects with
the Camera--It illustrates the nature of vision--Revolving camera
obscura--_Portable_ camera

  _page_ 184-196.


THE DAGUERREOTYPE.

An important discovery for fixing the images produced by the
camera--Description of the Daguerreotype process--Preparation of
the plate, fixing the impression, &c.--Preparation of photogenic
paper--Beneficial effects which this art may produce--Representations
of objects in the heavens, &c.

  _page_ 196-205.


CHAPTER III.

ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS.

Various illustrations of the apparent magnitude of objects--Fallacies
in relation to apparent magnitudes--Apparent magnitudes in the
heavens--Difference between _absolute_ and apparent magnitudes

  _page_ 206-213.


CHAPTER IV.

ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES.

SECT. 1.--THE GALILEAN TELESCOPE.

Construction and peculiar properties of this instrument

  _page_ 214-217.


SECT. 2.--THE COMMON ASTRONOMICAL REFRACTING TELESCOPE.

Description of its nature and construction--How its magnifying power is
determined. Table of the linear aperture, magnifying powers, &c., of
astronomical telescopes from 1 to 120 feet in length--Summary view of
the properties of this telescope

  _page_ 218-224.


SECT. 3.--THE AERIAL TELESCOPE.

This telescope is used without a tube--Description of the apparatus
connected with it, illustrated with figures--Huygens’ Hartsocker’s and
Cassini’s large telescopes


SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL
OBJECTS.

Arrangement of its lenses--Magnifying power--Manner in which the rays
of light are refracted through the telescopes now described

  _page_ 228-231.


SECT. 5.--TELESCOPE FORMED BY A SINGLE LENS.

Various experiments in relation to this point--Experiments with a lens
26 focal distance, and 11-1/2 inches diameter

  _page_ 232-235.


SECT. 6.--THE ACHROMATIC TELESCOPE.

Imperfections of common refracting telescopes--Dollond’s
discovery--Newton’s error--Explanation of the principle of achromatic
telescopes--Combination of lenses--Difficulties in the construction
of such instruments--Difficulty in procuring large disks of flint
glass--Guinaud’s experiments

  _page_ 235-248.


NOTICES OF SOME LARGE ACHROMATIC TELESCOPES ON THE CONTINENT, AND IN
GREAT BRITAIN.

The Dorpat telescope--Sir J. South’s telescope--Captain Smyth’s--Rev.
Dr. Pearson’s--Mr. Lawson’s--Mr. Cooper’s--Mr. Bridges’,
&c.,--Achromatics in Cambridge and Paris observatories

  _pages_ 248-254.


ACHROMATIC TELESCOPES OF A MODERATE SIZE, WITH THEIR PRICES, AS SOLD BY
LONDON OPTICIANS.

The 2-1/2 feet Achromatic--The 3-1/2 feet--The powers applied to
it--and the views it gives of the heavenly bodies--The 5 feet
achromatic--_Stands_ for telescopes, illustrated by engravings

  _page_ 254-264.


PROPORTIONS OR CURVATURE OF THE LENSES WHICH FORM AN ACHROMATIC
OBJECT-GLASS.

Various tables and explanations

  _page_ 265-269.


ACHROMATIC TELESCOPES COMPOSED OF FLUID LENSES.

Blair’s fluid telescope, with an account of its performance--Barlow’s
large refracting telescope with a fluid concave lens--Its construction,
and the effect it produces on double stars, &c.--Rogers’ achromatic
telescope on a new plan--Wilson’s telescope, &c.

  _page_ 269-283.


CHAPTER V.

ON REFLECTING TELESCOPES.

SECT. 1.--HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF
THE CONSTRUCTION OF THESE INSTRUMENTS.

Gregory’s Reflector--Newtonian Reflector--Cassegrainian
Reflector--Magnifying powers of reflectors--Short’s Reflectors--Their
powers and prices--_General remarks on Gregorian reflectors_--Apertures
and magnifying powers of Newtonian telescopes--_Prices_ of Reflecting
telescopes

  _page_ 284-301.


SECT. 2.--THE HERSCHELIAN TELESCOPE.

Description of Sir W. Herschel’s 40 feet telescope, with its machinery,
apparatus, and the discoveries made by it--Sir J. Herschel’s 20 feet
reflector _page_ 301-308.


SECT. 3.--RAMAGE’S LARGE REFLECTING TELESCOPE.

  _page_ 308-311.


SECT. 4.--THE AERIAL REFLECTOR--CONSTRUCTED BY THE AUTHOR.

Construction of this telescope, and the manner of using
it--Illustrated by figures--Its properties and advantages--Tube not
necessary in reflecting telescopes--How a large reflector might be
constructed without a tube--How the form of a telescope may be used for
viewing perspectives

  _page_ 311-325.


SECT. 4.--EARL OF ROSSE’S REFLECTING TELESCOPES.

His mode of forming a large speculum, &c., see also, Appendix

  _page_ 325-328.


SECT. 5.--REFLECTING TELESCOPES WITH GLASS SPECULA.

Various experiments on this subject, with their results

  _page_ 329-331.


SECT. 6.--A REFLECTING TELESCOPE WITH A SINGLE MIRROR AND NO
EYE-PIECE.

Experiments illustrative of this construction

  _page_ 332-334.


ON THE EYE-PIECES OF TELESCOPES. ASTRONOMICAL EYE-PIECES.

Huygenian eye-piece--Ramsden’s eye-piece--Aberration of
lenses--Celestial eye-pieces with variable powers. _Diagonal_
eye-pieces--Various forms of them described--Various aspects in which
objects may be viewed by them

  _page_ 335-347.


TERRESTRIAL EYE-PIECES.

Eye-pieces with four lenses--Proportions of the focal lengths of these
lenses--Dimensions and powers of several eye-pieces stated

  _page_ 347-353.


DESCRIPTION OF AN EYE-PIECE, &C., OF AN OLD DUTCH ACHROMATIC TELESCOPE.

This telescope supposed to have been _invented_ in Holland before
Dollond’s discovery was known--Peculiarity of its eye-piece

  _page_ 354-357.


DESCRIPTION OF THE PANCRATIC EYE-TUBE.

  _page_ 357-360.


CHAPTER VI.

MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.

1. Adjustments requisite to be attended to in the use of
telescopes--2. State of the atmosphere most proper for observing
terrestrial and celestial objects--Average number of hours in the year
fit for celestial observations.--3. On the magnifying powers requisite
for observing the phenomena of the different planets--Comets--Double
stars, &c.--Illustrated at large from p. 369-380.--4. Mode of
exhibiting the solar spots--Eye-pieces best adapted for this
purpose--How they may be exhibited to a large company--Mode in which
their dimensions may be determined.--5. On the _space-penetrating_
power of telescopes--Herschel’s observations on space-penetrating
powers--Comparison of achromatic and Gregorian reflectors.--6. On
choosing telescopes, and ascertaining their properties--Various modes
of ascertaining the goodness of telescopes--General remarks and
cautions on this point--A circumstance which requires to be attended
to in using achromatics.--7. On the mode of determining the magnifying
power of telescopes--Various experiments in relation to this point.--8.
On cleaning the lenses of telescopes

  _page_ 361-407.


ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.

Mode of adapting a telescope for this purpose--objects to which they
may be applied

  _page_ 407-411.


REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF
TELESCOPES.

Wonderful and mysterious nature of light--The organ of vision, and its
expansive range--Wonderful nature of the telescope, and the objects it
has disclosed to view--No boundaries should be set to the discoveries
of science and the improvement of art--The telescope is a machine which
virtually transports us to the distant regions of space--It enlarges
our views of the sublime scenes of creation--It has tended to amplify
our conceptions of the empire and the attributes of the Deity--Various
uses of this instrument in relation to science and common life

  _page_ 411-431.


CHAPTER VII.

ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND
SPECULA.

1. Directions for grinding lenses for eye-glasses, microscopes,
&c.--2. Method of casting and grinding the specula of reflecting
telescopes--Compositions for speculum metal--To try the figure of the
metal--To adjust the eye-hole of Gregorian reflectors--To center the
specula--To center lenses. _page_ 432-442.


PART III.

ON VARIOUS ASTRONOMICAL INSTRUMENTS.


CHAPTER I.

ON MICROMETERS.

Various descriptions of micrometers--_Cavallo’s_ micrometer
described--To ascertain the value of its divisions--Practical uses
of this micrometer--Problems which may be solved by it--Tables for
facilitating its use

  _page_ 443-452.


CHAPTER II.

ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY.

History of equatorials--Description of one of the simplest construction
of these instruments--To adjust the equatorial for observation--To
adjust the line of sight--Description of the _nonius_--To find the
meridian line by one observation--Manner of observing stars and planets
in the day-time

  _page_ 453-464.


OBSERVATIONS, BY THE AUTHOR, ON THE FIXED STARS AND PLANETS, MADE IN
THE DAY-TIME, BY THE EQUATORIAL.

Object of these observations--stars of the first and second
magnitudes--General deductions from these observations

  _page_ 464-469.


OBSERVATIONS ON THE PLANETS IN THE DAY-TIME.

Series of observations on _Venus_, when near the sun--Seen at the time
of her superior conjunction in 1843--Conclusions deduced from these
observations--phenomena observed during these observations--Remarkable
phenomenon during an eclipse of the sun

  _page_ 469-480.


OBSERVATIONS ON JUPITER AND OTHER PLANETS.

General conclusions, &c.

  _page_ 480-485.

UTILITY OF CELESTIAL DAY OBSERVATIONS.

  _page_ 485-491.

ON THE ASTRONOMICAL QUADRANT.

  _page_ 492-496.

THE ASTRONOMICAL CIRCLE.

  _page_ 496-502.

THE TRANSIT INSTRUMENT.

  _page_ 502-505.


CHAPTER III.

ON OBSERVATORIES.

Leading features of a spot adapted for celestial observations--Public
and private observatories--Greenwich observatory--Instruments with
which an observatory should be furnished--The Author’s private
observatory--Revolving domes for observatories--Cautions to be attended
to in celestial observations

  _page_ 506-516.


CHAPTER IV.

ON ORRERIES OR PLANETARIUMS.

History of such machines--Sphere of Archimedes and Posidonius--Dr.
Long’s _Uranium_--Wheel-work of the common Planetarium--Figure
representing this machine--Problems which may be performed by it

  _page_ 517-527.


DR. HENDERSON’S PLANETARIUM.

Section of its wheel-work--Number of teeth in the wheels and pinions
which move the different planets--Extreme accuracy of these movements.
_page_ 527-538.


ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SATURN’S RING,
ILLUSTRATED WITH 13 VIEWS.

When and by whom its true figure was discovered.

  _page_ 538-543.


ON THE SUPPOSED DIVISION OF THE EXTERIOR RING OF SATURN.

Kater’s, Short’s, Quetelet’s and Decuppis’s observations

  _page_ 543-547.


APPENDIX.

1. DESCRIPTION OF THE EARL OF ROSSE’S LARGEST TELESCOPE.

Composition of the speculum, and the process of casting it--Mode of
grinding and polishing it--Manner in which it is filled up--Expenses
incurred in its construction--Results of observations which have been
made with it--Two views representing this instrument and the buildings
connected with it--Sir J. South’s remarks and anticipations

  _page_ 548-562.


2. HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF
TELESCOPES.

  _page_ 563.




LIST OF ENGRAVINGS.


  _Figure_                                                        _Page_

  1. Representation of the diminution of the intensity of light.      22

  2. Illustrative of the refraction of light.                         43

  3. Representing the angles of incidence and refraction.             44

  4. The refraction of the atmosphere.                                51

  5. Various forms of lenses.                                         65

  6, 7, 8. Parallel, converging, and diverging rays.                  66

  9, 10, 11. Passage of parallel, diverging, and
     converging rays through convex lenses.                           67

  12. Passage of parallel rays through concave lenses.                69

  13. Images formed by convex lenses.                                 71

  14. Angle of incidence and reflection.                              83

  15. Images as reflected from a plane mirror.                        84

  16. Illustrative of reflections from a plane mirror.                85

  17. Shewing how the image in a plane mirror is twice the
      length of the object.                                           86

  18. Reflection from _concave_ mirrors.                              87

  19. Reflection from _convex_ mirrors.                               89

  20. Parallel rays as reflected from concave mirrors.                91

  21. Diverging rays as reflected from concave mirrors.               91

  22. Images formed before concave mirrors.                           93

  23. Images formed behind concave mirrors.                           96

  24. Illustrating the magnifying power of concave mirrors.           97

  25. Inverted images formed in the front of concave mirrors.         98

  26. Illustrative of deceptions produced by concave mirrors.        100

  27, 28. Experiment with a bottle half filled with water.           101

  29. Effect of extraordinary refraction on ships at sea.            109

  30. Experiment for illustrating the causes of uncommon refraction. 117

  31. Prismatic spectrum.                                            127

  32. Different foci of  rays in convex lenses.              129

  33. Experiment to show the different foci of red and violet rays.  129

  34. Illustrative of the prismatic colours.                         136

  35. Explanatory of refraction and reflection from drops of rain.   147

  36. Explanatory of the rainbow.                                    149

  37. Images of objects formed in a dark chamber.                    187

  38. The revolving Camera Obscura.                                  194

  39, 40. The portable Camera Obscura.                          195, 196

  40, 41, 42. Illustrative of the angle of vision,
      and the apparent magnitude of objects.               206, 207, 208

  43. The Galilean telescope.                                        215

  44. The astronomical telescope.                                    218

  45, 46. The aerial refracting telescope.                           226

  47. The common refracting telescope.                               228

  48, 49, 50. Manner in which the rays of light are refracted in
      telescopes.                                                    231

  51. Telescope with a single lens.                                  234

  52. Illustrative of spherical aberration                           236

  53. Illustrative of the principle of achromatic telescopes.        241

  54, 55. Double and treble achromatic object-glass.                 242

  57. Common stand for achromatic telescopes.                        260

  58. Equatorial stand for achromatic telescopes.                    262

  59. Dollond’s stand for achromatic telescopes.                     264

  60. Blair’s fluid achromatic object-glass.                         271

  61. Barlow’s fluid telescope.                                      274

  62, 63, 64, 65, 66. Various forms of reflecting telescopes.        288

  67. Gregorian reflecting telescope.                                293

  69. The aerial reflector.                                          313

  70. Front view of the aerial reflector.                            314

  71. Construction of large reflecting telescope                     322

  72. Reflecting telescope with a single mirror                      332

  73. Huygenian eye-piece.                                           336

  74. Ramsden’s eye-piece.                                           339

  75, 76. Combination of lenses for achromatic eye-pieces.           340

  77, 78. Diagonal eye-pieces.                                  344, 345

  79. Terrestrial eye-piece with four lenses.                        349

  80. Eye-piece of an old Dutch achromatic telescope.                356

  81. Pancratic eye-piece.                                           359

  82. Manner of exhibiting the solar spots.                          384

  84. Mode of measuring distances from one station.                  430

  85. Cavallo’s micrometer.                                          446

  86. The equatorial telescope, or portable observatory.             455

  87. Figure to illustrate the principle of the quadrant.            491

  88. The astronomical quadrant.                                     493

  89. The astronomical circle.                                       496

  90. The transit instrument.                                        502

  91. Plan of a private observatory.                                 511

  92. Rotatory dome for an observatory.                              513

  93. Wheel-work of a planetarium.                                   521

  94. Perspective view of a planetarium.                             522

  95. Apparatus for exhibiting the retrograde motions of the
      planets.                                                       525

  96. Section of the wheel-work of Dr. Henderson’s planetarium.      528

  97. Thirteen views of the supposed form of Saturn’s ring.          539

  98. Earl of Rosse’s Great Telescope.                               559

  99. Section of the machinery connected with the telescope.         560

  100. Perspective view of the author’s observatory--to front the title.




THE

PRACTICAL ASTRONOMER.




PART I.

ON LIGHT.


INTRODUCTION.

Light is that invisible etherial matter which renders objects
perceptible by the visual organs. It appears to be distributed
throughout the immensity of the universe, and is essentially requisite
to the enjoyment of every rank of perceptive existence. It is by
the agency of this mysterious substance, that we become acquainted
with the beauties and sublimities of the universe, and the wonderful
operations of the Almighty Creator. Without its universal influence,
an impenetrable veil would be thrown over the distant scenes of
creation; the sun, the moon, the planets, and the starry orbs, would
be shrouded in the deepest darkness, and the variegated surface of
the globe on which we dwell, would be almost unnoticed and unknown.
Creation would disappear, a mysterious gloom would surround the mind
of every intelligence, all around would appear a dismal waste, and an
undistinguished chaos. To whatever quarter we might turn, no form nor
comeliness would be seen, and scarcely a trace of the perfections and
agency of an All Wise and Almighty Being could be perceived throughout
the universal gloom. In short, without the influence of light, no world
could be inhabited, no animated being could subsist in the manner it
now does, no knowledge could be acquired of the works of God, and
happiness, even in the lowest degree, could scarcely be enjoyed by any
organized intelligence.

We have never yet known what it is to live in a world deprived of this
delightful visitant; for in the darkest night we enjoy a share of its
beneficial agency, and even in the deepest dungeon its influence is
not altogether unfelt.[1] The blind, indeed, do not directly enjoy
the advantages of light, but its influence is reflected upon them,
and their knowledge is promoted through the medium of those who enjoy
the use of their visual organs. Were all the inhabitants of the world
deprived of their eye-sight, neither knowledge nor happiness, such as
we now possess, could possibly be enjoyed.

There is nothing which so strikingly displays the beneficial and
enlivening effects of light, as the dawn of a mild morning after a
night of darkness and tempest. All appears gloom and desolation, in
our terrestrial abode, till a faint light begins to whiten the eastern
horizon. Every succeeding moment brings along with it something new
and enlivening. The crescent of light towards the east, now expands its
dimensions and rises upwards towards the cope of heaven; and objects,
which a little before were immersed in the deepest gloom, begin to be
clearly distinguished. At length the sun arises, and all nature is
animated by his appearance; the magnificent scene of creation, which
a little before was involved in obscurity, opens gradually to view,
and every object around excites sentiments of wonder, delight, and
adoration. The radiance which emanates from this luminary, displays
before us a world strewed with blessings and embellished with the most
beautiful attire. It unveils the lofty mountains and the forests with
which they are crowned--the fruitful fields with the crops that cover
them--the meadows, with the rivers which water and refresh them--the
plains adorned with verdure, the placid lake and the expansive ocean.
It removes the curtain of darkness from the abodes of men, and shows us
the cities, towns and villages, the lofty domes, the glittering spires,
and the palaces and temples with which the landscape is adorned. The
flowers expand their buds and put forth their colours, the birds awake
to melody, man goes forth to his labour, the sounds of human voices are
heard, and all appears life and activity, as if a new world had emerged
from the darkness of Chaos.

The whole of this splendid scene, which light produces, may be
considered as a new creation, no less grand and beneficent than the
first creation, when the command was issued, “Let there be light, and
light was.” The aurora and the rising sun cause the earth and all
the objects which adorn its surface, to arise out of that profound
darkness and apparent desolation which deprived us of the view of them,
as if they had been no more. It may be affirmed, in full accordance
with truth, that the efflux of light in the dawn of the morning, after
a dark and cloudy night, is even more magnificent and exhilarating
than at the first moment of its creation. At that period, there
were no spectators on earth to admire its glorious effects; and no
objects, such as we now behold, to be embellished with its radiance.
The earth was a shapeless chaos, where no beauty or order could be
perceived; the mountains had not reared their heads; the seas were not
collected into their channels; no rivers rolled through the valleys, no
verdure adorned the plains; the atmosphere was not raised on high to
reflect the radiance, and no animated beings existed to diversify and
enliven the scene. But now, when the dawning of the morning scatters
the darkness of the night, it opens to view a scene of beauty and
magnificence. The heavens are adorned with azure, the clouds are tinged
with the most lively colours, the mountains and plains are clothed with
verdure, and the whole of this lower creation stands forth arrayed with
diversified scenes of beneficence and grandeur, while the contemplative
eye looks round and wonders.

Such, then, are the important and beneficent effects of that _light_
which every moment diffuses its blessings around us. It may justly
be considered as one of the most essential substances connected with
the system of the material universe, and which gives efficiency to
all the other principles and arrangements of nature. Hence we are
informed, in the sacred history, that light was the first production
of the Almighty Creator, and the first born of created beings; for
without it the universe would have presented nothing but an immense
blank to all sentient existences. Hence, likewise, the Divine Being
is metaphorically represented under the idea of light, as being the
source of knowledge and felicity to all subordinate intelligences: “God
is _light_, and in Him is no darkness at all;” and he is exhibited as
“dwelling in light unapproachable and full of glory, whom no man hath
seen or can see.” In allusion to these circumstances, Milton, in his
Paradise Lost, introduces the following beautiful apostrophe:--

    ‘Hail holy light! offspring of heaven first born,
    Or of the eternal co-eternal beam!
    May I express thee unblam’d? since God is light,
    And never but in unapproached light
    Dwelt from eternity; dwelt then in thee,
    Bright effluence of bright essence increate.
    ----Before the sun
    Before the heavens thou wert, and at the voice
    Of God, as with a mantle, did’st invest
    The rising world of waters dark and deep
    Won from the void and formless infinite.’

As light is an element of so much importance and utility in the
system of nature, so we find that arrangements have been made for its
universal diffusion throughout all the worlds in the universe. The sun
is one of the principal sources of light to this earth on which we
dwell, and to all the other planetary bodies. And, in order that it may
be _equally_ distributed over every portion of the surfaces of these
globes, to suit the exigencies of their inhabitants, they are endowed
with a motion of rotation, by which every part of their surfaces is
alternately turned towards the source of light; and when one hemisphere
is deprived of the direct influence of the solar rays, its inhabitants
derive a portion of light from luminaries in more distant regions, and
have their views directed to other suns and systems dispersed, in
countless numbers, throughout the remote spaces of the universe. Around
several of the planets, satellites, or moons, have been arranged for
the purpose of throwing light on their surfaces in the absence of the
sun, while at the same time the primary planets themselves reflect an
effulgence of light upon their satellites. All the stars which our
unassisted vision can discern in the midnight sky, and the millions
more which the telescope alone enables us to descry, must be considered
as so many fountains of light, not merely to illuminate the voids of
immensity, but to irradiate with their beams surrounding worlds with
which they are more immediately connected, and to diffuse a general
lustre throughout the amplitudes of infinite space. And, therefore,
we have every reason to believe, that, could we fly, for thousands of
years, with the swiftness of a seraph, through the spaces of immensity,
we should never approach a region of absolute darkness, but should find
ourselves, every moment encompassed with the emanations of light, and
cheered with its benign influences. That Almighty Being who inhabiteth
immensity and “dwells in light inaccessible,” evidently appears to
have diffused light over the remotest spaces of his creation, and to
have thrown a radiance upon all the provinces of his wide and eternal
empire, so that every intellectual being, wherever existing, may feel
its beneficent effects, and be enabled, through its agency, to trace
his wonderful operations, and the glorious attributes with which he is
invested.

As the science of astronomy depends solely on the influence of light
upon the organ of vision, which is the most noble and extensive of
all our senses; and as the construction of telescopes and other
astronomical instruments is founded upon our knowledge of the
nature of light and the laws by which it operates--it is essentially
requisite, before proceeding to a description of such instruments, to
take a cursory view of its nature and properties, in so far as they
have been ascertained, and the effects it produces when obstructed by
certain bodies, or when passing through different mediums.




CHAPTER I.

GENERAL PROPERTIES OF LIGHT.


It is not my intention to discuss the subject of light in minute
detail--a subject which is of considerable extent, and which would
require a separate treatise to illustrate it in all its aspects and
bearings. All that I propose is to offer a few illustrations of
its general properties, and the laws by which it is refracted and
reflected, so as to prepare the way for explaining the nature and
construction of telescopes, and other optical instruments.

There is no branch of natural science more deserving of our study and
investigation than that which relates to light--whether we consider its
beautiful and extensive effects--the magnificence and grandeur of the
objects it unfolds to view--the numerous and diversified phenomena it
exhibits--the optical instruments which a knowledge of its properties
has enabled us to construct--or the daily advantages we derive, as
social beings, from its universal diffusion. If air, which serves as
the medium of sound, and the vehicle of speech, enables us to carry on
an interchange of thought and affection with our fellow-men; how much
more extensively is that intercourse increased by light, which presents
the images of our friends and other objects as it were immediately
before us, in all their interesting forms and aspects--the speaking
eye--the rosy cheeks--the benevolent smile, and the intellectual
forehead! The eye, more susceptible of multifarious impressions than
the other senses, ‘takes in at once the landscape of the world,’ and
enables us to distinguish, in a moment, the shapes and forms of all
its objects, their relative positions, the colours that adorn them,
their diversified aspect, and the motions by which they are transported
from one portion of space to another. Light, through the medium of the
eye, not only unfolds to us the persons of others, in all their minute
modifications and peculiarities, but exhibits us to ourselves. It
presents to our own vision a faithful portrait of our peculiar features
behind reflecting substances, without which property we should remain
entirely ignorant of those traits of countenance which characterize us
in the eyes of others.

But, what is the nature of this substance we call _light_, which thus
unfolds to us the scenes of creation? On this subject two leading
opinions have prevailed in the philosophical world. One of those
opinions is, that the whole sphere of the universe is filled with a
subtle matter, which receives from luminous bodies an agitation which
is incessantly continued, and which, by its vibratory motion, enables
us to perceive luminous bodies. According to this opinion, light may
be considered as analogous to sound, which is conveyed to the ear by
the vibratory motions of the air. This was the hypothesis of Descartes,
which was adopted, with some modifications, by the celebrated Euler,
Huygens, Franklin, and other philosophers, and has been admitted by
several scientific gentlemen of the present day. The other opinion is,
that light consists of the emission or emanation of the particles of
luminous bodies, thrown out incessantly on all sides, in consequence
of the continued agitation it experiences. This is the hypothesis of
the illustrious Newton, and has been most generally adopted by British
philosophers.

To the first hypothesis, it is objected that, if true, ‘light would
not only spread itself in a direct line, but its motion would be
transmitted in every direction like that of sound, and would convey
the impression of luminous bodies in the regions of space beyond the
obstacles that intervene to stop its progress.’ No wall or other opaque
body could obstruct its course, if it undulated in every direction like
sound; and it would be a necessary consequence, that we should have no
night, nor any such phenomena as eclipses of the sun or moon, or of
the satellites of Jupiter and Saturn. This objection has never been
very satisfactorily answered. On the other hand, Euler brings forward
the following objections against the Newtonian doctrine of emanation.
1. That, were the sun emitting continually, and in all directions,
such floods of luminous matter with a velocity so prodigious, he must
speedily be exhausted, or at least, some alteration must, after the
lapse of so many ages, be perceptible. 2. That the sun is not the only
body that emits rays, but that all the stars have the same quality; and
as every where the rays of the sun must be crossing the rays of the
stars, their collision must be violent in the extreme, and that their
direction must be changed by such a collision.[2]

To the first of these objections it is answered--that so vast is the
tenuity of light, that it utterly exceeds the power of conception:
the most delicate instrument having never been certainly put in
motion by the impulse of the accumulated sun-beams. It has been
calculated that in the space of 385,130,000 Egyptian years, (of 360
days,) the sun would lose only the 1/1,217,420th of his bulk from the
continual efflux of his light. And, therefore, if in 385 _millions_ of
years the sun’s diminution would be so extremely small, it would be
altogether insensible during the comparatively short period of five
or six thousand years. To the second objection it is replied--that
the particles of light are so extremely rare that their distance from
one another is incomparably greater than their diameters--that all
objections of this kind vanish when we attend to the continuation of
the impression upon the retina, and to the small number of luminous
particles which are on that account necessary for producing constant
vision. For it appears, from the accurate experiments of M. D’Arcy,
that the impression of light upon the retina continues _eight thirds_,
and as a particle of light would move through 26,000 miles in that
time, constant vision would be maintained by a succession of luminous
particles twenty-six thousand miles distant from each other.

Without attempting to decide on the merits of these two hypotheses, I
shall leave the reader to adopt that opinion which he may judge to be
attended with the fewest difficulties, and proceed to illustrate some
of the _properties of light_:--and in the discussion of this subject, I
shall generally adhere to the terms employed by those who have adopted
the hypothesis of the _emanation_ of light.

1. _Light emanates or radiates from luminous bodies in a straight
line._ This property is proved by the impossibility of seeing light
through bent tubes, or small holes pierced in metallic plates placed
one behind another, except the holes be placed in a straight line.
If we endeavour to look at the sun or a candle through the bore of a
bended pipe, we cannot perceive the object, nor any light proceeding
from it, but through a straight pipe the object may be perceived. This
is likewise evident from the form of the rays of light that penetrate
a dark room, which proceed straight forward in lines proceeding from
the luminous body; and from the form of the _shadows_ which bodies
project, which are bounded by right lines passing from the luminous
body, and meeting the lines which terminate the interposing body. This
property may be demonstrated to the eye, by causing light to pass
through small holes into a dark room filled with smoke or dust. It is
to be understood, however, that in this case, the rays of light are
considered as passing through the same medium; for when they pass from
air into water, glass, or other media, they are bent at the point where
they enter a different medium, as we shall afterwards have occasion to
explain.

2. _Light moves with amazing velocity._ The ancients believed
that it was propagated from the sun and other luminous bodies
_instantaneously_; but the observations of modern astronomers have
demonstrated that this is an erroneous hypothesis, and that light, like
other projectiles, occupies a certain time in passing from one part of
space to another. Its velocity, however, is prodigious, and exceeds
that of any other body with which we are acquainted. It flies across
the earth’s orbit--a space 190 millions of miles in extent, in the
course of sixteen and a half minutes, which is at the rate of 192,000
miles every second, and more than a million of times swifter than a
cannon ball flying with its greatest velocity. It appears from the
discoveries of Dr. Bradley, respecting the aberration of the stars,
that light flies from those bodies, with a velocity similar, if not
exactly the same; so that the light of the sun, the planets, the stars,
and every luminous body in the universe is propagated with _uniform_
velocity.[3] But, if the velocity of light be so very great, it may be
asked, how does it not strike against all objects with a force equal
to its velocity? If the finest sand were thrown against our bodies
with the hundredth part of this velocity, each grain would pierce us
as certainly as the sharpest and swiftest arrows from a bow. It is a
principle in mechanics that the force with which all bodies strike,
is in proportion to the size of these bodies, or the quantity of
matter they contain, multiplied by the velocity with which they move.
Therefore if the particles of light were not almost infinitely small,
they would, of necessity prove destructive in the highest degree. If a
particle of light were equal in size to the twelve hundred thousandth
part of a small grain of sand,--supposing light to be material--we
should be no more able to withstand its force than we should that of
sand shot point blank from the mouth of a cannon. Every object would
be battered and perforated by such celestial artillery, till our world
were laid in ruins, and every living being destroyed. And herein
are the wisdom and benevolence of the Creator displayed in making
the particles of light so extremely small as to render them in some
degree proportionate to the greatness of the force with which they are
impelled; otherwise, all nature would have been thrown into ruin and
confusion, and the great globes of the universe shattered to atoms.

We have many proofs, besides the above, that the particles of light are
next to infinitely small. We find that they penetrate with facility the
hardest substances, such as crystal, glass, various kinds of precious
stones, and even the diamond itself, though among the hardest of
stones; for such bodies could not be transparent, unless light found
an easy passage through their pores. When a candle is lighted in an
elevated situation, in the space of a second or two, it will fill a
cubical space (if there be no interruption) of two miles around it, in
every direction, with luminous particles, before the least sensible
part of its substance is lost by the candle:--that is, it will in a
short instant, fill a sphere four miles in diameter, twelve and a half
miles in circumference, and containing thirty-three and a half cubical
miles with particles of light; for an eye placed in any part of this
cubical space would perceive the light emitted by the candle. It has
been calculated that the number of particles of light contained in
such a space cannot be less than _four hundred septillions_--a number
which is _six billions_ of times greater than the number of grains of
sand which could be contained in the whole earth considered as a solid
globe, and supposing each cubic inch of it to contain ten hundred
thousand grains. Such is the inconceivable tenuity of that substance
which emanates from all luminous bodies, and which gives beauty and
splendour to the universe! This may also be evinced by the following
experiment. Make a small pin-hole in a piece of black paper, and hold
the paper upright facing a row of candles placed near each other, and
at a little distance behind the black paper, place a piece of white
pasteboard. On this pasteboard the rays which flow from all the candles
through the small hole in the black paper, will form as many specks
of light as there are candles, each speck being as clear and distinct
as if there were only one speck from a single candle. This experiment
shows that the streams of light from the different candles pass through
the small hole without confusion, and consequently, that the particles
of light are exceedingly small. For the same reason we can easily see
through a small hole not more than 1/100th of an inch in diameter, the
sky, the trees, houses, and nearly all the objects in an extensive
landscape, occupying nearly an entire hemisphere, the light of all
which may pass through this small aperture.

3. _Light is sent forth in all directions from every visible point
of luminous bodies._ If we hold a sheet of paper before a candle,
or the sun, or any other source of light, we shall find that the
paper is illuminated in whatever position we hold it, provided the
light is not obstructed by its edge or by any other body. Hence,
wherever a spectator is placed with regard to a luminous body,
every point of that part of its surface which is toward him will
be visible, when no intervening object intercepts the passage of
the light. Hence, likewise, it follows, that the sun illuminates,
not only an immense plane extending along the paths of the planets,
from the one side of the orbit of Uranus to the other, but the whole
of that sphere, or solid space, of which the distance of Uranus
is the radius. The diameter of this sphere is three thousand six
hundred _millions_ of miles, and it, consequently, contains about
24,000,000,000,000,000,000,000,000,000, or twenty-four thousand
_quartillions_ of cubical miles,--every point of which immense space
is filled with the solar beams. Not only so, but the whole cubical
space which intervenes between the sun and the nearest fixed stars
is more or less illuminated by his rays. For, at the distance of
Sirius, or any other of the nearest stars, the sun would be visible,
though only as a small twinkling orb; and consequently, his rays
must be diffused, however faint, throughout the most distant spaces
whence he is visible. The diameter of this immense sphere of light
cannot be less than _forty billions_ of miles, and its solid contents
33,500,000,000,000,000,000,000,000,000,000,000,000,000 or, thirty-three
thousand, five hundred _sextillions_ of cubical miles. All this
immense, and incomprehensible space is filled with the radiations of
the solar orb; for were an eye placed in any one point of it, where
no extraneous body interposed, the sun would be visible either as a
large luminous orb, or as a small twinkling star. But he can be visible
only by the rays he emits, and which enter the organs of vision. How
inconceivably immense, then, must be the quantity of rays which are
thrown off in all directions from that luminary which is the source of
our day! Every star must likewise be considered as emitting innumerable
streams of radiance over a space equally extensive, so that no point in
the universe can be conceived where absolute darkness prevails, unless
in the _interior_ regions of planetary bodies.

4. _The effect of light upon the eye is not instantaneous, but
continues for a short space of time._ This may be proved and
illustrated by the following examples:--If a stick--or a ball
connected with a string--be whirled round in a circle, and a certain
degree of velocity given it, the object will appear to fill the whole
circle it describes. If a lighted firebrand be whirled round in the
same rapid manner, a complete circle of light will be exhibited.
This experiment obviously shows that the impression made on the
eye by the light from the ball or the firebrand--when in any given
point of the circle--is sufficiently lasting to remain till it has
described the whole circle, and again renews its effect, as often as
the circular motion is continued. The same is proved by the following
considerations:--We are continually shutting our eyes, _or winking_;
and, during the time our eyes are shut, on such occasions, we should
lose the view of surrounding objects, if the impression of light did
not continue a certain time while the eye-lid covers the pupil; but
experience proves that during such vibrations of the eye-lids, the
light from surrounding objects is not sensibly intercepted. If we look
for some time steadily at the light of a candle, and particularly, if
we look directly at the sun, without any interposing medium, or if we
look for any considerable time at this luminary, through a telescope
with a  glass interposed--in all these cases, if we shut our
eyes immediately after viewing such objects, we shall still perceive a
faint image of the object, by the impression which its light has made
upon our eyes.

‘With respect to the _duration_ of the impression of light, it has been
observed that the teeth of a cog-wheel in a clock were still visible
in succession, when the velocity of rotation brought 246 teeth through
a given fixed point in a second. In this case it is clear that if the
impression made on the eye by the light reflected from any tooth, had
lasted without sensible diminution for the 246th part of a second, the
teeth would have formed one unbroken line, because a new tooth would
have continually arrived in the place of the interior one before its
image could have disappeared. If a live coal be whirled round, it is
observed that the luminous circle is complete, when the rotation is
performed in the (8-1/2)/60th of a second. In this instance we see
that the impression was much more durable than the former. Lastly, if
an observer sitting in a room direct his sight through a window, to
any particular object out of doors, for about half a minute, and then
shut his eyes and cover them with his hands, he will still continue to
see the window, together with the outline of the terrestrial objects
bordering on the sky. This appearance will remain for near a minute,
though occasionally vanishing and changing colour in a manner that
brevity forbids our minutely describing. From these facts we are
authorized to conclude, that all impressions of light on the eye, last
a considerable time, that the brightest objects make the most lasting
impressions; and that, if the object be very bright, or the eye weak,
the impression may remain for a time so strong, as to mix with and
confuse the subsequent impressions made by other objects. In the last
case the eye is said to be _dazzled_ by the light.’[4]

The following experiment has likewise been suggested as a proof of the
impression which light makes upon the eye. If a card, on both sides of
which a figure is drawn, for example, a bird and a cage, be made to
revolve rapidly on the straight line which divides it symmetrically,
the eye will perceive both figures at the same time, provided they
return successively to the same place. M. D’Arcy found by various
experiments, that, in general, the impression which light produces on
the eye, lasts about _the eighth of a second_. M. Plateau, of Brussels,
found that the impression of different colours lasted the following
periods; the numbers here stated being the decimal parts of a _second_.
_Flame_, 0.242. or nearly one fourth of a second; _Burning coal_,
0.229; _White_, 0.182, or, a little more than one sixth of a second;
_Blue_, 0.186; _Yellow_, 0.173; _Red_, 0.184.

5. _Light_, though extremely minute, _is supposed to have a certain
degree of force or momentum_. In order to prove this, the late
ingenious Mr. Mitchell contrived the following experiment. He
constructed a small vane in the form of a common weather-cock, of a
_very thin_ plate of copper, about an inch square, and attached to
one of the finest harpsicord wires, about ten inches long, and nicely
balanced at the other end of the wire, by a grain of very small shot.
The instrument had also fixed to it in the middle, at right angles to
the length of the wire, and in an horizontal direction, a small bit
of a very slender sewing needle, about half an inch long, which was
made magnetical. In this state the whole instrument might weigh about
ten grains. The vane was supported in the manner of the needle in the
mariner’s compass, so that it could turn with the greatest ease; and
to prevent its being affected by the vibrations of the air, it was
enclosed in a glass case or box. The rays of the sun were then thrown
upon the broad part of the vane or copper plate, from a concave mirror
of about two feet diameter, which, passing through the front glass of
the box, were collected into the focus of the mirror upon the copper
plate. In consequence of this the plate began to move with a slow
motion of about an inch in a second of time, till it had moved through
a space of about two inches and a half, when it struck against the
back of the box. The mirror being removed, the instrument returned to
its former situation, and the rays of the sun being again thrown upon
it, it again began to move, and struck against the back of the box as
before. This was repeated three or four times with the same success.

On the above experiment, the following calculation has been founded:
If we impute the motion produced in this experiment to the impulse of
the rays of light, and suppose that the instrument weighed ten grains,
and acquired a velocity of one inch in a second, we shall find that the
quantity of matter contained in the rays falling upon the instrument in
that time amounted to no more than one twelve hundred-millionth part
of a grain, the velocity of light exceeding the velocity of one inch
in a second in the proportion of about 12,000,000,000 to 1. The light
in this experiment was collected from a surface of about three square
feet, which reflecting only about half what falls upon it, the quantity
of matter contained in the rays of the sun incident upon a foot and a
half of surface in one second of time, ought to be no more than the
twelve hundred-millionth part of a grain. But the density of the rays
of light at the surface of the sun is greater than that at the earth
in the proportion of 45,000 to 1; there ought therefore to issue from
one square foot of the sun’s surface in one second of time, in order to
supply the waste by light 1/45,000th part of a grain of matter, that
is, a little more than two grains a day, or about 4,752,000 grains, or
670 pounds avoirdupoise, nearly, in 6,000 years, a quantity which would
have shortened the sun’s diameter no more than about ten feet, if it
were formed of the density of water only.

If the above experiment be considered as having been accurately
performed, and if the calculations founded upon it be correct, it
appears that there can be no grounds for apprehension that the sun can
ever be sensibly diminished by the immense and incessant radiations
proceeding from his body on the supposition that light is a material
emanation. For the diameter of the sun is no less than 880,000 miles;
and, before this diameter could be shortened, by the emission of
light, one English mile, it would require three millions, one hundred
and sixty-eight thousand years, at the rate now stated; and, before
it could be shortened ten miles, it would require a period of above
thirty-one millions of years. And although the sun were thus actually
diminished, it would produce no sensible effect or derangement
throughout the planetary system. We have no reason to believe that
the system, _in its present state and arrangements_, was intended to
endure for ever, and before that luminary could be so far reduced,
during the revolutions of eternity, as to produce any irregularities
in the system, new arrangements and modifications might be introduced
by the hand of the All Wise and Omnipotent Creator. Besides, it is not
improbable that a system of means is established by which the sun and
all the luminaries in the universe receive back again a portion of the
light which they are continually emitting, either from the planets from
whose surfaces it is reflected, or from the millions of stars whose
rays are continually traversing the immense spaces of creation, or
from some other sources to us unknown.

6. _The intensity of light is diminished in proportion to the square
of the distance from the luminous body._ Thus, a person at two feet
distance from a candle, has only the fourth part of the light he would
have at one foot, at three feet distance the ninth part, at four feet
the sixteenth part, at five feet the twenty fifth part, and so on for
other distances. Hence the light received by the planets of the Solar
system decreases in proportion to the squares of the distances of these
bodies from the sun. This may be illustrated by the following figure,

[Illustration: _Figure 1._]

Suppose the light which flows from a point A, and passes through a
square hole B, is received upon a plane C, parallel to the plane of the
hole--or, let the figure C be considered as the shadow of the plane B.
When the distance of C is double of B, the length and breadth of the
shadow C will be each double of the length and breadth of the plane B,
and treble when AD is treble of AB, and so on, which may be easily
examined by the light of a candle placed at A. Therefore the surface of
the shadow C, at the distance AC--double of AB, is divisible into four
squares, and at a treble distance, into nine squares, severally equal
to the square B, as represented in the figure. The light, then, which
falls upon the plane B being suffered to pass to double that distance,
will be uniformly spread over four times the space, and consequently
will be four times thinner in every part of that space. And at a treble
distance it will be nine times thinner, and at a quadruple distance
sixteen times thinner than it was at first. Consequently the quantities
of this rarified light received upon a surface of any given size and
shape when removed successively to these several distances, will be but
one-fourth, one-ninth, one-sixteenth, of the whole quantity received by
it at the first distance AB.

In conformity with this law, the relative quantities of light on the
surfaces of the planets may be easily determined, when their distances
from the sun are known. Thus, the distance of Uranus from the sun is
1,800,000,000 miles, which is about nineteen times greater than the
distance of the earth from the same luminary. The square of 19 is 361;
consequently the earth enjoys 361 times the intensity of light when
compared with that of Uranus; in other words, this distant planet
enjoys only the 1/361 part of the quantity of light which falls upon
the earth. This quantity, however, is equivalent to the light we should
enjoy from the combined effulgence of 348 full moons; and if the
pupils of the eyes of the inhabitants of this planet be much larger
than ours, and the _retina_ of the eye be endued with a much greater
degree of nervous sensibility, they may perceive objects with as great
a degree of splendour as we perceive on the objects which surround
us in this world. Following out the same principle, we find that the
quantity of light enjoyed by the planet Mercury is nearly _seven_ times
greater than that of the Earth, and that of Venus nearly _double_ of
what we enjoy--that Mars has less than the one half--Jupiter the _one
twenty-seventh_ part--and Saturn only the _one ninetieth_ part of the
light which falls upon the Earth. That the light of these distant
planets, however, is not so weak as we might at first imagine appears
from the brilliancy they exhibit, when viewed in our nocturnal sky,
either with the telescope or with the unassisted eye--and likewise from
the circumstance that a very small portion of the Sun--such as the one
fortieth or one fiftieth part diffuses a quantity of light sufficient
for most of the purposes of life, as is found in the case of total
eclipses of the Sun, when his western limb begins to be visible, only
like a fine luminous thread, for his light is then sufficient to render
distinctly visible all the parts of the surrounding landscape.

7. _It is by light reflected from opake bodies that most of the objects
around us are rendered visible._ When a lighted candle is brought
into a dark room, not only the candle but all other bodies in the
room become visible. Rays of the sun passing into a dark room render
luminous a sheet of paper on which they fall, and this sheet in its
turn enlightens, to a certain extent, the whole apartment, and renders
objects in it visible, so long as it receives the rays of the sun. In
like manner, the moon and the planets are opake bodies, but the light
of the sun falling upon them, and being reflected from their surfaces,
renders them visible. Were no light to fall on them from the sun, or
were they not endued with a power of reflecting it, they would be
altogether invisible to our sight. When the moon comes between us and
the sun, as in a total eclipse of that luminary, as no solar light is
reflected from the surface next the earth, she is invisible--only the
curve or outline of her figure being distinguished by her shadow. In
this case, however, there is a certain portion of reflected light on
the lunar hemisphere next the earth, though not distinguishable during
a solar eclipse. The earth is enlightened by the sun, and a portion of
the rays which fall upon it is reflected upon the dark hemisphere of
the moon which is then towards the earth. This reflected light from the
earth is distinctly perceptible, when the moon appears as a slender
crescent, two or three days after new moon--when the earth reflects its
light back on the moon, in the same manner as the full moon reflects
her light on the earth. Hence, even at this period of the moon, her
whole face becomes visible to us, but its light is not uniform or of
equal intensity. The thin crescent on which the full blaze of the solar
light falls, is very brilliant and distinctly seen, while the other
part, on which falls only a comparatively feeble light from the earth,
appears very faint, and is little more than visible to the naked eye,
but with a telescope of moderate power,--if the atmosphere be very
clear--it appears beautifully distinct, so that the relative positions
of many of the lunar spots may be distinguished.

The intensity of reflected light is very small, when compared with
that which proceeds directly from luminous bodies. M. Bouguer, a
French philosopher, who made a variety of experiments to ascertain the
proportion of light emitted by the heavenly bodies, concluded from
these experiments, that the light transmitted from the sun to the
earth is at least 300,000 times as great as that which descends to us
from the full moon--and that, of 300,000 rays which the moon receives,
from 170,000 to 200,000 are absorbed. Hence we find that, however
brilliant the moon may appear at night--in the day time she appears as
obscure as a small portion of dusky cloud to which she happens to be
adjacent, and reflects no more light than a portion of whitish cloud of
the same size. And as the full moon fills only the ninety thousandth
part of the sky, it would require at least ninety thousand moons to
produce as much light as we enjoy in the day-time under a cloudy sky.

As the moon and the planets are rendered visible to us only by light
reflected from their surfaces, so it is in the same way that the images
of most of the objects around us are conveyed to our organs of vision.
We behold all the objects which compose an extensive landscape,--the
hills and vales, the woods and lawns, the lakes and rivers, and the
habitations of man--in consequence of the capacity with which they
are endued of sending forth reflected rays to the eye, from every
point of their surfaces and in all directions. In connection with the
reflection of light, the following curious observation may be stated.
Baron Funk, visiting some silver mines in Sweden, observed, that,
‘in a clear day, it was as dark as pitch underground in the eye of a
pit, at sixty or seventy fathoms deep; whereas, in a cloudy or rainy
day, he could see to read even at 106 fathoms deep. Enquiring of the
miners, he was informed that this is always the case; and reflecting
upon it, he imagined it arose from this circumstance, that when the
atmosphere is full of clouds, light is reflected from them into the
pit in all directions, and that thereby a considerable proportion of
the rays are reflected perpendicularly upon the earth: whereas when the
atmosphere is clear, there are no opaque bodies to reflect the light in
this manner, at least in a sufficient quantity; and rays from the sun
himself can never fall perpendicularly in that country.’--The reason
here assigned is, in all probability, the true cause of the phenomenon
now described.

8. It is supposed by some philosophers that _light is subject to the
same laws of attraction that govern all other material substances_--and
that _it is imbibed and forms a constituent part of certain bodies_.
This has been inferred from the phenomena of the _Bolognian stone_, and
what are generally called the _solar phosphori_. The Bolognian stone
was first discovered about the year 1680, by Leascariolo, a shoe-maker
of Bologna. Having collected together some stones of a shining
appearance at the bottom of Monte Paterno, and being in quest of some
alchemical secret, he put them into a crucible to calcine them--that
is, to reduce them to the state of cinders. Having taken them out of
the crucible, and exposed them to the light of the sun, he afterwards
happened to carry them into a dark place, when to his surprise, he
observed that they possessed a self-illuminating power, and continued
to emit faint rays of light for some hours afterwards. In consequence
of this discovery, the Bolognian spar came into considerable demand
among natural philosophers and the curious in general; and the best
way of preparing it seems to have been hit upon by the family of
Zagoni, who supplied all Europe with Bolognian phosphorus, till the
discovery of more powerful phosphoric substances put an end to their
monopoly.--In the year 1677, Baldwin, a native of Misnia, observed
that chalk dissolved in aqua-fortis exactly resembled the Bolognian
stone in its property of imbibing light, and emitting it after it was
brought into the dark ; and hence it has obtained the name of Baldwin’s
phosphorus.

In 1730 M. du Fay directed his attention to this subject, and observed
that all earthy substances susceptible of calcination, either by
mere fire, or when assisted by the previous action of nitrous acid,
possessed the property of becoming more or less luminous, when calcined
and exposed for a short time in the light--that the most perfect of
these phosphori were limestones, and other kinds of carbonated lime,
gypsum, and particularly the topaz, and that some diamonds were also
observed to be luminous by simple exposure to the sun’s rays. Sometime
afterwards, Beccaria discovered that a great variety of other bodies
were convertible into phosphori by exposure to the mere light of the
sun, such as, organic animal remains, most compound salts, nitre and
borax--all the farinaceous and oily seeds of vegetable substances,
all the gums and several of the resins--the white woods and vegetable
fibre, either in the form of paper or linen; also starch and loaf-sugar
proved to be good phosphori, after being made thoroughly dry, and
exposed to the direct rays of the sun. Certain animal substances by
a similar treatment were also converted into phosphori; particularly
bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The
same property was communicated to rock crystal and some other of
the gems, by rubbing them against each other so as to roughen their
surfaces, and then placing them for some minutes in the focus of a
lens, by which the rays of light were concentrated upon them, at the
same time that they were also moderately heated.

In the year 1768 Mr. Canton contributed some important facts in
relation to solar phosphori, and communicated a method of preparing
a very powerful one, which, after the inventor, is usually called
_Canton’s phosphorus_. He affirms that his phosphorus, enclosed in a
glass flask, and hermetically sealed, retains its property of becoming
luminous for at least four years, without any apparent decrease of
activity. It has also been found that, if a common box smoothing-iron,
heated in the usual manner, be placed for half a minute on a sheet
of dry, white paper, and the paper be then exposed to the light, and
afterwards examined in a dark closet, it will be found that the whole
paper will be luminous, that part, however, on which the iron had stood
being much more shining than the rest.

From the above facts it would seem that certain bodies have the power
of imbibing light and again emitting it, in certain circumstances,
and that this power may remain for a considerable length of time. It
is observed that the light which such bodies emit bears an analogy to
that which they have imbibed. In general, the illuminated phosphorus is
reddish; but when a weak light only has been admitted to it, or when
it has been received through pieces of white paper, the emitted light
is pale or whitish.--Mr. Morgan, in the seventy-fifth volume of the
Philosophical Transactions, treats the subject of light at considerable
length; and as a foundation for his reasoning, he assumes the following
data:--1. That light is a body, and like all others, subject to the
laws of attraction. 2. That light is a heterogeneous body; and that
the same attractive power operates with different degrees of force
on its different parts. To the principle of attraction, likewise,
Sir Isaac Newton has referred the most extraordinary phenomena of
light, Refraction and Inflection. He has also endeavoured to show that
light is not only subject to the law of attraction but of repulsion
also, since it is repelled or reflected from certain bodies. If
such principles be admitted, then, it is highly probable that the
phosphorescent bodies to which we have adverted have a power of
attracting or imbibing the substance of light, and of retaining or
giving it out under certain circumstances, and that the matter of light
is incorporated at least with the surface of such bodies. But on this
subject, as on many others, there is a difference of opinion among
philosophers.[5]

9. _Light is found to produce a remarkable effect on Plants and
Flowers, and other vegetable productions._ Of all the phenomena which
living vegetables exhibit there are few that appear more extraordinary
than the energy and constancy with which their stems incline toward the
light. Most of the discous flowers follow the sun in his course. They
attend him to his evening retreat, and meet his rising lustre in the
morning with the same unerring law. They unfold their flowers on the
approach of this luminary; they follow his course by turning on their
stems, and close them as soon as he disappears. If a plant, also, is
shut up in a dark room, and a small hole afterwards opened by which the
light of the sun may enter, the plant will turn towards that hole, and
even alter its own shape in order to get near it; so that though it
was straight before, it will in time become crooked, that it may get
near the light. Vegetables placed in rooms where they receive light
only in one direction, always extend themselves in that direction. If
they receive light in two directions, they direct their course towards
that which is strongest. It is not the _heat_ but the _light_ of the
sun which the plant thus covets; for, though a fire be kept in the
room, capable of giving a much stronger heat than the sun, the plant
will turn away from the fire in order to enjoy the solar light. Trees
growing in thick forests, where they only receive light from above,
direct their shoots almost invariably upwards, and therefore become
much taller and less spreading than such as stand single.

The _green_ colour of plants is likewise found to depend on the sun’s
light being allowed to shine on them; for without the influence of
the solar light, they are always of a _white_ colour. It is found by
experiment that, if a plant which has been reared in darkness be
exposed to the light of day, in two or three days it will acquire a
green colour perceptibly similar to that of plants which have grown
in open day-light. If we expose to the light one part of the plant,
whether leaf or branch, this part alone will become green. If we cover
any part of a leaf with an opake substance, this place will remain
white, while the rest becomes green. The whiteness of the inner leaves
of cabbages is a partial effect of the same cause, and many other
examples of the same kind might easily be produced. M. Decandolle,
who seems to have paid particular attention to this subject, has the
following remarks: ‘It is certain, that between the white state of
plants vegetating in darkness, and complete verdure, every possible
intermediate degree exists, determined by the intensity of the light.
Of this any one may easily satisfy himself by attending to the colour
of a plant exposed to the full day-light; it exhibits in succession
all the degrees of verdure. I had already seen the same phenomenon,
in a particular manner, by exposing plants reared in darkness to the
light of lamps. In these experiments, I not only saw the colour come on
gradually, according to the continuance of the exposure to light; but
I satisfied myself, that a certain intensity of permanent light never
gives to a plant more than a certain degree of colour. The same fact
readily shows itself in nature, when we examine the plants that grow
under shelter or in forests, or when we examine in succession the state
of the leaves that form the heads of cabbages.’[6]

It is likewise found that the _perspiration_ of vegetables is increased
or diminished, in a certain measure by the degree of _light_ which
falls upon them. The experiments of Mr. P. Miller and others, prove
that plants uniformly perspire most in the forenoon, though the
temperature of the air in which they are placed should be unvaried.
M. Guettard likewise informs us that a plant exposed to the rays of
the sun, has its perspiration increased to a much greater degree than
if it had been exposed to the same heat under the shade. Vegetables
are likewise found to be indebted to light for their smell, taste,
combustibility, maturity, and the resinous principle, which equally
depend upon this fluid. The aromatic substances, resins, and volatile
oil are the productions of southern climates, where the light is more
pure, constant, and intense. In fine, another remarkable property of
light on the vegetable kingdom is that, when vegetables are exposed to
open day-light, or to the sun’s rays, they emit oxygen gas or vital
air. It has been proved that, in the production of this effect, the
sun does not act as a body that heats. The emission of the gas is
determined by the light: pure air is therefore separated by the action
of light, and the operation is stronger as the light is more vivid.
By this continual emission of vital air, the Almighty incessantly
purifies the atmosphere, and repairs the loss of pure air occasioned by
respiration, combustion, fermentation, putrefaction, and numerous other
processes which have a tendency to contaminate this fluid so essential
to the vigor and comfort of animal life; so that, in this way, by the
agency of light, a due equilibrium is always maintained between the
constituent parts of the atmosphere.

In connection with this subject the following curious phenomenon may
be stated, as related by M. Haggern, a Lecturer on Natural History in
Sweden. One evening he perceived a faint flash of light repeatedly
dart from a marigold. Surprised at such an uncommon appearance, he
resolved to examine it with attention; and, to be assured it was no
deception of the eye, he placed a man near him, with orders to make
a signal at the moment when he observed the light. They both saw
it constantly at the same moment. The light was most brilliant on
marigolds of an orange or flame colour, but scarcely visible on pale
ones. The flash was frequently seen on the same flower two or three
times in quick succession; but more commonly at intervals of several
minutes; and when several flowers in the same place emitted their
light together, it could be observed at a considerable distance. The
phenomenon was remarked in the months of July and August at sun-set,
and for half an hour when the atmosphere was clear; but after a
rainy day, or when the air was loaded with vapours, nothing of it
was seen. The following flowers emitted flashes more or less vivid,
in this order:--1. The Marigold, 2. Monk’s hood, 3. The Orange Lily,
4. The Indian Pink. As to the _cause_ of this phenomenon, different
opinions may be entertained. From the rapidity of the flash and other
circumstances, it may be conjectured that electricity is concerned in
producing this appearance. M. Haggern, after having observed the flash
from the orange lily, the antheræ of which are at considerable distance
from the petals, found that the light proceeded from the petals only;
whence he concludes, that this electrical light is caused by the pollen
which, in flying off, is scattered on the petals. But, perhaps, the
true cause of it still remains to be ascertained.

10. _Light has been supposed to produce a certain degree of influence
on the_ PROPAGATION OF SOUND?--M. Parolette, in a long
paper in the ‘Journal de Physique,’ vol. 68, which is copied into
‘Nicholson’s Philosophical Journal,’ vol. 25, pp. 28-39,--has offered
a variety of remarks, and detailed a number of experiments on this
subject. The author states the following circumstances as having
suggested the connection between light and sound. ‘In 1803, I lived
in Paris, and being accustomed to rise before day to finish a work on
which I had long been employed, I found myself frequently disturbed
by the sound of carriages, as my windows looked into one of the most
frequented streets in that city. This circumstance which disturbed me
in my studies every morning, led me to remark, that the appearance of
day-break peculiarly affected the propagation of the sound: from dull
and deep, which it was before day, it seemed to me to acquire a more
sonorous sharpness in the period that succeeded the dissipation of
darkness. The rolling of the wheels seemed to announce the friction
of some substances grown more elastic; and my ear on attending to it
perceived this difference diminish, in proportion as the sound of
wheels was confounded with those excited by the tumult of objects
quitting their nocturnal silence. Struck with this observation, I
attempted to discover whether any particular causes had deceived my
ears. I rose several times before day for this purpose alone, and was
every time confirmed in my suspicion, that light must have a peculiar
influence on the propagation of sound. This variation, however, in
the manner in which the air gave sounds might be the effect of the
agitation of the atmosphere produced by the rarefaction the presence
of the sun occasioned; but the situation of my windows, and the usual
direction of the morning breeze, militated against this argument.’

The author then proceeds to give a description of a very delicate
instrument, and various apparatus for measuring the propagation and
intensity of sound, and the various experiments both in the dark, and
in day-light, and likewise under different changes of the atmosphere,
which were made with his apparatus--all of which tended to prove
that light had a sensible influence in the propagation of sound. But
the detail of these experiments and their several results would be
too tedious to be here transcribed.--The night has generally been
considered as more favourable than the day for the transmission of
sound. ‘That this is the case (says Parolette) with respect to our ears
cannot be doubted; but this argues nothing against my opinion. We hear
further by night on account of the silence, and this always contributes
to it, while the noise of a wind favourable to the propagation of a
sound, may prevent the sound from being heard.’ In reference to the
cause which produces the effect now stated, he proposes the following
queries. ‘Is the atmospheric air more dense on the appearance of light
than in darkness? Is this greater density of the air or of the elastic
fluid that is subservient to the propagation of sound, the effect of
aeriform substances kept in this state through the medium of light?’
He is disposed, on the whole, to conclude, that the effect in question
is owing to the action of light upon the oxygen of the atmosphere,
since oxygen gas is found by experiment to be best adapted to the
transmission of sound.

Our author concludes his communication with the following
remarks:--‘Light has a velocity 900,000 times as rapid as that of
sound. Whether it emanate from the sun and reach to our earth, or
act by means of vibrations agitating the particles of a fluid of a
peculiar nature--the particles of this fluid must be extremely light,
elastic and active. Nor does it appear to me unreasonable, to ascribe
to the mechanical action of these particles set in motion by the sun,
the effects its presence occasions in the vibrations that proceed from
sonorous bodies. The more deeply we investigate the theory of light,
the more we must perceive, that the powers by which the universe is
moved reside in the imperceptible particles of bodies; and that the
grand results of nature are but an assemblage of an order of actions
that take place in its infinitely small parts; consequently, we cannot
institute a series of experiments more interesting than those which
tend to develope the properties of light. Our organs of sense are so
immediately connected with the fluid that enlightens us, that the
notion of having acquired an idea of the mode of action of this fluid
presents itself to our minds, as the hope of a striking advance in
the knowledge of what composes the organic mechanism of our life, and
of that of beings which closely follow the rank assigned to the human
species.’

       *       *       *       *       *

Such is a brief description of some of the leading properties of light.
Of all the objects that present themselves to the philosophic and
contemplative mind, light is one of the noblest and most interesting.
The action it exerts on all the combinations of matter, its extreme
divisibility, the rapidity of its propagation, the sublime wonders it
reveals, and the office it performs in what constitutes the life of
organic beings, lead us to consider it as a substance acting the first
part in the economy of nature. The magic power which this emanation
from the heavens exerts on our organs of vision, in exhibiting to our
view the sublime spectacle of the universe, cannot be sufficiently
admired. Nor is its power confined to the organs of sight; all our
senses are, in a greater or less degree, subjected to the action of
light, and all the objects in this lower creation--whether in the
animal, the vegetable, or the mineral kingdoms--are, to a certain
extent, susceptible of its influence. Our globe appears to be little
more than an accumulation of terrestrial materials introduced into the
boundless ocean of the _solar light_, as a theatre on which it may
display its exhaustless power and energy, and give animation, beauty
and sublimity to every surrounding scene--and to regulate all the
powers of nature, and render them subservient to the purposes for which
they were ordained. This elementary substance appears to be universal
in its movements, and in its influence. It descends to us from the
solar orb. It wings its way through the voids of space, along a course
of ninety-five millions of miles, till it arrives at the outskirts of
our globe; it passes freely through the surrounding atmosphere, it
strikes upon the clouds and is reflected by them; it irradiates the
mountains, the vales, the forests, the rivers, the seas, and all the
productions of the vegetable kingdom, and adorns them with a countless
assemblage of colours. It scatters and disperses its rays from one end
of creation to another, diffusing itself throughout every sphere of the
universe. It flies without intermission from star to star, and from
suns to planets, throughout the boundless sphere of immensity, forming
a connecting chain and a medium of communication among all the worlds
and beings within the wide empire of Omnipotence.

When the sun is said “to rule over the day,” it is intimated that he
acts as the vicegerent of the Almighty, who has invested him with a
mechanical power of giving light, life and motion to all the beings
susceptible of receiving impressions from his radiance. As the servant
of his creator he distributes blessings without number among all the
tribes of sentient and intelligent existence. When his rays illumine
the eastern sky in the morning, all nature is enlivened with his
presence. When he sinks beneath the western horizon, the flowers droop,
the birds retire to their nests, and a mantle of darkness is spread
over the landscape of the world. When he approaches the equinox in
spring, the animal and vegetable tribes revive, and nature puts on a
new and a smiling aspect. When he declines towards the winter solstice,
dreariness and desolation ensue, and a temporary death takes place
among the tribes of the vegetable world.--This splendid luminary,
whose light embellishes the whole of this lower creation, forms the
most lively representation of Him who is the source and the centre of
all beauty and perfection. “God is a sun,” the sun of the moral and
spiritual universe, from whom all the emanations of knowledge, love and
felicity descend. “He covereth himself with light as with a garment.”
and “dwells in light inaccessible and full of glory.” The felicity
and enjoyments of the future world are adumbrated under the ideas of
_light_ and _glory_. “The glory of God enlightens the celestial city,”
its inhabitants are represented as “the saints _in light_,” it is
declared that “their _sun_ shall no more go down,” and that “the Lord
God is their _everlasting light_.” So that light not only cheers and
enlivens all beings throughout the material creation, but is the emblem
of the Eternal Mind, and of all that is delightful and transporting in
the scenes of a blessed immortality.

In the formation of light, and the beneficent effects it produces,
the wisdom and goodness of the Almighty are conspicuously displayed.
Without the beams of the sun and the influence of light, what were all
the realms of this world, but an undistinguished chaos and so many
dungeons of darkness? In vain should we roll our eyes around to behold,
amidst the universal gloom, the flowery fields, the verdant plains, the
flowing streams, the expansive ocean, the moon walking in brightness,
the planets in their courses, or the innumerable host of stars. All
would be lost to the eye of man, and the “blackness of darkness” would
surround him for ever. And with how much wisdom has every thing been
arranged in relation to the motion and minuteness of light? Were it
capable of being transformed into a solid substance, and retain its
present velocity, it would form the most dreadful and appalling element
in nature, and produce universal terror and destruction throughout the
universe. That this is not impossible, and could easily be effected by
the hand of Omnipotence, appears from such substances as _phosphorus_,
where light is supposed to be concentrated in a solid state. But in all
its operations and effects, as it is now directed by unerring wisdom
and beneficence, it exhibits itself as the most benign and delightful
element connected with the constitution of the material system,
diffusing splendour and felicity wherever its influence extends.




CHAPTER II.


ON THE REFRACTION OF LIGHT.

Refraction is the turning or bending of the rays of light out of their
natural course.

Light, when proceeding from a luminous body--without being reflected
from any opake substance or inflected by passing near one--is
invariably found to proceed in straight lines without the least
deviation. But if it happens to pass obliquely from one medium to
another, it always leaves the direction it had before and assumes a
new one. This change of direction, or _bending_ of the rays of light,
is what is called _Refraction_--a term which probably had its origin
from the broken appearance which a staff or a long pole exhibits, when
a portion of it is immersed in water--the word, derived from the Latin
_frango_, literally signifying _breaking_ or bending.

When light is thus refracted, or has taken a new direction, it then
proceeds invariably in a straight line till it meets with a different
medium,[7] when it is again turned out of its course. It must be
observed, however, that though we may by this means cause the rays of
light to make any number of angles in their course, it is impossible
for us to make them describe a curve, except in one single case,
namely, where they pass through a medium, the density of which either
uniformly increases or diminishes. This is the case with the light of
the celestial bodies, which passes downwards through our atmosphere,
and likewise with that which is reflected upwards through it by
terrestrial objects. In both these cases it describes a curve of the
hyperbolic kind; but at all other times, it proceeds in straight lines,
or in what may be taken for straight lines without any sensible error.

There are two circumstances essential to refraction. 1. That the rays
of light shall pass out of one medium into another of a different
density, or of a greater or less degree of resistance. 2. That they
pass in an _oblique_ direction. The denser the refracting medium, or
that into which the ray enters, the greater will be its refracting
power; and of two refracting mediums of the same density, that which
is of an oily or inflammable nature will have a greater refracting
power than the other. The nature of refraction may be more particularly
explained and illustrated by the following figure and description.

Let ADHI fig. 2, be a body of water, AD its surface, C a point in which
a ray of light BC enters from the air into the water. This ray, by the
greater density of the water, instead of passing straight forward in
its first direction to K, will be bent at the point C, and pass along
in the direction CE, which is called the _refracted_ ray. Let the line
FG be drawn perpendicular to the surface of the water in C, then it
is evident that the ray BC, in passing out of air, a _rare_ medium,
into a _dense_ medium, as water, is refracted into a ray CE which is
_nearer_ to the perpendicular CG than the incident ray BC, and on the
contrary, the ray EC passing out of a denser medium into a rarer will
be refracted into CB, which is _farther_ from the perpendicular.

[Illustration: _figure 2._]

The same thing may be otherwise illustrated as follows:--suppose a hole
made in one of the sides of the vessel as at _a_, and a lighted candle
placed within two or three feet of it, when empty, so that its flame
may be at L, a ray of light proceeding from it will pass through the
hole _a_ in a straight line LBCK till it reach the bottom of the vessel
at K, where it will form a small circle of light. Having put a mark at
the point K, pour water into the vessel till it rise to the height AD,
and the round spot that was formerly at K, will appear at E; that is,
the ray which went straight forward, when the vessel was empty, to K,
has been bent at the point C, where it falls into the the water, into
the line CE. In this experiment it is necessary that the front of the
vessel should be of glass, in order that the course of the ray may be
seen; and if a little soap be mixed with the water so as to give it a
little mistiness, the ray CE will be distinctly perceived. If, in place
of fresh water we pour in salt water, it will be found that the ray BC
is more bent at C. In like manner alcohol will refract the ray BC more
than salt water, and oil more than alcohol, and a piece of solid glass,
of the shape of the water, would refract the light still more than the
oil.

The angle of refraction depends on the obliquity of the rays falling on
the refracting surface being always such, that the sine of the incident
angle is to the sine of the refracted angle, in a given proportion. The
_incident_ angle is the angle made by a ray of light and a line drawn
perpendicular to the refracting surface, at the point where the light
enters the surface. The _refracted_ angle is the angle made by the ray
in the refracting medium with the same perpendicular produced. The
_sine_ of the angle is a line which serves to measure the angle, being
drawn from a point in one leg perpendicular to the other. The following
figure (fig. 3.) will tend to illustrate these definitions.

[Illustration: _figure 3._]

In this figure BC is the incident ray, CE the refracted ray, DG the
perpendicular, AD the sine of the angle of incidence ACD, and HR the
sine of the angle of refraction GCE. Now, it is a proposition in optics
that,--the sine AD of the angle of incidence BCD is either accurately
or very nearly in a given proportion to the sine HR of the angle of
refraction GCE. This ratio of the sines is as four to three, when the
refraction is made out of air into water, that is AD is to HR as four
to three. When the refraction is out of air into glass, the proportion
is about as thirty-one to twenty, or nearly as three to two. If the
refraction be out of air into diamond it is as five to two, that is
AD : HR :: 5 : 2. The denser the medium is, the less is the angle and
sine of refraction. If a ray of light MC, were to pass from air into
water, or from empty space into air, in the direction MC perpendicular
to the plane NO which separates the two mediums, it would suffer no
refraction, because one of the essentials to that effect is wanting,
namely, the _obliquity_ of the incidence.

It may be also proper to remark, that a ray of light cannot pass out
of a denser medium into a rarer, if the angle of incidence exceed a
certain limit. Thus a ray of light will not pass out of glass into air,
if the angle of incidence exceed 40° 11´; or out of glass into water,
if the angle of incidence exceed 59° 20´. In such cases refraction will
be changed into reflection.

The following common experiments, which are easily performed, will
illustrate the doctrine of refraction. Put a shilling or any other
small object which is easily distinguished, into a bason or any other
similar vessel, and then retire to such a distance as that the edge
of the vessel shall just hide it from your sight. If then you cause
another person to fill the vessel with water, you will then find that
the shilling is rendered perfectly visible, although you have not in
the slightest degree changed your position. The reason of this is,
that the rays of light, by which it is rendered visible, _are bent
out of their course_. Thus, suppose the shilling to have been placed
in the bottom of the bason at E, (fig. 2.) the ray of light BC which
passes obliquely from the air into water at C, instead of continuing
its course to K, takes the direction CE, and consequently an object
at E would be rendered visible by rays proceeding in that direction,
when they would not have touched it had they proceeded in their direct
course.

The same principle is illustrated by the following experiment. Place
a bason or square box on a table, and a candle at a small distance
from it; lay a small rod or stick across the sides of the bason, and
mark the place where the extremity of the shadow falls, by placing a
shilling or other object at the point; then let water be poured into
the bason, and the shadow will then fall much nearer to the side next
the candle than before. This experiment may likewise be performed by
simply observing the change produced on the shadow of the side of
the bason itself. Again, put a long stick obliquely into deep water,
and the stick will seem to be broken at the point where it appears
at the surface of the water--the part which is immersed in the water
appearing to be bent upwards. Hence every one must have observed that,
in rowing a boat, the ends of the oars appear bent or broken every time
they are immersed in the water, and their appearance at such times is
a representation of the course of the refracted rays. Again, fill a
pretty deep jar with water, and you will observe the bottom of the jar
considerably elevated, so that it appears much shallower than it did
before the water was poured in, in the proportion of nearly a third of
its depth, which is owing to the same cause as that which makes the
end of a stick immersed in water appear more elevated than it would do
if there were no refraction. Another experiment may be just mentioned.
Put a sixpence in a wine-glass, and pour upon it a little water.
When viewed in a certain position, two sixpences will appear in the
glass--one image of the sixpence from below, which comes directly to
the eye, and another which appears considerably raised above the other,
in consequence of the rays of light rising through the water, and being
refracted. In this experiment the wine-glass should not be more than
half filled with water.

The refraction of light explains the causes of many curious and
interesting phenomena both in the heavens and on the earth. When we
stand on the banks of a river, and look obliquely through the waters
to its bottom, we are apt to think it is much shallower than it really
is. If it be eight feet deep in reality, it will appear from the bank
to be only six feet; if it be five feet and a half deep, it will appear
only about four feet. This is owing to the effects of refraction, by
which the bottom of the river is apparently raised by the refraction
of the light passing through the water into air, so as to make the
bottom appear higher than it really is, as in the experiment with the
jar of water. This is a circumstance of some importance to be known
and attended to in order to personal safety. For many school-boys and
other young persons have lost their lives by attempting to ford a
river, the bottom of which appeared to be within their reach, when they
viewed it from its banks: and even adult travellers on horseback have
sometimes fallen victims to this optical deception; and this is not
the only case in which a knowledge of the laws of nature may be useful
in guarding us against dangers and fatal accidents.

It is likewise owing to this refractive power in water, that a skilful
marksman who wishes to shoot fish under water, is obliged to take aim
considerably _below_ the fish as it appears, because it seems much
nearer the top of the water than it really is. An acquaintance with
this property of light is particularly useful to divers, for, in any
of their movements or operations, should they aim directly at the
object, they would arrive at a point considerably beyond it; whereas,
by having some idea of the depth of the water, and the angle which a
line drawn from the eye to the object makes with its surface, the point
at the bottom of the water, between the eye and the object at which the
aim is to be taken, may be easily determined. For the same reason, a
person below water does not see objects distinctly. For, as the aqueous
humour of the eye has the same refractive power as water, the rays of
light from any object under water will undergo no refraction in passing
through the cornea, and aqueous humour, and will therefore meet in a
point far behind the retina. But if any person accustomed to go below
water should use a pair of spectacles, consisting of two convex lenses,
the radius of whose surface is three tenths of an inch--which is nearly
the radius of the convexity of the cornea--he will see objects as
distinctly below water as above it.

It is owing to refraction, that we cannot judge so accurately of
magnitudes and distances in water as in air. A fish looks considerably
larger in water than when taken out of it. An object plunged
_vertically_ into water always appears contracted, and the more so as
its upper extremity approaches nearer the surface of the water. Every
thing remaining in the same situation, if we take the object gradually
out of the water, and it be of a slender form, we shall see it become
larger and larger, by a rapid developement, as it were, of all its
parts. The distortion of objects, seen through a crooked pane of glass
in a window, likewise arises from its unequal refraction of the rays
that pass through it. It has been calculated that in looking through
the common glass of a window, objects appear about the one thirtieth of
an inch out of their real place, by means of the refraction.

Refraction likewise produces an effect upon the _heavenly bodies_, so
that their apparent positions are generally different from their real.
By the refractive power of the atmosphere, the sun is seen before he
comes to the horizon in the morning, and after he sinks beneath it in
the evening; and hence this luminary is never seen in the place in
which it really is, except when it passes the zenith at noon, to places
within the torrid zone. The sun is visible, when actually thirty-two
minutes of a degree below the horizon, and when the opake rotundity of
the earth is interposed between our eye and that orb, just on the same
principle as, in the experiment with the shilling and basin of water,
the shilling was seen when the edge of the basin interposed between it
and the sight. The refractive power of the atmosphere has been found
to be much greater, in certain cases, than what has been now stated.
In the year 1595 a company of Dutch sailors having been wrecked on
the shores of Nova Zembla, and having been obliged to remain in that
desolate region during a night of more than three months--beheld the
sun make his appearance in the horizon about sixteen days before the
time in which he should have risen according to calculation, and when
his body was actually more than four degrees below the horizon; which
circumstance has been attributed to the great refractive power of the
atmosphere in those intensely cold regions. This refraction of the
atmosphere, which renders the apparent rising and setting of the sun
both earlier and later than the real, produces at least one important
beneficial effect. It procures for us the benefit of a much longer day,
at all seasons of the year, than we should enjoy, did not this property
of the atmosphere produce this effect. It is owing to the same cause
that the disks of the sun and moon appear elliptical or oval, when seen
in the horizon, their horizontal diameters appearing longer than their
vertical--which is caused by the greater refraction of the rays coming
from the lower limb, which is immersed in the densest part of the
atmosphere.

The illumination of the heavens which precedes the rising of the sun,
and continues sometime after he is set--or, what is commonly called
the morning and evening _twilight_--is likewise produced by the
atmospherical refraction--which circumstance forms a very pleasing and
beneficial arrangement in the system of nature. It not only prolongs
to us the influence of the solar light, and adds nearly two hours to
the length of our day, but prevents us from being transported all at
once from the darkness of midnight to the splendour of noon-day, and
from the effulgence of day to the gloom and horrors of the night--which
would bewilder the traveller and navigator in their journeys by sea or
land, and strike the living world with terror and amazement.

The following figure will illustrate the position now stated, and
the manner in which the refraction of the atmosphere produces these
effects. Let A _a_ C, fig. 4, represent one half of our globe, and the
dark space between that curve and B _r_ D, the atmosphere. A person
standing on the earth’s surface at _a_ would see the sun rise at _b_,
when that luminary was in reality only at _c_--more than half a degree
below the horizon. When the rays of the sun, after having proceeded
in a straight line through empty space, strike the upper part of the
atmosphere at the point _d_, they are bent out of their right-lined
course, by the refraction of the atmosphere, into the direction _d a_,
so that the body of the sun, though actually intercepted by the curve
of the earth’s convexity consisting of a dense mass of land or water,
is actually beheld by the spectator at _a_. The refractive power of the
atmosphere gradually diminishes from the horizon to the zenith, and
increases from the zenith to the horizon, in proportion to the density
of its different strata, being densest at its lower extremity next the
earth, and more rare towards its higher regions. If a person at _a_
had the sun, _e_, in his zenith, he would see him where he really
is; for his rays coming perpendicularly through the atmosphere, would
be equally attracted in all directions, and would therefore suffer no
inflection. But, about two in the afternoon, he would see the sun at
_i_, though, in reality, he was at _k_, thirty-three seconds lower than
his apparent situation. At about four in the afternoon he would see him
at _m_, when he is at _n_, one minute and thirty-eight seconds from his
apparent situation. But at six o’clock, when we shall suppose he sets,
he will be seen at _o_, though he is at that time at _p_, more than
thirty-two minutes below the horizon. These phenomena arise from the
different refractive powers of the atmosphere at different elevations,
and from the obliquity with which the rays of light fall upon it; for
we see every object along that line in which the rays from it are
directed by the last medium through which they passed.

[Illustration: _figure 4._]

The same phenomena happen in relation to the moon, the planets, the
comets, the stars, and every other celestial body, all of which appear
more elevated, especially when near the horizon, than their true
places. The variable and increasing refraction from the zenith to the
horizon, is a source of considerable trouble and difficulty in making
astronomical observations, and in nautical calculations. For, in order
to determine the real altitudes of the heavenly bodies, the exact
degree of refraction, at the observed elevation, must be taken into
account. To the same cause we are to ascribe a phenomenon that has
sometimes occurred--namely, that the moon has been seen rising totally
eclipsed, while the sun was still visible in the opposite quarter of
the horizon. At the middle of a total eclipse of the moon, the sun
and moon are in opposition, or 180 degrees asunder; and, therefore,
were no atmosphere surrounding the earth, these luminaries, in such a
position, could never be seen above the horizon at the same time. But,
by the refraction of the atmosphere near the horizon, the bodies of the
sun and moon are raised more than 32 minutes above their true places,
which is equal, and sometimes more than equal to the apparent diameters
of these bodies.


_Extraordinary cases of refraction in relation to terrestrial objects._

In consequence of the accidental condensation of certain strata of
the atmosphere, some very singular effects have been produced in the
apparent elevation of terrestrial objects to a position much beyond
that in which they usually appear. The following instance is worthy of
notice. It is taken from the Philosophical Transactions of London for
1798, and was communicated by W. Latham, Esq., F.R.S., who observed
the phenomenon from Hastings, on the south coast of England:--‘On July
26, 1797, about five o’clock in the afternoon, as I was sitting in my
dining-room in this place, which is situated upon the Parade, close
to the sea-shore, nearly fronting the south, my attention was excited
by a number of people running down to the sea-side. Upon inquiring
the reason, I was informed, that the coast of France was plainly to
be distinguished by the naked eye. I immediately went down to the
shore, and was surprised to find that, even without the assistance
of a telescope, I could very plainly see the cliffs on the opposite
coast, which, at the nearest part, are between forty and fifty miles
distant, and are not to be discerned from that low situation by the
aid of the best glasses. They appeared to be only a few miles off,
and seemed to extend for some leagues along the coast. I pursued my
walk along the shore eastward, close to the water’s edge, conversing
with the sailors and fishermen upon the subject. They at first would
not be persuaded of the reality of the appearance; but they soon
became so thoroughly convinced by the cliffs gradually appearing more
elevated, and approaching nearer, as it were, that they pointed out and
named to me the different places they had been accustomed to visit,
such as the Bay, the Old Head, or Man, the Windmill, &c. at Boulogne,
St. Vallery, and other places on the coast of Picardy, which they
afterwards confirmed, when they viewed them through their telescopes.
Their observations were, that the places appeared as near as if they
were sailing, at a small distance, into the harbours. The day on which
this phenomenon was seen was extremely hot; it was high water at
Hastings about two o’clock, P.M., and not a breath of wind was
stirring the whole day.’ From the summit of an adjacent hill, a most
beautiful scene is said to have presented itself. At one glance the
spectators could see Dungeness, Dover Cliffs, and the French coast, all
along from Calais to St. Vallery, and, as some affirmed, as far to the
westward as Dieppe, which could not be much less than eighty or ninety
miles. By the telescope, the French fishing-boats were plainly seen at
anchor, and the different colours of the land on the heights, with the
buildings, were perfectly discernible.

This singular phenomenon was doubtless occasioned by an extraordinary
refraction produced either by an unusual expansion, or condensation
of the lower strata of the atmosphere, arising from circumstances
connected with the extreme heat of the season. The objects seem to
have been apparently raised far above their natural positions; for,
from the beach at Hastings, a straight line drawn across towards the
French coast, would have been intercepted by the curve of the waters.
They seem also to have been magnified by the refraction, and brought
apparently four or five times nearer the eye than in the ordinary state
of the atmosphere.

The following are likewise instances of unusual refraction:--When
Captain Colby was ranging over the coast of Caithness, with the
telescope of his great Theodolite, on the 21st of June, 1819, at
eight o’clock, P.M. from Corryhabbie Hill, near Mortlich, in
Banffshire, he observed a brig over the land of Caithness, sailing to
the westward in the Pentland Frith, between the Dunnet and Duncansby
heads. Having satisfied himself as to the fact, he requested his
assistants, Lieutenants Robe and Dawson, to look through the telescope,
which they immediately did, and observed the brig likewise. It was
very distinctly visible for several minutes, while the party continued
to look at it, and to satisfy themselves as to its position. The brig
could not have been less than from ninety to one hundred miles distant;
and, as the station on Corryhabbie is not above 850 yards above the
sea, the phenomenon is interesting. The thermometer was at 44°. The
night and day preceding the sight of the brig had been continually
rainy and misty, and it was not till 7 o’clock of the evening of the
21st that the clouds cleared off the hill.[8]

Captain Scoresby relates a singular phenomenon of this kind, which
occurred while he was traversing the Polar seas. His ship had been
separated by the ice from that of his father for a considerable time,
and he was looking out for her every day, with great anxiety. At
length, one evening, to his utter astonishment, he saw her suspended in
the air, in an inverted position, traced on the horizon in the clearest
colours, and with the most distinct and perfect representation. He
sailed in the direction in which he saw this visionary phenomenon, and
actually found his father’s vessel by its indication. He was divided
from him by immense masses of icebergs, and at such a distance, that
it was quite impossible to have seen the ship in her actual situation,
or to have seen her at all, if her spectrum had not been thus raised
several degrees above the horizon into the sky by this extraordinary
refraction. She was reckoned to be seventeen miles beyond the visible
horizon, and thirty miles distant.

Mrs. Somerville states, that a friend of her’s, while standing on
the plains of Hindostan, saw the whole upper chain of the Himalaya
mountains start into view, from a sudden change in the density of the
air, occasioned by a heavy shower, after a long course of dry and hot
weather. In looking at distant objects through a telescope, over the
top of a ridge of hills, about two miles distant, I have several times
observed, that some of the more distant objects which are sometimes
hid by the interposition of a ridge of hills, are, at other times,
distinctly visible above them. I have sometimes observed, that objects
near the middle of the field of view of a telescope, which was in a
fixed position, have suddenly appeared to descend to the lower part,
or ascend to the upper part of the field, while the telescope remained
unaltered. I have likewise seen, with a powerful telescope, the Bell
Rock Lighthouse, at the distance of about twenty miles, to appear as if
contracted to less than two-thirds of its usual apparent height, while
every part of it was quite distinct and well-defined, and in the course
of an hour or less, it appeared to shoot up to its usual apparent
elevation--all which phenomena are evidently produced by the same cause
to which we have been adverting.

Such are some of the striking effects produced by the refraction of
light. It enables us to see objects in a direction where they are not;
it raises, apparently, the bottoms of lakes and rivers: it magnifies
objects when their light passes through dense mediums: it makes the
sun appear above the horizon, when he is actually below it, and thus
increases the length of our day: it produces the Aurora and the evening
twilight, which forms, in many instances, the most delightful part of a
summer day: it prevents us from being involved in total darkness, the
moment after the sun has descended beneath the horizon: it modifies the
appearances of the celestial bodies, and the directions in which they
are beheld: it tinges the sun, moon, and stars, as well as the clouds,
with a ruddy hue, when near the horizon: it elevates the appearance of
terrestrial objects, and, in certain extraordinary cases, brings them
nearer to our view, and enables us to behold them when beyond the line
of our visible horizon. In combination with the power of reflection,
it creates visionary landscapes, and a variety of grotesque and
extraordinary appearances, which delight and astonish, and sometimes
appal the beholders. In short,--as we shall afterwards see more
particularly--the refraction of light through glasses of different
figures, forms the principle on which telescopes and microscopes
are constructed, by which both the remote and the minute wonders of
creation have been disclosed to view. So that had there been no bodies
capable of refracting the rays of light, we should have remained for
ever ignorant of many sublime and august objects in the remote regions
of the universe, and of the admirable mechanism and the countless
variety of minute objects which lie beyond the range of the unassisted
eye in our lower creation, all of which are calculated to direct our
views, and to enlarge our conceptions of the Almighty Creator.

In the operation of the law of refraction in these and numerous other
instances, we have a specimen of the diversified and beneficent effects
which the Almighty can produce by the agency of a single principle in
nature. By the influence of the simple law of gravitation, the planets
are retained in their orbits, the moon directed in her course around
the earth, and the whole of the bodies connected with the sun preserved
in one harmonious system. By the same law the mountains of our globe
rest on a solid basis, the rivers flow through the plains toward the
seas, the ocean is confined to its prescribed boundaries, and the
inhabitants of the earth are retained to its surface and prevented
from flying upwards through the voids of space. In like manner the law
by which light is refracted produces a variety of beneficial effects
essential to the present constitution of our world and the comfort
of its inhabitants. When a ray of light enters obliquely into the
atmosphere, instead of passing directly through, it bends a little
downwards, so that the greater portion of the rays which thus enter the
atmospheric mass, descend by inflection to the earth. We then enjoy
the benefit of that light which would otherwise have been totally
lost. We perceive the light of day an hour before the solar orb makes
its appearance, and a portion of its light is still retained when
it has descended nearly eighteen degrees below our horizon. We thus
enjoy, throughout the year, seven hundred and thirty hours of light
which would have been lost, had it not been refracted down upon us
from the upper regions of the atmosphere. To the inhabitants of the
polar regions this effect is still more interesting and beneficial.
Were it not for their twilight, they would be involved, for a much
longer period than they now are, in perpetual darkness; but by the
powerful refraction of light which takes place in the frigid zones, the
day sooner makes its appearance towards spring, and their long winter
nights are, in certain cases, shortened by a period of thirty days.
Under the poles, where the darkness of night would continue six months
without intermission, if there were no refraction, total darkness does
not prevail during the one half of this period. When the sun sets, at
the North pole about the 23rd of September, the inhabitants (if any)
enjoy a perpetual aurora, till he has descended 18 degrees below the
horizon. In his course through the ecliptic the sun is two months
before he can reach this point, during which time there is a perpetual
twilight. In two months more he arrives again at the same point, namely
18 degrees below the horizon, when a new twilight commences, which is
continually increasing in brilliancy, for other two months, at the end
of which the body of this luminary is seen rising in all its glory.
So that, in this region, the light of day is enjoyed, in a greater or
less degree, for ten months without interruption, by the effects of
atmospheric refraction; and, during the two months when the influence
of the solar light is entirely withdrawn, the moon is shining above the
horizon for two half months without intermission; and thus it happens,
that no more than two separate fortnights are passed in absolute
darkness; and this darkness is alleviated by the light of the stars and
the frequent coruscations of the Aurora Borealis. Hence, it appears,
that there are no portions of our globe that enjoy, throughout the
year, so large a portion of the solar light, as these northern regions,
which is chiefly owing to the refraction of the atmosphere.

The refraction of light by the atmosphere, combined with its power
of reflecting it, is likewise the cause of that universal light and
splendour which appears on all the objects around us. Were the earth
disrobed of its atmosphere, and exposed naked to the solar beams--in
this case, we might see the sun without having day, strictly so called.
His rising would not be preceded by any twilight as it now is. The most
intense darkness would cover us till the very moment of his rising;
he would then suddenly break out from under the horizon with the same
splendour he would exhibit at the highest part of his course, and would
not change his brightness till the very moment of his setting, when
in an instant all would be black as the darkest night. At noon day we
should see the sun like an intensely brilliant globe shining in a sky
as black as ebony, like a clear fire in the night seen in the midst of
an extensive field, and his rays would show us the adjacent objects
immediately around us; but the rays which fall on the objects remote
from us would be for ever lost in the expanse of the heavens. Instead
of the beautiful azure of the sky, and the colours which distinguish
the face of nature by day, we should see nothing but an abyss of
darkness, and the stars shining from a vault as dark as chaos. Thus
there would be no day, such as we now enjoy, without the atmosphere:
since it is by the refraction and reflections connected with this
aerial fluid that light is so modified and directed, as to produce all
that beauty, splendour and harmony, which appear on the concave of the
sky, and on the objects which diversify our terrestrial abode.

The effect of refraction, in respect to _terrestrial_ objects, is
likewise of a beneficial nature. The quantity of this refraction
is estimated by Dr. Maskelyne at one-tenth of the distance of the
object observed, expressed in degrees of a great circle. Hence, if
the distance be 10,000 fathoms, its tenth part 1000 fathoms, is the
sixtieth part of a degree, or one minute, which is the refraction
in altitude. Le Gendre estimates it at one fourteenth; De Lambre at
one eleventh; and others at a twelfth of the distance; but it must
be supposed to vary at different times and places according to the
varying state of the atmosphere. This refraction, as it makes objects
appear to be raised higher than they really are, enlarges the extent
of our landscapes, and enables us to perceive distant objects which
would otherwise have been invisible. It is particularly useful to
the navigator at sea. It is one important object of the mariner when
traversing his course, to look out for capes and headlands, rocks and
islands, so as to descry them as soon as they are within the reach
of his eye. Now, by means of refraction, the tops of hills and the
elevated parts of coasts, are apparently raised into the air, so that
they may be discovered several leagues further off on the sea than
they would be, did no such refractive power exist. This circumstance
is therefore a considerable benefit to the science of navigation, in
enabling the mariner to steer his course aright, and to give him the
most early warning of the track he ought to take, or of the dangers to
which he may be exposed.

In short, the effects produced by the refraction and reflection of
light on the scenery connected with our globe, teach us that these
principles, in the hand of the Almighty, might be so modified and
directed, as to produce the most picturesque, the most glorious and
wonderful phenomena, such as mortal eyes have never yet seen, and of
which human imagination can form no conception; and in other worlds,
more resplendent and magnificent than ours, such scenes may be fully
realized, in combination with the operation of physical principles and
agents, with which we are at present unacquainted. From what we already
know of the effects of the reflection and the refraction of light, it
is not beyond the bounds of probability to suppose, that in certain
regions of the universe, light may be reflected and refracted through
different mediums, in such a manner, as to present to the view of
their inhabitants the prominent scenes connected with distant systems
and worlds, and to an extent, as shall infinitely surpass the effects
produced by our most powerful telescopes.




CHAPTER III.


ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT SUBSTANCES, OR
LENSES.

It is to the refraction of light that we are indebted for the use
of lenses or artificial glasses to aid the powers of vision. It
lays the foundation of telescopes, microscopes, camera obscuras,
phantasmagorias, and other optical instruments, by which so many
beautiful, useful, and wonderful effects have been produced. In order
therefore to illustrate the principles on which such instruments are
constructed, it is necessary to explain the manner in which the rays
of light are refracted and modified, when passing through spherical
mediums of different forms. I do not intend however to enter into
the minutiæ of this subject, nor into any abstract mathematical
demonstrations, but shall simply offer a few explanations of general
principles, and several experimental illustrations, which may enable
the general reader to understand the construction of the optical
instruments to be afterwards described.

A lens is a transparent substance of a different density from the
surrounding medium, and terminating in two surfaces, either both
spherical, or one spherical and the other plain. It is usually made
of _glass_, but may also be formed of any other transparent substance,
as ice, crystal, diamond, pebbles, or by fluids of different densities
and refractive powers, enclosed between concave glasses. Lenses are
ground into various forms, according to the purpose they are intended
to serve. They may be generally distinguished as being either _convex_
or _concave_. A convex glass is thickest in the middle, and thinner
towards the edges. A concave glass is thin in the middle, and thicker
towards the extremities. Of these there are various forms, which are
represented in fig. 5. A, is a _plano-convex_ lens, which has one side
plane, and the other spherical or convex. B, is a _plano-concave_,
which is plane on the one side and concave on the other. C, is a
_double-convex_, or one which is spherical on both sides. D, a
_double-concave_, or concave on both sides. E, is called a _meniscus_,
which is convex on one side and concave on the other. F, is a
_concavo-convex_, the convex side of which is of a smaller sphere than
the concave. In regard to the _degree_ of convexity or concavity in
lenses, it is evident that there may be almost an infinite variety. For
every convex surface is to be considered as the segment of a circle,
the diameter and radius of which may vary to almost any extent. Hence,
lenses have been formed by opticians, varying from one-fiftieth of an
inch in radius, to two hundred feet. When we speak of the length of
the radius of a lens,--as for instance, when we say that a lens is two
inches or forty inches radius, we mean, that the convex surface of the
glass is the part of a circle the radius of which, or half the diameter
is two inches or forty inches; or in other words, were the portion of
the sphere on which it is ground formed into a globe of corresponding
convexity, it would be four inches or eighty inches in diameter.

[Illustration: _figure 5._]

[Illustration: _figure 6._]

[Illustration: _figure 7._]

[Illustration: _figure 8._]

The _axis_ of a lens is a straight line drawn through the center of its
spherical surface; and as the spherical sides of every lens are arches
of circles the axis of the lens would pass through the centre of that
circle of which its sides are segments. _Rays_ are those emanations
of light which proceed from a luminous body, or from a body that is
illuminated. The _Radiant_ is that body or object which emits the rays
of light--whether it be a self-luminous body, or one that only reflects
the rays of light. Rays may proceed from a Radiant in different
directions. They may be either parallel, converging, or diverging.
_Parallel_ rays are those which proceed _equally distant_ from each
other through their whole course. Rays proceeding from the sun, the
planets, the stars, and distant terrestrial objects are considered as
parallel, as in fig. 6. _Converging_ rays are such as, proceeding from
a body, approach nearer and nearer in their progress, tending to a
certain point where they all unite. Thus, the rays proceeding from the
object AB, (fig. 7.) to the point F, are said to converge towards that
point. All convex glasses cause parallel rays, which fall upon them
to converge in a greater or less degree; and they render converging
rays still more convergent. If AB, fig. 7. represent a convex lens,
and H G I parallel rays falling upon it, they will be refracted and
converge towards the point F, which is called the _focus_, or burning
point; because, when the sun’s rays are thus converged to a point by a
large lens, they set on fire combustible substances. In this point the
rays meet and intersect each other. _Diverging rays_ are those which,
proceeding from any point as A, fig. 8, continually recede from each
other as they pass along in their course towards BC. All the rays which
proceed from near objects as a window in a room, or an adjacent house
or garden are more or less divergent. The following figures show the
effects of parallel, converging and diverging rays in passing through
a double convex lens.

[Illustration: _figure 11._]

[Illustration: _figure 9._]

[Illustration: _figure 10._]

Fig. 9, shows the effects of parallel rays, KA, DE, LB, falling on a
convex glass AB. The rays which fall near the extremities at A and B,
are bent or refracted towards CF, the focus, and centre of convexity.
It will be observed, that they are less refracted as they approach
the center of the lens, and the central ray DEC, which is called the
_axis_ of the lens, and which passes through its center, suffers
no refraction. Fig. 10, exhibits the course of _converging_ rays,
when passing through a similar lens. In this case the rays converge
to a focus _nearer_ to the lens than the center; for a convex lens
uniformly increases the convergence of converging rays. The converging
rays here represented, may be conceived as having been refracted by
another convex lens of a longer focus, and, passing on towards a point
of convergence, were intercepted by the lens AB. The point D is the
place where the rays would have converged to a focus, had they not been
thus intercepted. Fig. 11, represents the course of diverging rays
when falling on a double convex glass. In this case the rays D B, D A,
&c., after passing through the lens, converge to a focus at a point
considerably farther from the lens than its centre, as at F. Such rays
must be considered as proceeding from near objects, and the fact may be
illustrated by the following experiment. Take a common reading-glass,
and hold it in the rays of the sun, opposite a sheet of writing-paper
or a white wall, and observe _at what distance_ from the glass the rays
on the paper converge to a small distinct white spot. This distance
gives the focal length of the lens by parallel rays. If now, we hold
the glass within a few feet of a window, or a burning candle, and
receive its image on the paper, the focal distance of the image from
the glass will be found to be longer. If, in the former case, the focal
distance was twelve inches,--in the latter case it will be thirteen,
fifteen, or sixteen inches, according to the distance of the window or
the candle from the glass.

If the lens A B, fig. 9, on which parallel rays are represented as
falling, were a _plano-convex_, as represented at A, fig, 5, the rays
would converge to a point P, at double the radius, or the whole
diameter of the sphere of which it is a segment. If the thickness of a
plano-convex be considered, and if it be exposed on its convex side to
parallel rays, as those of the sun, the focus will be at the distance
of _twice the radius, wanting two-thirds of the thickness of the lens_.
But if the same lens be exposed with its plane side to parallel rays,
the focus will then be precisely at the distance of twice the radius
from the glass.

The effects of _concave_ lenses are directly opposite to those of
convex. Parallel rays, striking one of those glasses, instead of
converging towards a point, are made to _diverge_. Rays already
divergent are rendered more so, and convergent rays are made less
convergent. Hence objects seen through concave glasses appear
considerably smaller and more distant than they really are. The
following diagram, fig. 12, represents the course of parallel rays
through a double concave lens, where the parallel rays T A, D E, I B,
&c., when passing through the concave glass A B, diverge into the rays
G L, E C, H P, &c., as if they proceeded from F, a point before the
lens, which is the principal focus of the lens.

[Illustration: _figure 12._]

The principal focal distance E F, is the same as in convex lenses.
Concave glasses are used to correct the imperfect vision of
short-sighted persons. As the form of the eye of such persons is too
convex, the rays are made to converge before they reach the optic
nerve; and therefore a concave glass, causing a little divergency,
assists this defect of vision, by diminishing the effect produced
by the too great convexity of the eye, and lengthening its focus.
These glasses are seldom used, in modern times, in the construction
of optical instruments, except as eye-glasses for small pocket
perspectives, and opera glasses.

_To find the focal distance of a concave glass._ Take a piece of
paste-board or card paper, and cut a round hole in it, not larger
than the diameter of the lens; and, on another piece of paste-board,
describe a circle whose diameter is just double the diameter of the
hole. Then apply the piece with the hole in it to the lens, and hold
them in the sun-beams, with the other piece at such a distance behind,
that the light proceeding from the hole may spread or diverge so as
precisely to fill the circle; then the distance of the circle from the
lens is equal to its virtual focus, or to its radius, if it be a double
concave, and to its diameter, if a plano-concave. Let _d, e_, (fig.
12,) represent the diameter of the hole, and _g, i_, the diameter of
the circle, then the distance C, I, is the virtual focus of the lens.[9]

The _meniscus_ represented at E, fig. 5, is like the crystal of a
common watch, and as the convexity is the same as the concavity,
it neither magnifies nor diminishes. Sometimes, however, it is
made in the form of a crescent, as at F, fig. 5, and is called a
_concavo-convex_ lens; and, when the convexity is greater than the
concavity, or, when it is thickest in the middle, it acts nearly in the
same way as a double or plano-convex lens of the same focal distance.


_Of the_ IMAGES _formed by convex lenses._

It is a remarkable circumstance, and which would naturally excite
admiration, were it not so common and well known, that _when the rays
of light from any object are refracted through a convex lens, they
paint a distinct and accurate picture of the object before it, in all
its colours, shades, and proportions_. Previous to experience, we
could have had no conception that light, when passing through such
substances, and converging to a point, could have produced so admirable
an effect,--an effect on which the construction and utility of all our
optical instruments depend. The following figure will illustrate this
position. Let L, N, represent a double convex lens, A, C, _a_, its
axis, and OB, an object perpendicular to it. A ray passing from the
extremity of the object at O, after being refracted by the lens at F,
will pass on in the direction FI, and form an image of that part of the
object at I. This ray will be the axis of all the rays which fall on
the lens from the point O, and I will be the focus where they will all
be collected. In like manner BCM, is the axis of that parcel of rays
which proceed from the extremity of the object B, and their focus will
be at M; and since all the points in the object between O, and B, must
necessarily have their foci between I and M, a complete picture of the
points from which they come will be depicted, and consequently an image
of the whole object OB.

[Illustration: _figure 13._]

It is obvious, from the figure, that the image of the object is
formed in the focus of the lens, in an _inverted position_. It must
necessarily be in this position, as the rays cross at C, the centre of
the lens; and as it is impossible that the rays from the upper part
of the object O, can be carried by refraction to the upper end of
the image at M. This is a universal principle in relation to convex
lenses of every description, and requires to be attended to in the
construction and use of all kinds of telescopes and microscopes. It
is easily illustrated by experiment. Take a convex lens of eight,
twelve, or fifteen inches focal distance, such as a reading glass, or
the glass belonging to a pair of spectacles, and holding it, at its
focal distance from a white wall, in a line with a burning candle, the
flame of the candle will be seen depicted on the wall in an inverted
position, or turned upside down. The same experiment may be performed
with a window-sash, or any other bright object. But, the most beautiful
exhibition of the images of objects formed by convex lenses, is made
by darkening a room, and placing a convex lens of a long focal distance
in a hole cut out of the window-shutter; when a beautiful inverted
landscape, or picture of all the objects before the window, will be
painted on a white paper or screen placed in the focus of the glass.
The image thus formed exhibits not only the proportions and colours,
but also the motions of all the objects opposite the lens, forming as
it were a living landscape. This property of lenses lays the foundation
of the camera obscura, an instrument to be afterwards described.

The following principles in relation to images formed by convex lenses
may be stated. 1. That _the image subtends the same angle at the centre
of the glass as the object itself does_. Were an eye placed at C, the
centre of the lens LN, fig. 13, it would see the object OB, and the
image IM under the same optical angle, or, in other words, they would
appear equally large. For, whenever right lines intersect each other,
as OI and BM, the opposite angles are always equal, that is, the angle
MCI is equal to the angle OCB. 2. _The length of the image formed by
a convex lens, is to the length of the object, as the distance of the
image is to the distance of the object from the lens_: that is, MI is
to OB :: as CA to CA. Suppose the distance of the object CA
from the lens, to be forty-eight inches, the length of the object OB
= sixteen inches, and the distance of the image from the lens, six
inches, then the length of the image will be found by the following
proportion, 48 : 16 :: 6 : 2, that is, the length of the image, in such
a case, is two inches. 3. _If the object be at an infinite distance,
the image will be formed exactly in the focus._ 4. _If the object be
at the same distance from the lens as its focus, the image is removed
to an infinite distance on the opposite side_; in other words, the
rays will proceed in a _parallel_ direction. On this principle, lamps
on the streets are sometimes directed to throw a bright light along a
foot-path where it is wanted, when a large convex glass is placed at
its focal distance from the burner; and on the same principle, light
is thrown to a great distance from lighthouses, either by a very large
convex lens of a short focal distance, or by a concave reflector. 5.
_If the object be at double the distance of the focus from the glass,
the image will also be at double the distance of the focus from the
glass._ Thus, if a lens of six inches focal distance be held at twelve
inches distance from a candle, the image of the candle will be formed
at twelve inches from the glass on the other side. 6. _If the object
be a little further from the lens than its focal distance, an image
will be formed, at a distance from the object, which will be greater
or smaller in proportion to the distance._ For example, if a lens
five inches focus, be held at a little more than five inches from a
candle, and a wall or screen at five feet six inches distant, receive
the image, a large and inverted image of the candle will be depicted,
which will be magnified in proportion as the distance of the wall
from the candle exceeds the distance of the lens from the candle.
Suppose the distance of the lens to be five and a half inches, then
the distance of the wall where the image is formed, being twelve times
greater, the image of the candle will be magnified twelve times. If
MI (fig. 13.) be considered as the object, then OB will represent the
magnified image on the wall. On this principle the image of the object
is formed by the small object glass of a compound microscope. On the
same principle the large pictures are formed by the Magic Lantern and
the Phantasmagoria; and in the same way small objects are represented
in a magnified form, on a sheet or wall by the Solar microscope. 7.
_All convex lenses magnify the objects seen through them, in a greater
or less degree._ The shorter the focal distance of the lens, the
greater is the magnifying power. A lens four inches focal distance,
will magnify objects placed in the focus, two times in length and
breadth; a lens two inches focus will magnify four times, a lens one
inch focus eight times; a lens half an inch focus sixteen times, &c.
supposing eight inches to be the least distance at which we see near
objects distinctly. In viewing objects with small lenses, the object
to be magnified should be placed exactly at the focal distance of the
lens, and the eye at about the same distance on the other side of
the lens. When we speak of magnifying power, as, for example, that a
lens one inch focal distance magnifies objects eight times, it is to
be understood of the _lineal_ dimensions of the object. But as every
object at which we look has breadth as well as length, the _surface_
of the object is in reality magnified sixty-four times, or the square
of its lineal dimensions; and for the same reason a lens half an inch
focal distance magnifies the _surfaces_ of objects 256 times.


_Reflections deduced from the preceding subject._

Such are some of the leading principles which require to be recognised
in the construction of refracting telescopes, microscopes, and
other dioptric instruments whose performance chiefly depends on the
_refraction_ of light.--It is worthy of particular notice that all the
phenomena of optical lenses now described, depend upon that peculiar
property which the Creator has impressed upon the rays of light, that,
_when they are refracted to a focus by a convex transparent substance,
they depict an accurate image of the objects whence they proceed_.
This, however common, and however much overlooked by the bulk of
mankind, is indeed a very wonderful property with which light has been
endued. Previous to experience we could have had no conception that
such an effect would be produced; and, in the first instance, we could
not possibly have traced it to all its consequences. All the objects
in creation might have been illuminated as they now are, for aught
we know, without sending forth either direct or reflected rays _with
the property of forming exact representations of the objects whence
they proceeded_. But this we find to be a universal law in regard to
light of every description, whether as emanating directly from the
sun, or as reflected from the objects he illuminates, or as proceeding
from bodies artificially enlightened. It is a law or a property of
light not only in our own system, but throughout all the systems of
the universe to which mortal eyes have yet penetrated. The rays from
the most distant star which astronomers have descried, are endued
with this property, otherwise they could never have been perceived by
means of our optical instruments; for it is by the pictures or images
formed in these instruments that such distant objects are brought to
view. Without this property of light, therefore, we should have had
no telescopes, and consequently we could not have surveyed, as we can
now do, the hills and vales, the deep caverns, the extensive plains,
the circular ranges of mountains, and many other novel scenes which
diversify the surface of our moon. We should have known nothing of the
stupendous spots which appear on the surface of the sun--of the phases
of Venus--of the satellites and belts of Jupiter--of the majestic rings
of Saturn--of the existence of Uranus and his six moons,--or of the
planets Vesta, Juno, Ceres, and Pallas, nor could the exact bulks of
any of these bodies have been accurately determined. But, above all,
we should have been entirely ignorant of the wonderful phenomena of
double stars--which demonstrate that suns revolve around suns--of the
thousands and millions of stars which crowd the profundities of the
Milky Way and other regions of the heavens--of the thousands of Nebulæ
or starry systems which are dispersed throughout the immensity of the
firmament, and many other objects of sublimity and grandeur, which fill
the contemplative mind with admiration and awe, and raise its faculties
to higher conceptions than it could otherwise have formed of the
omnipotence and grandeur of the Almighty Creator.

Without this property of the rays of light we should likewise have
wanted the use of the microscope--an instrument which has disclosed a
world invisible to common eyes, and has opened to our view the most
astonishing exhibitions of Divine mechanism, and of the wisdom and
intelligence of the Eternal Mind. We should have been ignorant of
those tribes of living beings, invisible to the unassisted eye, which
are found in water, vinegar, and many other fluids--many of which are
twenty thousand times smaller than the least visible point, and yet
display the same admirable skill and contrivance in their construction,
as are manifested in the formation of the larger animals. We should
never have beheld the purple tide of life, and even the globules of the
blood rolling with swiftness through veins and arteries smaller than
the finest hair; or had the least conception that numberless species
of animated beings, so minute that a million of them are less than
a grain of sand, could have been rendered visible to human eyes, or
that such a number of vessels, fluids, movements, diversified organs
of sensation, and such a profusion of the richest ornaments and the
gayest colours could have been concentrated in a single point. We
should never have conceived that even the atmosphere is replenished
with invisible animation, that the waters abound with countless myriads
of sensitive existence, that the whole earth is full of life, and
that there is scarcely a tree, plant, or flower, but affords food and
shelter to a species of inhabitants peculiar to itself, which enjoy
the pleasures of existence and share in the bounty of the Creator. We
could have formed no conception of the beauties and the varieties of
mechanism which are displayed in the scenery of that invisible world
to which the microscope introduces us--beauties and varieties, in
point of ornament and delicate contrivance, which even surpass what is
beheld in the visible operations and aspect of nature around us. We
find joints, muscles, a heart, stomach, entrails, veins, arteries, a
variety of motions, a diversity of forms, and a multiplicity of parts
and functions--in breathing atoms. We behold in a small fibre of a
peacock’s feather, not more than one-eighth of an inch in length, a
profusion of beauties no less admirable than is presented by the whole
feather to the naked eye--a stem sending out multitudes of lateral
branches, each of which emits numbers of little sprigs, which consist
of a multitude of bright shining globular parts, adorned with a rich
variety of colours. In the sections of plants, we see thousands and
ten thousands of tubes and pores, and other vessels for the conveyance
of air and juices for the sustenance of the plant; in some instances,
more than ten hundred thousand of these being compressed within the
space of a quarter of an inch in diameter, and presenting to the eye
the most beautiful configurations. There is not a weed, nor a moss, nor
the most insignificant vegetable, which does not show a multiplicity
of vessels disposed in the most curious manner for the circulation of
sap for its nourishment, and which is not adorned with innumerable
graces for its embellishment. All these and ten thousands of other
wonders which lie beyond the limits of natural vision, in this new and
unexplored region of the universe, would have been for ever concealed
from our view, had not the Creator endued the rays of light with the
power of _depicting the images of objects_, when refracted by convex
transparent substances.

In this instance, as well as in many others, we behold a specimen of
the admirable and diversified effects, which the Creator can produce
from the agency of a single principle in nature. By means of optical
instruments, we are now enabled to take a more minute and expansive
view of the amazing operations of nature, both in heaven and on earth,
than former generations could have surmised. These views tend to raise
our conceptions of the attributes of that Almighty Being, who presides
over all the arrangements of the material system, and to present them
to our contemplation in a new, a more elevated, and expansive point of
view. There is, therefore, a connection which may be traced between the
apparently accidental principle of the rays of light forming images of
objects, and the comprehensive views we are now enabled to take of the
character and perfections of the Divinity. Without the existence of the
law or principle alluded to, we could not, in the present state, have
formed precisely the same conceptions either of the Omnipotence, or of
the wisdom and intelligence of the Almighty. Had no microscope ever
been invented, the idea never could have entered into the mind of man,
that worlds of living beings exist beyond the range of natural vision,
that organized beings possessed of animation exist, whose whole bulk
is less than the ten hundred thousandth part of the smallest grain of
sand; that, descending from a visible point to thousands of degrees
beyond it, an invisible world exists, peopled with tribes of every form
and size, the extent of which, and how far it verges towards infinity
downwards, mortals have never yet explored, and perhaps will never be
able to comprehend. This circumstance alone presents before us the
perfections of the divinity in a new aspect, and plainly intimates that
it is the will and the intention of the Deity, that we should explore
his works, and investigate the laws by which the material world is
regulated, that we may acquire more expansive views of his character
and operations. The inventions of man in relation to art and science,
are not therefore to be considered as mere accidental occurrences, but
as special arrangements in the divine government, for the purpose of
carrying forward the human mind to more clear and ample views of the
scenes of the universe, and of the attributes and the agency of Him
“who is wonderful in counsel and excellent in working.”




CHAPTER IV.


ON THE REFLECTION OF LIGHT.

The _reflection_ of the rays of light is that property by which--after
approaching the surfaces of bodies, they are thrown back, or repelled.
It is in consequence of this property that all the objects around
us, and all the diversified landscapes on our globe, are rendered
visible. It is by light reflected from their surfaces that we perceive
the planetary bodies and their satellites, the belts of Jupiter, the
rings of Saturn, the various objects which diversify the surface of
the Moon, and all the bodies in the universe which have no light of
their own. When the rays of light fall upon rough and uneven surfaces,
they are reflected very irregularly and scattered in all directions,
in consequence of which thousands of eyes, at the same time, may
perceive the same objects, in all their peculiar colours, aspects,
and relations. But, when they fall upon certain smooth and polished
surfaces, they are reflected with regularity, and according to certain
laws. Such surfaces, when highly polished, are called _Mirrors_ or
_Speculums_; and it is to the reflection of light from such surfaces,
and the effects it produces, that I am now to direct the attention of
the reader.

Mirrors or Specula, may be distinguished into three kinds, _plane_,
_concave_, and _convex_, according as they are bounded by plane or
spherical surfaces. These are made either of _metal_ or of _glass_,
and have their surfaces highly polished for the purpose of reflecting
the greatest number of rays. Those made of glass are foliated or
quicksilvered on one side; and the metallic specula are generally
formed of a composition of different metallic substances, which,
when accurately polished, is found to reflect the greatest quantity
of light. I shall, in the first place, illustrate the phenomena of
reflection produced by _plane-mirrors_.

When light impinges, or falls, upon a polished flat surface,
rather more than the half of it is reflected, or thrown back in a
direction similar to that of its approach; that is to say, if it fall
_perpendicularly_ on the polished surface, it will be perpendicularly
reflected; but if it fall _obliquely_, it will be reflected with
the same obliquity. Hence, the following fundamental law, regarding
the reflection of light, has been deduced both from experiment and
mathematical demonstration, namely, that _the angle of reflection is,
in all cases, exactly equal to the angle of incidence_. This is a law
which is universal in all cases of reflection, whether it be from
plane or spherical surfaces, or whether these surfaces be concave or
convex, and which requires to be recognized in the construction of all
instruments which depend on the reflection of the rays of light. The
following figure (fig. 14) will illustrate the position now stated.

Let AB represent a plane mirror, and CD a line or ray of light
perpendicular to it. Let FD represent the _incident_ ray from any
object, then DE will be the reflected ray, thrown back in the
direction from D to E, and it will make with the perpendicular CD the
same angle which the incident ray FD did with the same perpendicular,
that is, the angle FDC will be equal to the angle EDC, in all cases of
obliquity. The incident ray of light may be considered as rebounding
from the mirror, like a tennis ball from a marble pavement, or the wall
of a court.

[Illustration: _figure 14._]

In viewing objects by reflection we see them in a different direction
from that in which they really are, namely, along the line in which the
rays come to us last. Thus, if AB (fig. 15) represent a plane mirror,
the image of an object C appears to the eye at E behind the mirror, in
the direction EG, and always in the intersection G of the perpendicular
CG, and the reflected ray EG--and consequently at G as far behind
the mirror, as the object C is before it. We therefore see the image
in the line EG, the direction in which the reflected rays proceed. A
plane mirror does not alter the figure or size of objects; but the
whole image is equal and similar to the whole object, and has a like
situation with respect to one side of the plane, that the object has
with respect to the other.

[Illustration: _figure 15._]

Mr. Walker illustrates the manner in which we see our faces in a mirror
by the following figure (16). AB represents a mirror, and
OC, a person looking into it. If we conceive a ray proceeding from
the forehead CE, it will be sent to the eye at O,
agreeably to the angle of incidence and reflection. But the mind puts
CEO into one line, and the forehead is seen at H,
as if the lines CEO had turned on a hinge at E.--It
seems a wonderful faculty of the mind to put the two oblique lines
CE and OE into one straight line OH, yet
it is seen every time we look at a mirror. For the ray has really
travelled from C to E, and from E to
O, and it is that journey which determines the distance of the
object; and hence we see ourselves as far beyond the mirror as we stand
from it. Though a ray is here taken only from one part of the face, it
may be easily conceived that rays from every other part of the face
must produce a similar effect.

[Illustration: _figure 16._]

In every plain mirror, the image is always equal to the object, at what
distance soever it may be placed; and as the mirror is only at half
the distance of the image from the eye, it will completely receive an
image of _twice_ its own length. Hence a man six feet high may view
himself completely in a looking glass of three feet in length, and
half his own breadth; and this will be the case at whatever distance
he may stand from the glass. Thus, the man AC (fig. 17) will see the
whole of his own image in the glass AB, which is but one half
as large as himself. The rays from the head pass to the mirror in the
line AA, perpendicular to the mirror, and are returned to the
eye in the same line; consequently, having travelled twice the length
AA, the man must see his head at B. From his feet C rays
will be sent to the bottom of the mirror at B; these will be
reflected at an equal angle to the eye in the direction BA, as
if they had proceeded in the direction DBA, so that the man
will see his foot at D, and consequently his whole figure at BD.

[Illustration: _figure 17._]

A person when looking into a mirror, will always see his own image as
far beyond the mirror as he is before it, and as he moves to or from
it, the image will, at the same time, move towards or from him on the
other side; but apparently with a double velocity, because the two
motions are equal and contrary. In like manner, if while the spectator
is at rest, an object be in motion, its image behind the mirror will
be seen to move at the same time. And if the spectator moves, the
images of objects that are at rest will appear to approach, or recede
from him, after the same manner as when he moves towards real objects;
plane mirrors reflecting not only the object, but the distance also,
and that exactly in its natural dimensions--The following principle is
sufficient for explaining most of the phenomena seen in a plane mirror,
namely;--_That the image of an object seen in a plane mirror, is always
in a perpendicular to the mirror joining the object and the image, and
that the image is as much on one side the mirror, as the object is on
the other_.


_Reflection by Convex and Concave Mirrors._

Both convex and concave mirrors are formed of portions of a sphere.
A convex speculum is ground and polished in a _concave_ dish or tool
which is a portion of a sphere, and a concave speculum is ground upon
a convex tool. The inner surface of a sphere brings parallel rays
to a focus at _one fourth_ of its diameter, as represented in the
following figure, where C is the centre of the sphere on which the
concave speculum AB is formed, and F the focus where parallel rays from
a distant object would be united, after reflection, that is, at one
half the radius, or one fourth of the diameter from the surface of the
speculum. Were a speculum of this kind presented to the sun, F would
be the point where the reflected rays would be converged to a focus,
and set fire to combustible substances if the speculum be of a large
diameter, and of a short focal distance. Were a candle placed in that
focus, its light would be reflected parallel as represented in the
figure. These are properties of concave specula which require to be
particularly attended to in the construction of reflecting telescopes.
It follows, from what has been now stated, that if we intend to form
a speculum of a certain focal distance,--for example, two feet, it is
necessary that _it should be ground upon a tool whose radius is double
that distance_, or four feet.

[Illustration: _figure 18._]


_Properties of Convex Mirrors._

From a convex surface, parallel rays when reflected are made to
diverge; convergent rays are reflected less convergent; and divergent
rays are rendered more divergent. It is the nature of all convex
mirrors and surfaces to scatter or _disperse_ the rays of light, and in
every instance to impede their convergence. The following figure shows
the course of parallel rays as reflected from a convex mirror. AEB is
the convex surface of the mirror; and KA, IE, LB, parallel rays falling
upon it. These rays, when they strike the mirror, are made to diverge
in the direction AG, BH, &c. and both the parallel and divergent rays
are here represented as they appear in a dark chamber, when a convex
mirror is presented to the solar rays. The dotted lines denote only the
course or tendency of the reflected rays, towards the _virtual_ focus
F, were they not intercepted by the mirror. This virtual focus is just
equal to half the radius CE.

[Illustration: _figure 19._]

The following are some of the properties of convex mirrors: 1. The
image appears always erect, and behind the reflecting surface. 2. _The
image is always smaller than the object_, and the diminution is greater
in proportion as the object is further from the mirror, but if the
object touch the mirror, the image at the point of contact is of the
same size as the object. 3. The image does not appear so far behind the
reflecting surface as in a plain mirror. 4. The image of a straight
object, placed either parallel or oblique to the mirror is seen
_curved_ in the mirror; because the different points of the object are
not all at an equal distance from the surface of the mirror. 5. Concave
mirrors have a _real_ focus where an image is actually formed; but
convex specula have only a _virtual_ focus, and this focus is behind
the mirror; no image of any object being formed before it.

The following are some of the purposes to which convex mirrors are
applied. They are frequently employed by painters for reducing the
proportions of the objects they wish to represent, as the images of
objects diminish in proportion to the smallness of the radius of
convexity, and to the distances of objects from the surface of the
mirror. They form a fashionable part of modern furniture, as they
exhibit a large company assembled in a room, with all the furniture it
contains, in a very small compass, so that a large hall with all its
objects, and even an extensive landscape, being reduced in size, may
be seen from one point of view. They are likewise used as the small
specula of those reflecting telescopes which are fitted up on the
_Cassegrainian_ plan, and in the construction of Smith’s Reflecting
Microscope. But on the whole, they are very little used in the
construction of optical instruments.


_Properties of Concave speculums._

Concave specula have properties very different from those which are
convex; they are of more importance in the construction of reflecting
telescopes and other optical instruments; and therefore require more
minute description and illustration. Concave mirrors cause parallel
rays to converge; they increase the convergence of rays that are
already converging; they diminish the divergence of diverging rays;
and, in some cases, render them parallel and even convergent; which
effects are all in proportion to the concavity of the mirror. The
following figures show the course of diverging and parallel rays as
reflected from concave mirrors.

Fig. 20 represents the course of _parallel_ rays, and AB, the concave
mirror on which they fall. In this case, they are reflected so as to
unite at F, which point is distant from its surface _one fourth_ of the
diameter of the sphere of the mirror. This point is called the focus of
parallel rays, or _the true focus of the mirror_. And, since the sun
beams are parallel among themselves, if they are received on a concave
mirror, they will all be reflected to that point, and there burn in
proportion to the quantity of rays collected by the mirror. Fig. 21.
shows the direction of _diverging_ rays, or those which proceed from
a near object. These rays proceeding from an object further from
the mirror than the true focal point, as from D to A and to B, are
reflected converging and meet at a point F, _further from the mirror_
than the focal point of parallel rays. If the distance of the radiant,
or object D, be equal to the radius CE, then will the focal distance
be likewise equal to the radius: That is, if an object be placed in
the center of a concave speculum, the image will be reflected upon the
object, or they will seem to meet and embrace each other in the centre.
If the distance of the radiant be equal to half the radius, its image
will be reflected to an infinite distance, for the rays will then be
parallel. If, therefore, a luminous body be placed at half the radius
from a concave speculum, it will enlighten places directly before it
at great distances. Hence their use when placed behind a candle in a
common lantern; hence their utility in throwing light upon objects in
the Magic Lantern and Phantasmagoria, and hence the vast importance
of very large mirrors of this description, as now used in most of our
Light Houses, for throwing a brilliant light to great distances at sea
to guide the mariner when directing his course under the cloud of night.

[Illustration: _figure 20._]

[Illustration: _figure 21._]

When _converging_ rays fall upon a concave mirror, they are reflected
more converging and unite at a point between the focus of parallel rays
and the mirror; that is, nearer the mirror than one half the radius;
and their precise degree of convergency will be greater than that
wherein they converged before reflection.


_Of the images formed by Concave Mirrors._

If rays proceeding from a distant object fall upon a concave speculum,
they will paint an image or representation of the object on its focus
_before_ the mirror. This image will be inverted, because the rays
cross at the points where the image is formed. We have already seen
that a convex glass forms an image of an object _behind_ it; the rays
of light from objects _pass through_ the glass, and the picture is
formed on the side farthest from the object. But in concave mirrors
the images of distant objects--and of all objects that are farther
from its surface than its principal focus--are formed _before_ the
mirror, or on the same side as the object. In almost every other
respect, however, the effect of a concave mirror is the same as that
of a convex lens, in regard to the formation of images, and the course
pursued by the rays of light, except that the effect is produced in the
one case by refraction, and in the other by reflection. The following
figure represents the manner in which images are formed by concave
mirrors. GF represents the reflecting surface of the mirror; OAB, the
object; and IAM, the image formed by the mirror. The rays
proceeding from O, will be carried to the mirror, in the direction OG,
and according to the law that the angle of incidence is equal to the
angle of reflection, will be reflected to I, in the direction GI. In
like manner the rays from B, will be reflected from F to M, the rays
from A, will be reflected to a, and so of all the intermediate rays, so
that an inverted image of the object OB, will be formed at IM. If the
rays proceeded from objects at a very great distance the image would be
formed in the real focus of the mirror, or at one-fourth the diameter
of the sphere from its surface; but near objects, which send forth
diverging rays, will have their images formed a little farther from the
surface of the mirror.

[Illustration: _figure 22._]

If we suppose a real object placed at IM, then OB will represent its
magnified image, which will be larger than the object, in proportion to
its distance from the mirror. This may be experimentally illustrated
by a concave mirror and a candle. Suppose a concave mirror whose focal
distance is five inches, and that a candle is placed before it, at
a little beyond its focus, (as at IM)--suppose at five and a half
inches,--and that a wall or white screen receives the image, at the
distance of five feet six inches from the mirror, an image of the
candle will be formed on the wall which will be twelve times longer
and broader than the candle itself. In this way concave mirrors may be
made to magnify the images of objects to an indefinite extent. This
experiment is an exact counterpart of what is effected in similar
circumstances by a convex lens, as described p. 74; the mirror
performing the same thing by reflection, as the lens did by refraction.

From what has been stated in relation to concave mirrors it will be
easily understood how they make such powerful burning-glasses. Suppose
the focal distance of a concave mirror to be twelve inches, and its
diameter or breadth twelve inches. When the sun’s rays fall on such
a mirror, they form an image of the sun at the focal point whose
diameter is found to be about one-tenth of an inch. All the rays
which fall upon the mirror are converged into this small point; and
consequently their intensity is in proportion as the square of the
surface of the mirror is to the square of the image. The squares of
these diameters are as 14,400 to 1; and consequently the density of the
sun’s rays, in the focus, is to their density on the surface of the
mirror as 14,400 to 1. That is, the heat of the solar rays in the focus
of such a mirror will be fourteen thousand four hundred times greater
than before--a heat which is capable of producing very powerful effects
in melting and setting fire to substances of almost every description.

Were we desirous of forming an image by a concave speculum which shall
be exactly equal to the object, the object must be placed exactly in
the centre; and, by an experiment of this kind, the centre of the
concavity of a mirror may be found.

In the cases now stated, the images of objects are all formed in the
front of the mirror, or between it and the object. But there is a case
in which the image is formed behind the mirror. This happens when the
object is placed between the mirror and the focus of parallel rays,
and then the image is larger than the object. In fig. 23, GF is a
concave mirror, whose focus of parallel rays is at E. If an object OB
be placed a little within this focus, as at A, a large image IM will
be seen _behind_ the mirror, somewhat curved and erect, which will be
seen by an eye looking directly into the front of the mirror. Here
the image appears at a greater distance behind the mirror than the
object is before it, and the object appears magnified in proportion
to its distance from the focus and the mirror. If the mirror be one
inch focal distance, and the object be placed eight-tenths of an inch
from its surface, the image would be five times as large as the object
in length and breadth, and consequently twenty-five times larger in
surface. In this way small objects may be magnified by reflection, as
such objects are magnified by refraction, in the case of deep convex
lenses. When such mirrors are large, for example six inches diameter,
and eight or ten inches focal distance, they exhibit the human face
as of an enormous bulk. This is illustrated by the following figure.
Let C N, Fig. 24, represent the surface of a concave mirror,
and A a human face looking into it, the face will appear magnified
as represented by the image behind the mirror D Q. Suppose
a ray A C proceeding from the forehead, and another M
N from the chin; these rays are reflected to the person’s eye
at O, which consequently sees the image in the lines of
reflection O D, O Q, and in the angle D O
Q, and consequently magnified much beyond the natural size, and at
a small distance behind the mirror.

[Illustration: _figure 23._]

[Illustration: _figure 24._]

If we suppose the side T U to represent a _convex_ mirror,
and the figure D Q a head of an ordinary size, then the
figure A will represent the diminished appearance which a person’s
face exhibits, when viewed in such a mirror. It will not only appear
reduced, but somewhat distorted; because from the form of the mirror,
one part of the object is nearer to it than another, and consequently
will be reflected under a different angle.

The effect we have now mentioned as produced by _concave_ mirrors,
will only take place when the eye is nearer the mirror than its
principal focus. If the spectator retire beyond this focus--suppose to
the distance of five or six feet, he will not see the image _behind_
the mirror; but he will see his image in a diminished form, hanging
upside down, and suspended in the air, in a line between his eye and
the mirror. In this case, his image is formed _before_ the mirror as
represented at IM fig. 22. In this situation, if you hold out your hand
towards the mirror, the hand of the image will come out towards your
hand, and, when at the centre of concavity, it will be of an equal size
with it, and you may shake hands with this aerial image. If you move
your hand farther, you will find the hand of the image pass by your
hand, and come between it and your body. If you move your hand towards
either side, the hand of the image will move towards the other side;
the image moving always in a contrary direction to the object. All
this while the by-standers, if any, see nothing of the image, because
none of the reflected rays that form it can enter their eyes.--The
following figure represents a phenomenon produced in the same manner.
A B is a concave mirror of a large size; C represents
a hand presented before the mirror, at a point farther distant than
its focus. In this case, an inverted image of the hand is formed which
is seen hanging in the air at M. The rays C and
D go diverging from the two opposite points of the object, and
by the action of the mirror, they are again made to converge to points
at O and S where they cross, form an image, and again
proceed divergent to the eye.[10]

[Illustration: _figure 25._]

In consequence of the properties of concave mirrors, now described,
many curious experiments and optical deceptions have been exhibited.
The appearance of images in the air, suspended between the mirror and
the object, have sometimes been displayed with such dexterity and an
air of mystery, as to have struck with astonishment those who were
ignorant of the cause. In this way birds, flying angels, spectres and
other objects have been exhibited, and when the hand attempts to lay
hold on them, it finds them to be nothing, and they seem to vanish
into air. An apple or a beautiful flower is presented, and when a
spectator attempts to touch it, it instantly vanishes, and a death’s
head immediately appears, and seems to snap at his fingers. A person
with a drawn sword appears before him, in an attitude as if about to
run him through, or one terrific phantom starts up after another, or
sometimes the resemblances of deceased persons are made to appear, as
if, by the art of conjuration, they had been forced to return from the
world of spirits. In all such exhibitions, a very large concave mirror
is requisite, a brilliant light must be thrown upon the objects, and
every arrangement is made, by means of partitions, &c., to prevent
either the light, the mirror, or the object from being seen by the
spectators. The following representation (fig. 26.) shows one of the
methods by which this is effected: A is a large concave
mirror, either of metal or of glass, placed on the back part of a dark
box, D is the performer, concealed from the spectators by
the cross partition C; E is a strong light, which
is likewise concealed by the partition I, which is thrown
upon the actor D, or upon any thing he may hold in his hand.
If he hold a book, as represented in the figure, the light reflected
from it will pass between the partitions C and I to
the mirror, and will be reflected from thence to Z, where
the image of the book will appear so distinct and tangible, that a
spectator looking through the opening at X, will imagine that it is in
his power to take hold of it. In like manner, the person situated at
D, may exhibit his own head or body--a portrait, a painting,
a spectre, a landscape, or any object or device which he can strongly
illuminate.

[Illustration: _figure 26._]

[Illustration: _figure 27._]

[Illustration: _figure 28._]

There is another experiment, made with a concave mirror, which has
somewhat puzzled philosophers to account for the phenomena. Take a
glass bottle AC, (fig. 27) and fill it with water to the point B;
leave the upper part BC empty, and cork it in the common manner. Place
this bottle opposite a concave mirror, and beyond its focus, that
it may appear reversed, and, before the mirror place yourself still
further distant from the bottle, and it will appear in the situation
A B C. Now, it is remarkable in this apparent bottle, that
the _water_, which, according to the laws of catoptrics, should
appear at A B, appears on the contrary at B C, and
consequently, the part A B appears empty. If the bottle be
inverted and placed before the mirror, its image will appear in its
natural erect position, and the water which is in reality at BC (fig.
28) is seen at A B. If while the bottle is inverted, it be
uncorked, and the water run gently out, it will appear, that, while the
part BC is _emptying_, that of A B in the image is filling,
and, what is remarkable, as soon as the bottle is empty, the illusion
ceases, the image also appearing entirely empty.--The remarkable
circumstances in this experiment are, first, not only to see the object
where it is not, but also where its _image_ is not; and secondly, that
of two objects which are really in the same place, as the surface of
the bottle and the water it contains, the one is seen at one place, and
the other at another; and to see the bottle in the place of its image,
and the water where neither it nor its image are.

The following experiments are stated by Mr. Ferguson in his “Lectures
on select Subjects,” &c. “If a fire be made in a large room, and a
smooth mahogany table be placed at a good distance near the wall,
before a large concave mirror, so placed that the light of the fire may
be reflected from the mirror to its focus upon the table; if a person
stand by the table, he will see nothing upon it but a longish beam of
light: but if he stand at a distance toward the fire, not directly
between the fire and mirror, he will see an image of the fire upon the
table, large and erect. And if another person who knows nothing of
the matter beforehand should chance to come into the room, and should
look from the fire toward the table, he would be startled at the
appearance; for the table would seem to be on fire, and by being near
the wainscot, to endanger the whole house. In this experiment there
should be no light in the room but what proceeds from the fire; and the
mirror ought to be at least fifteen inches in diameter. If the fire be
darkened by a screen, and a large candle be placed at the back of the
screen, a person standing by the candle will see the appearance of a
very fine large star, or rather planet, upon the table, as bright as
Venus or Jupiter. And if a small wax taper--whose flame is much less
than the flame of the candle--be placed near the candle, a satellite to
the planet will appear on the table; and if the taper be moved round
the candle, the satellite will go round the planet.”

Many other illustrations of the effects of concave specula might have
been given, but I shall conclude this department by briefly stating
some of the _general properties of speculums_.

1. There is a great resemblance between the properties of _convex_
lenses and _concave_ mirrors. They both form an inverted focal image
of any remote object, by the convergence of the pencil of rays. In
those instruments whose performances are the effects of reflection, as
reflecting telescopes, the concave mirror is substituted in the place
of the convex lens. The whole effect of these instruments, in bringing
to view remote objects in heaven and on earth, entirely depends on the
property of a concave mirror in forming _images_ of objects in its
focus. 2. The image of an object placed beyond the centre, is less
than the object; if the object be placed between the principal focus
and the centre, the image is greater than the object. In both cases
the image is inverted. 3. When the object is placed between the focus
and the mirror, the image situated _behind_ the mirror is greater
than the object, and it has the same direction: in proportion as the
object approaches the focus, the image becomes larger and more distant.
These and similar results are proved by placing a lighted candle at
different distances from a concave mirror. 4. An eye cannot see an
image in the air except it be placed in the diverging rays; but if the
image be received on a piece of white paper, it may be seen in any
position of the eye, as the rays are then reflected in every direction.
5. If a picture drawn according to the rules of perspective, be placed
before a large concave speculum, a little nearer than its principal
focus, the image of the picture will appear extremely natural, and
very nearly like the real objects whence it was taken. Not only are
the objects considerably magnified, so as to approach to their natural
size, but they have also different apparent distances, as in nature, so
that the view of the inside of a church appears very like what it is
in reality, and representations of landscapes appear very nearly, as
they do from the spot whence they were taken. In this respect a large
concave speculum may be made to serve nearly the same purpose, as the
Optical Diagonal Machine, in viewing perspective prints. 6. The concave
speculum is that alone which is used as the great mirror which forms
the first image in reflecting telescopes; and it is likewise the only
kind of speculum used as the small mirror, in that construction of the
instrument called the _Gregorian Reflector_.


_Quantity of light reflected by polished surfaces._

As this is a circumstance connected with the construction of reflecting
telescopes, it may not be improper, in this place, to state some
of the results of the accurate experiments of M. Bonguer on this
subject. This philosopher ascertained that of the light reflected from
mercury, or quicksilver, more than _one-fourth_ is lost, though it is
probable that no substances reflect more light than this. The rays
were received at an angle of eleven and a half degrees of incidence,
measured from the surface of the reflecting body, and not from the
perpendicular. The reflection from _water_ was found to be almost
as great as that of quicksilver; so that in very small angles it
reflects nearly three-fourths of the direct light. This is the reason
why so strong a reflection appears on water, when one walks, in still
weather, on the brink of a lake opposite to the sun. The direct light
of the sun diminishes gradually as it approaches the horizon, while
the reflected light at the same time grows stronger; so that there
is a certain elevation of the sun in which the united force of the
direct and reflected light will be the greatest possible, and this is
when he is twelve or thirteen degrees in altitude. On the other hand,
light reflected from water at _great angles_ of incidence is extremely
small. When the light was perpendicular, it reflected no more than the
thirty-seventh part which mercury does in the same circumstances, and
only the fifty-fifth part of what fell upon it in this case.

Using a smooth piece of glass, one line in thickness, he found that,
when it was placed at an angle of fifteen degrees with the incident
rays, it reflected 628 parts of 1000 which fell upon it; at the same
time, a metallic mirror which he tried in the same circumstances,
reflected only 561 of them. At a less angle of incidence much more
light was reflected; so that at an angle of three degrees, the glass
reflected 700 parts, and the metal something less, as in the former
case. The most striking observations made by this experimenter relate
to the very great difference in the quantity of light reflected at
different angles of incidence. He found that for 1000 incident rays,
the reflected rays, at different angles of incidence, were as follows.

  Angles of           Rays reflected          Rays reflected
  incidence             by water                by glass

     5°                   501                     549
    10                    333                     412
    15                    211                     299
    30                     65                     112
    50                     22                      34
    70                     18                      25
    90                     18                      25

With regard to such mirrors as the specula of reflecting telescopes, it
will be found, in general, that they reflect little more than the _one
half_ of the rays which fall upon them.


_Uncommon appearances in nature produced by the combined influences of
Reflection and Refraction._

The reflection and refraction of the rays of light frequently produce
phenomena which astonish the beholders, and which have been regarded
by the ignorant and the superstitious, as the effects of supernatural
agency. Of these phenomena I shall state a few examples.

One of the most striking appearances of this kind is what has been
termed the _Fata Morgana_, or optical appearances of figures in the
sea and the air, as seen in the Faro of Messina. The following account
is translated from a work of Minasi, who witnessed the phenomenon, and
wrote a dissertation on the subject. “When the rising sun shines from
that point whence its incident ray forms an angle of about forty-five
degrees to the sea of Riggio, and the bright surface of the water
in the bay is not disturbed either by the wind or the current, the
spectator being placed on an eminence of the city, with his back to the
sun and his face to the sea;--on a sudden there appear on the water,
as in a catoptric theatre, various multiplied objects, that is to
say, numberless series of pilasters, arches, castles well delineated,
regular columns, lofty towers, superb palaces, with balconies and
windows, extended alleys of trees, delightful plains with herds and
flocks, armies of men on foot and horseback, and many other strange
images, in their natural colours and proper actions, passing rapidly in
succession along the surface of the sea, during the whole of the short
period of time, while the above mentioned causes remain.--But, if in
addition to the circumstances now described, the atmosphere be highly
impregnated with vapour and dense exhalations, not previously dispersed
by the winds or the sun, it then happens that, in this vapour, as in
a curtain extended along the channel, at the height of about thirty
palms, and nearly down to the sea, the observer will behold the scene
of the same objects, not only reflected from the surface of the sea,
but likewise in the air, though not so distant or well defined, as the
former objects from the sea.--Lastly, if the air be slightly hazy or
opake, and at the same time dewy and adapted to form the iris, the then
above-mentioned objects will appear only at the surface of the sea, as
in the first case, but all vividly  or fringed with red, green,
blue and other prismatic colours.”[11]

It is somewhat difficult to account for all the appearances here
described; but, in all probability, they are produced by a calm sea,
and one or more strata of superincumbent air differing in refractive
and consequently in reflective power. At any rate reflection and
refraction are some of the essential causes which operate in the
production of the phenomena.

The _Mirage_, seen in the deserts of Africa, is a phenomenon, in all
probability produced by a similar cause. M. Monge, who accompanied
the French army to Egypt, relates that, when in the desert between
Alexandria and Cairo, the mirage of the blue sky was inverted, and
so mingled with the sand below, as to give to the desolate and arid
wilderness an appearance of the most rich and beautiful country. They
saw, in all directions, green islands, surrounded with extensive lakes
of pure, transparent water. Nothing could be conceived more lovely and
picturesque than the landscape. In the tranquil surface of the lakes,
the trees and houses with which the islands were covered, were strongly
reflected with vivid and varied hues, and the party hastened forward to
enjoy the cool refreshments of shade and stream which these populous
villages proffered to them. When they arrived, the lake on whose bosom
they floated, the trees among whose foliage they were embowered, and
the people who stood on the shore inviting their approach, had all
vanished, and nothing remained but an uniform and irksome desert of
sand and sky, with a few naked huts and ragged Arabs. Had they not been
undeceived by their nearer approach, there was not a man in the French
army who would not have sworn that the visionary trees and lakes had a
real existence in the midst of the desert.

Dr. Clark observed precisely the same appearances at Rosetta. The city
seemed surrounded with a beautiful sheet of water; and so certain was
his Greek interpreter--who was unacquainted with the country--of this
fact, that he was quite indignant at an Arab who attempted to explain
to him that it was a mere optical delusion. At length they reached
Rosetta in about two hours, without meeting with any water; and on
looking back on the sand they had just crossed, it seemed to them as if
they had waded through a vast blue lake.

[Illustration: _figure 29._]

On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which
appeared as at A, (fig. 29.) the topmast being the only part
of it that was seen above the horizon. An inverted image of it was seen
at B, immediately above the real ship A, and an erect
image at C, both of them being complete and well defined.
The sea was distinctly seen between them, as at V W. As the
ship rose to the horizon the image C gradually disappeared,
and while this was going on, the image B descended, but the
mainmast of B did not meet the mainmast of A. The
two images BC were perfectly visible when the whole ship was
actually below the horizon. Dr. Vince then directed his telescope to
another ship whose hull was just in the horizon, and he observed a
complete inverted image of it, the mainmast of which just touched the
mainmast of the ship itself. He saw at the same time several other
ships whose images appeared in nearly a similar manner, in one of
which the two images were visible when the whole ship was beneath the
horizon. These phenomena must have been produced by the same causes
which operated in the case formerly mentioned, in relation to Captain
Scoresby, when he saw the figure of his father’s ship inverted in the
distant horizon. Such cases are, perhaps not uncommon, especially in
calm and sultry weather, but they are seldom observed, except when a
person’s attention is accidentally directed to the phenomenon, and,
unless he use a telescope, it will not be so distinctly perceived.

The following phenomenon, of a description nearly related to the above,
has been supposed to be chiefly owing to _reflection_. On the 18th
of November, 1804, Dr. Buchan, when watching the rising sun, about
a mile to the east of Brighton, just as the solar disk emerged from
the surface of the water, saw the face of the cliff on which he was
standing, a windmill, his own figure and the figure of his friend,
distinctly represented, precisely opposite, at some distance from the
ocean. This appearance lasted about ten minutes, till the sun had
risen nearly his own diameter above the sea. The whole then seemed to
be elevated into the air and successively disappeared. The surface of
the sun was covered with a dense fog of many yards in height, which
gradually receded from the rays of the sun as he ascended from the
horizon.

The following appearance most probably arose chiefly from the
_refraction_ of the atmosphere. It was beheld at Ramsgate, by Dr.
Vince of Cambridge and another gentleman. It is well known that
the four turrets of Dover castle are seen at Ramsgate, over a hill
which intervenes between a full prospect of the whole. On the 2nd of
August, 1806, not only were the four turrets visible, but the castle
itself appeared as though situated on that side of the hill nearest
Ramsgate, and so striking was the appearance, that for a long time the
Doctor thought it an illusion; but at last, by accurate observation,
was convinced that it was an actual image of the castle. He, with
another individual, observed it attentively for twenty minutes, but
were prevented by rain from making further observations. Between the
observers and the land from which the hill rises, there were about six
miles of sea, and from thence to the top of the hill there was about
the same distance, their own height above the surface of the water was
about seventy feet.--The cause of this phenomenon was, undoubtedly,
_unequal refraction_. The air being more dense near the ground and
above the sea than at greater heights, reached the eye of the observer,
not in straight but in curvilinear lines. If the rays from the castle
had in their path struck an eye at a much greater distance than
Ramsgate, the probability is, that the image of the castle would have
been inverted in the air; but in the present case,the rays from the
turret and the base of the castle had not crossed each other.

To similar causes as those now alluded to are to be attributed such
phenomena as the following:

_The Spectre of the Brocken._ This is a wonderful and, at first sight,
a terrific phenomenon, which is sometimes seen from the summit of one
of the Hartz mountains in Hanover, which is about 3,300 feet above the
level of the sea, and overlooks all the country fifteen miles round.
From this mountain the most gigantic and terrific spectres have been
seen, which have terrified the credulous, and gratified the curious, in
a very high degree. M. Hawé who witnessed this phenomenon, says, the
sun rose about four o’clock, after he had ascended to the summit, in a
serene sky, free of clouds; and about a quarter past five, when looking
round to see if the sky continued clear, he suddenly beheld at a little
distance, a human figure of _a monstrous size_ turned towards him, and
glaring at him. While gazing on this gigantic spectre, with a mixture
of awe and apprehension, a sudden gust of wind nearly carried off his
hat, and he clapt his hand to his head to detain it, when to his great
delight, the colossal spectre did the same. He changed his body into a
variety of attitudes,all which the spectre exactly imitated, and then
suddenly vanished without any apparent cause, and, in a short time as
suddenly appeared. Being joined by another spectator, after the first
visions had disappeared, they kept steadily looking for the aërial
spectres, when two gigantic monsters suddenly appeared. These spectres
had been long considered as preternatural, by the inhabitants of the
adjacent districts, and the whole country had been filled with awe
and terror. Some of the lakes of Ireland are found to be susceptible
of producing illusions, particularly the lake of _Killarney_. This
romantic sheet of water is bounded on one side, by a semicircle of
rugged mountains, and on the other by a flat morass; and the vapours
generated in the marsh, and broken by the mountains, continually
represent the most fantastic objects. Frequently men riding along
the shore are seen as if they were moving across the lake, which is
supposed to have given rise to the legend of O’Donougho, a magician who
is said to be visible on the lake every May morning.

There can be little doubt that most of those visionary appearances
which have been frequently seen in the sky and in mountainous regions,
are phantoms produced by the cause to which I am adverting, such as
armies of footmen and horsemen, which some have asserted to have been
seen in the air near the horizon. A well authenticated instance of this
kind occurred in the Highlands of Scotland:--Mr. Wren of Wetton Hall,
and D. Stricket his servant, in the year 1744, were sitting at the
door of the house in a summer evening, when they were surprised to see
opposite to them on the side of Sonterfell hill--a place so extremely
steep, that scarce a horse could _walk_ slowly along it--the figure of
a man with a dog pursuing several horses, all running at a most rapid
pace. Onwards they passed till at last they disappeared at the lower
end of the Fell. In expectation of finding the man dashed to pieces by
so tremendous a fall, they went early next morning and made a search,
but no trace of man or horse, or the prints of their feet on the turf
could be found. Sometime afterwards, about seven in the evening, on the
same spot, they beheld a troop of horsemen advancing in close ranks and
at a brisk pace. The inmates of every cottage for a mile round beheld
the wondrous scene, though they had formerly ridiculed the story told
by Mr. Wren and his servant, and were struck with surprise and fear.
The figures were seen for upwards of two hours, till the approach of
darkness rendered them invisible. The various evolutions and changes
through which the troops passed were distinctly visible, and were
marked by all the observers. It is not improbable that these aërial
troopers were produced by the same cause which made the castle of Dover
to appear on the side of the hill next to Ramsgate, and it is supposed
that they were the images of a body of rebels, on the other side of the
hill, exercising themselves previous to the rebellion in 1745.[12]

I shall mention only another instance of this description which lately
occurred in France, and for a time caused a powerful sensation among
all ranks. On Sunday the 17th of December, 1826, the clergy in the
parish of Migné, in the vicinity of Poictiers, were engaged in the
exercises of the Jubilee which preceded the festival of Christmas,
and a number of persons to the amount of 3000 souls assisted in the
service. They had planted as part of the ceremony, a large cross,
twenty-five feet high, and painted red, in the open air beside the
church. While one of the preachers, about five in the evening, was
addressing the multitude, he reminded them of the miraculous cross
which appeared in the sky to Constantine and his army, and the effect
it produced--when suddenly a similar celestial cross appeared in the
heavens just before the porch of the church about 200 feet above
the horizon, and 140 feet in length, and its breadth from three to
four feet, of a bright silver colour tinged with red. The curate
and congregation fixed their wondering gaze upon this extraordinary
phenomenon, and the effect produced on the minds of the assembly
was strong and solemn: they spontaneously threw themselves on their
knees; and many, who had been remiss in their religious duties, humbly
confessed their sins, and made vows of penance and reformation. A
commission was appointed to investigate the truth of this extraordinary
appearance, and a memorial stating the above and other facts was
subscribed by more than forty persons of rank and intelligence, so
that no doubt was entertained as to the reality of the phenomenon.
By many it was considered as strictly miraculous, as having happened
at the time and in the circumstances mentioned. But it is evident,
from what we have already stated, that it may be accounted for on
physical principles. The large cross of wood painted red was doubtless
the real object which produced the magnified image. The state of the
atmosphere, according to the descriptions given in the memorial, must
have been favourable for the production of such images. The spectrum
of the wooden cross must have been cast on the concave surface of
some atmospheric mirror, and so reflected back to the eyes of the
spectators, from an opposite place--retaining exactly the same shape
and proportions, but dilated in size; and what is worthy of attention,
it was tinged with red, the very colour of the object of which it was
the reflected image.

Such phenomena as we have now described, and the causes of them which
science is able to unfold, are worthy of consideration, in order to
divest the mind of superstitious terrors, and enable it clearly to
perceive the laws by which the Almighty directs the movements of the
material system. When any appearance in nature, exactly the reverse
of every thing we could have previously conceived--presents itself
to view, and when we know of no material cause by which it could be
produced, the mind must feel a certain degree of awe and terror, and
will naturally resort to supernatural agency as acting either in
opposition to the established laws of the universe, or beyond the range
to which they are confined. Besides the fears and apprehensions to
which such erroneous conceptions give rise, they tend to convey false
and distorted impressions of the attributes of the Deity and of his
moral government. Science, therefore, performs an invaluable service to
man, by removing the cause of superstitious alarms, by investigating
the laws and principles which operate in the physical system, and by
assigning reasons for those occasional phenomena, which at first sight
appeared beyond the range of the operation of natural causes.

The late ingenious Dr. Wollaston illustrated the causes of some of
the phenomena we have described, in the following manner. He looked
along the side of a red hot poker at a word or object ten or twelve
feet distant; and at a distance less than three eights of an inch from
the line of the poker, an _inverted_ image was seen, and within and
without that image, an _erect_ image, in consequence of the change
produced, by the heat of the poker, in the density of the air. He also
suggested the following experiment as another illustration of the
same principle, namely, viewing an object through a stratum of spirit
of wine lying above water, or a stratum of water laid above one of
syrup. He poured into a _square_ phial a small quantity of clear syrup,
and above this he poured an equal quantity of _water_ which gradually
combined with the syrup, as seen at A. fig. 30. The word ‘Syrup,’ on
a card held behind the bottle, appeared erect when seen through the
pure spirit, but inverted, when seen through the mixture of water and
syrup. He afterwards put nearly the same quantity of rectified spirits
of wine above the water, as seen at B, and he saw the appearance as
represented, namely, the true place of the word ‘Spirit,’ and the
inverted and erect images below. These substances, by their gradual
incorporation, produce refracting power, diminishing from the _spirit
of wine_ to the _water_, or from the _syrup_ to the _water_; so that by
looking through the mixed stratum, an inverted image of any object is
seen behind the bottle. These experiments show that the _mirage_ and
several other atmospherical phenomena may be produced by variations in
the refractive power of different strata of the atmosphere.

[Illustration: _figure 30._]

It is not unlikely that phenomena of a new and different description
from any we have hitherto observed, may be produced from the same
causes to which we have adverted. A certain optical writer remarks--‘If
the variation of the refractive power of the air takes place in a
horizontal line perpendicular to the line of vision, that is, from
right to left, then we may have a _lateral_ Mirage, that is, an image
of a ship may be seen on the right or left hand of the real ship, or
on both, if the variation of refractive power is the same on each side
of the line of vision, and a fact of this kind was once observed on
the Lake of Geneva. If there should happen at the same time, both a
vertical and a lateral variation of refractive power in the air, and
if the variation should be such as to expand or elongate the object in
both directions, then the object would be magnified as if seen through
a telescope, and might be seen and recognized at a distance at which it
would not otherwise have been visible. If the refracting power, on the
contrary, varied, so as to construct the object in both directions, the
image of it would be diminished as if seen through a concave lens.’


_Remarks and Reflections, in reference to the phenomena described
above._

Such, then, are some of the striking and interesting effects
produced by the refraction and the reflection of the rays of light.
As the formation of the _images_ of objects by convex lenses, lays
the foundation of the construction of refracting telescopes and
microscopes, and of all the discoveries they have brought to light, so
the property of _concave specula_, in forming similar images, is that
on which the construction of _Reflecting_ telescopes entirely depends.
To this circumstance Herschel was indebted for the powerful telescopes
he was enabled to construct--which were all formed on the principle of
reflection--and for all the discoveries they enabled him to make in
the planetary system, and in the sidereal heavens. The same principles
which operate in optical instruments, under the agency of man, we have
reason to believe, frequently act on a more expansive scale in various
parts of the system of nature. The magnificent _Cross_ which astonished
the preacher and the immense congregation assembled at Migné, was, in
all probability, formed by a vast atmospherical speculum formed by the
hand of nature, and representing its objects on a scale far superior to
that of human art; and probably, to the same cause is to be attributed
the singular phenomenon of the coast of France having been made to
appear within two or three miles of the town of Hastings, as formerly
described, (see p. 53.) Many other phenomena which we have never
witnessed, and of which we can form no conception, may be produced by
the same cause operating in an infinity of modes.

The facts we have stated above, and the variety of modes by which light
may be refracted and reflected by different substances in nature,
lead us to form some conceptions of the magnificent and diversified
scenes which light may produce in other systems and worlds, under
the arrangements of the all-wise and Beneficent Creator. Light, in
all its modifications and varieties of colour and reflection, may be
considered as the beauty and glory of the universe, and the source
of unnumbered enjoyments to all its inhabitants. It is a symbol of
the Divinity himself; for “GOD IS LIGHT, and in Him is no
darkness at all.” It is a representative of Him who is exhibited in
the Sacred oracles, as “The SUN of Righteousness,” and “the
LIGHT of the world.” It is an emblem of the glories and
felicities of that future world, where knowledge shall be perfected,
and happiness complete; for its inhabitants are designated “the saints
in _light_;” and it is declared in Sacred history, to have been the
first born of created beings. In our lower world, its effects on the
objects which surround us, and its influences upon all sensitive
beings, are multifarious and highly admirable. While passing from
infinitude to infinitude, it reveals the depth and immensity of
the heavens, the glory of the sun, the beauty of the stars, the
arrangements of the planets, the rainbow encompassing the sky with
its glorious circle, the embroidery of flowers, the rich clothing of
the meadows, the valleys standing thick with corn, “the cattle on a
thousand hills,” the rivers rolling through the plains, and the wide
expanse of the ocean. But in other worlds the scenes it creates may
be far more resplendent and magnificent. This may depend upon the
refractive and reflective powers with which the Creator has endowed
the atmospheres of other planets, and the peculiar constitution of the
various objects with which they are connected. It is evident, from
what we already know of the reflection of light, that very slight
modifications of certain physical principles, and very slight additions
to the arrangements of our terrestrial system, might produce scenes of
beauty, magnificence and splendour of which, at present, we can form
no conception. And, it is not unlikely that by such diversities of
arrangement, in other worlds, _an infinite variety_ of natural scenery
is produced throughout the universe.

In the arrangements connected with the planet Saturn, and the immense
rings with which it is encompassed, and in the various positions which
its satellites daily assume with regard to one another, to the planet
itself, and to these rings--there is, in all probability, a combination
of refractions, reflections, light, and shadows, which produce scenes
wonderfully diversified, and surpassing in grandeur what we can now
distinctly conceive. In the remote regions of the heavens, there are
certain bodies composed of immense masses of luminous matter, not yet
formed into any regular system, and which are known by the name of
_Nebulæ_. What should hinder us from supposing that certain exterior
portions of those masses form speculums of enormous size, as some
parts of our atmosphere are sometimes found to do? Such specula may
be conceived to be hundreds and even thousands of miles in diameter,
and that they may form images of the most distant objects in the
heavens, on a scale of immense magnitude and extent, and which may
be reflected, in all their grandeur, to the eyes of intelligences at
a vast distance. And, if the organs of vision of such beings, be far
superior to ours in acuteness and penetrating power, they may thus be
enabled to take a survey of an immense sphere of vision, and to descry
magnificent objects at distances the most remote from the sphere they
occupy. Whatever grounds there may be for such suppositions, it must be
admitted, that all the knowledge we have hitherto acquired respecting
the operation of light, and the splendid effects it is capable of
producing, is small indeed, and limited to a narrow circle, compared
with the immensity of its range, the infinite modifications it may
undergo, and the wondrous scenes it may create in regions of creation
to which human eyes have never yet penetrated,--and which may present
to view objects of brilliancy and magnificence such as, “Eye hath not
yet seen, nor ear heard, nor hath it entered into the heart of man to
conceive.”




CHAPTER V.


SECT. I.--ON THE COLOURS OF LIGHT.

We have hitherto considered light chiefly as a simple homogeneous
substance, as if all its rays were white, and as if they were all
refracted in the same manner by the different lenses on which they
fall. Investigations however, into the nature of this wonderful
fluid, have demonstrated that this is not the case, and that it is
possessed of certain additional properties, of the utmost importance
in the system of nature. Had every ray of light been a pure white, and
incapable of being separated into any other colours, the scene of the
universe would have exhibited a very different aspect from what we
now behold. One uniform hue would have appeared over the whole face
of nature, and one object could scarcely have been distinguished from
another. The different shades of verdure which now diversify every
landscape, the brilliant colouring of the flowery fields, and almost
all the beauties and sublimities which adorn this lower creation would
have been withdrawn. But it is now ascertained that every ray of white
light is composed of an assemblage of colours, whence proceed that
infinite variety of shade and colour with which the whole of our
terrestrial habitation is arrayed. Those colours are found not to be
in the objects themselves, but in the rays of light which fall upon
them, without which they would either be invisible, or wear an uniform
aspect. In reference to this point, Goldsmith has well observed: ‘The
blushing beauties of the rose, the modest blue of the violet, are not
in the flowers themselves, but in the light that adorns them. Odour,
softness, and beauty of figure are their own; but it is light alone
that dresses them up in those robes which shame the monarch’s glory.’

Many strange opinions and hypotheses were entertained respecting
colours, by the ancients, and even by many modern writers, prior to
the time of Sir Isaac Newton. The Pythagoreans called colour the
_superficies_ of bodies; Plato said that it was a flame issuing from
them. According to Zeno it is the first configuration of matter,
and according to Aristotle, it is that which moves bodies actually
transparent. Among the moderns, Des Cartes imagined that the difference
of colour proceeds from the prevalence of the direct or rotatory
motions of the particles of light. Grimaldi, Dechales, and others,
thought the differences of colour depended upon the quick or slow
vibrations of a certain elastic medium filling the whole universe.
Rohault imagined that the different colours were made by the rays of
light entering the eye at different angles with respect to the optic
axis; and Dr. Hook conceived that colour is caused by the sensation
of the oblique or uneven pulse of light; and this being capable of
no more than two varieties, he concluded that there could be no
more than two primary colours. Such were some of the crude opinions
which prevailed before the era of the illustrious Newton, by whose
enlightened investigations the true theory of colours was at last
discovered. In the year 1666 this philosopher began to investigate the
subject; and finding the  image of the sun, formed by a glass
prism, to be of an oblong and not of a circular form, as according to
the laws of refraction it ought to be, he was surprised at the great
disproportion between its length and breadth, the former being _five_
times the length of the latter; and he began to conjecture that light
is not _homogeneal_, but that it consists of rays some of which are
much more refrangible than others. Prior to this period, philosophers
supposed that _all_ light, in passing out of one medium into another
of different density was _equally_ refracted in the same or like
circumstances; but Newton discovered that this is not the fact; but
that there are _different species_ of light, and that each species is
disposed both to suffer a different degree of refrangibility in passing
out of one medium into another,--and to excite in us the idea of a
_different colour_ from the rest; and that bodies appear of that colour
which arises from the peculiar rays they are disposed to reflect. It is
now, therefore, universally acknowledged, that the light of the sun,
which to us seems perfectly homogeneal and white, is composed of no
fewer than _seven_ different colours, namely _Red_, _Orange_, _Yellow_,
_Green_, _Blue_, _Indigo and Violet_. A body which appears of a red
colour has the property of reflecting the red rays more powerfully
than any of the others; a body of a green colour reflects the green
rays more copiously than rays of any other colour, and so of the
orange, yellow, blue, purple and violet. A body which is of a _black_
colour, instead of reflecting--_absorbs_ all, or the greater part of
the rays that fall upon it; and, on the contrary, a body that appears
_white_ reflects the greater part of the rays indiscriminately without
separating the one from the other.

Before proceeding to describe the experiments by which the above
results were obtained, it may be proper to give some idea of the form
and effects of the _Prism_ by which such experiments are made. This
instrument is triangular and straight, and generally about three or
four inches long. It is commonly made of white glass, as free as
possible from veins and bubbles, and other similar defects, and is
solid throughout. Its lateral faces, or sides, should be perfectly
plane and of a fine polish. The angle formed by the two faces, one
receiving the ray of light that is refracted in the instrument, and the
other affording it an issue on its returning into the air, is called
the _refracting angle_ of the prism, as ACB, (fig. 31.) The manner in
which Newton performed his experiments, and established the discovery
to which we have alluded, is as follows.

In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was
made, of about one third of an inch diameter, and behind it was
placed a glass prism ACB, so that the beam of light, SF, proceeding
directly from the sun was made to pass through the prism. Before the
interposition of the prism, the beam proceeded in a straight line
towards T, where it formed a round white spot; but being now bent
out of its course by the prism, it formed an oblong image OP, upon
the white pasteboard, or screen LM, containing the seven colours
marked in the figure--the _red_ being the _least_, and the _violet_
the _most_ refracted from the original direction of the solar beam,
ST. This oblong image is called the _prismatic spectrum_. If the
refracting angle of the prism ACB, be 64 degrees, and the distance
of the pasteboard from the prism about 18 feet, the length of the
image OP will be about 10 inches, and the breadth 2 inches. The sides
of the spectrum are right lines distinctly bounded, and the ends are
semicircular. From this circumstance it is evident that it is still
the image of the sun, but elongated by the refractive power of the
prism. It is evident from the figure, that since some part of the
beam, RO, is refracted much further out of its natural course WT, than
some other part of the beam, as WP, the rays towards RO have a much
greater disposition to be refracted than those toward WP; and that this
disposition arises from the naturally different qualities of those
rays, is evident from this consideration, that the refracting angle or
power of the prism is the same in regard to the superior part of the
beam as to the inferior.

[Illustration: _figure 31._]

By making a hole in the screen LM opposite any one of the colours of
the spectrum, so as to allow that colour alone to pass--and by letting
the colour thus separated fall upon a second prism--Newton found that
the light of each of the colours was alike refrangible, because the
second prism could not separate them into an oblong image, or into
any other colour. Hence he called all the seven colours _simple_
or homogeneous, in opposition to _white_ light, which he called
_compound_ or heterogeneous. With the prism which this philosopher
used he found the lengths of the colours and spaces of the spectrum
to be as follows: Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue,
60; Indigo, 48; Violet, 80: or 360 in all. But these spaces vary a
little with prisms formed of different substances, and as they are
not separated by distinct limits, it is difficult to obtain any thing
like an accurate measure of their relative extents. Newton examined
the ratio between the sines of incidence and refraction of these
decompounded rays (see p. 30,) and found that each of the seven primary
colour-making rays, had certain limits within which they were confined.
Thus let the sine of incidence in glass be divided into 50 equal
parts, the sine of refraction into air of the _least_ refrangible,
and the _most_ refrangible rays will contain respectively 77 and 78
such parts. The sines of refraction of all the degrees of _red_ will
have the intermediate degrees of magnitude, from 77 to 77 one-eighth;
_Orange_ from 77 one-eighth to 77 one-fifth; _Yellow_ from 77 one-fifth
to 77 one-third; _Green_ from 77 one-third to 77 one-half; _Blue_
from 77 one-half to 77 two-thirds; _Indigo_ from 77 two-thirds to 77
seven-ninths; and _Violet_ from 77 seven-ninths to 78.

From what has been now stated, it is evident that, in proportion as any
part of an optic glass bears a resemblance to the form of a prism, the
component rays that pass through it must be necessarily separated, and
will consequently paint or tinge the object with colours. The edges of
every convex lens approach to this form, and it is on this account that
the extremities of objects when viewed through them are found to be
tinged with the prismatic colours. In such a glass, therefore, those
different  rays will have _different foci_, and will form their
respective images at different distances from the lens. Thus, suppose
LN (fig. 32.) to represent a double convex-lens, and OB an object at
some distance from it. If the object OB was of a pure red colour, the
rays proceeding from it would form a red image at RR; if the
object was of a violet colour, an image of that colour would be formed
at VV, _nearer_ the lens; and if the object was white or any
other combination of the colour-making rays, those rays would have
their respective foci at different distances from the lens, and form a
succession of images, in the order of the prismatic colours, between
the space RR and VV.

[Illustration: _figure 32._]

[Illustration: _figure 33._]

This may be illustrated by experiment in the following manner. Take
a card or slip of white pasteboard, as ABEF, (fig. 33.) and paint
one half ABCD _red_, the other half CF, _violet_ or indigo; and tying
black threads across it, set it near the flame of a candle G, then
take a lens HI, and holding a sheet of white paper behind it, move it
backwards and forwards upon the edge of a graduated ruler, till you
see the black threads most distinctly in the image, and you will find
the focus of the violet FE, much nearer than that of the red
AC, which plainly shows that bodies of different colours can
never be depicted by convex-lenses, without some degree of confusion.

The quantity of dispersion of the  rays in convex lenses
depends upon the focal length of the glass; the space which the
 images occupy being about the twenty-eighth part. Thus if
the lens be twenty-eight inches focal distance, the space between
RR and VV (fig 32) will be about one inch; if it
be twenty-eight feet focus, the same space will be about one foot,
and so on in proportion. Now, when such a succession of images formed
by the different  rays, is viewed through an eye-glass, it
will seem to form but one image, and consequently very indistinct,
and tinged with various colours, and as the red figure RR is
largest, or seen under the greatest angle--the extreme parts of this
confused image will be red, and a succession of the prismatic colours
will be formed within this red fringe, as is generally found in common
refracting-telescopes, constructed with a single object-glass. It is
owing to this circumstance that the common refracting telescope cannot
be much improved without having recourse to lenses of a very long
focal distance; and hence, about 150 years ago, such telescopes were
constructed of 80, and 100, and 120 feet in length. But still the image
was not formed so distinctly as was desired, and the aperture of the
object-glass was obliged to be limited. This is a defect which was
long regarded as without a remedy; and even Newton himself despaired
of discovering any means by which the defects of refracting telescopes
might be removed and their improvement effected. This, however, was
accomplished by Dollond to an extent far surpassing what could have
been expected, of which a particular account will be given in the
sequel.

It was originally remarked by Newton, and the fact has since
been confirmed by the experiments of Sir W. Herschel, that _the
different- rays have not the same illuminating power_. The
violet rays appear to have the least illuminating effect; the indigo
more, and the effect increases in the order of the colours,--the
_green_ being very great; between the green and the yellow the greatest
of all; the yellow the same as the green; but the red less than the
yellow. Herschel also endeavoured to determine whether the power of the
differently- rays to _heat_ bodies, varied with their power
to illuminate them. He introduced a beam of light into a dark room,
which was decomposed by a prism, and then exposed a very sensible
thermometer to all the rays in succession, and observed the heights
to which it rose in a given time. He found that their heating power
increased from the violet to the red. The mercury in the thermometer
rose higher when its bulb was placed in the Indigo than when it was
placed in the violet, still higher in blue, and highest of all at red.
Upon placing the bulb of the thermometer below the red, quite out of
the spectrum, he was surprised to find that the mercury rose highest
of all; and concluded that _rays proceed from the sun, which have the
power of_ HEATING, _but not of illuminating bodies_. These
rays have been called _invisible_ solar rays. They were about half an
inch from the commencement of the red rays; at a greater distance from
this point the heat began to diminish, but was very perceptible even
at the distance of 1-1/2 inch. He determined that the heating power of
the _red_ to that of the _green_ rays, was 2-3/4 to 1, and 3-1/2 to 1,
in red to _violet_. He afterwards made experiments to collect those
invisible calorific rays, and caused them to act independently of the
light, from which he concluded that they are sufficient to account
for all the effects produced by the solar rays in exciting heat; that
they are capable of passing through glass, and of being refracted and
reflected, after they have been finally detached from the solar beam.

M. Ritter of Jena, Wollaston, Beckman and others, have found
that the rays of the spectrum are possessed of certain _chemical
properties_--that beyond the least brilliant extremity, namely, a
little beyond the _violet_ ray, there are _invisible_ rays which
act chemically, while they have neither the power of heating nor
illuminating bodies. Muriate of silver exposed to the action of the red
rays becomes blackish; a greater effect is produced by the yellow: a
still greater by the violet, and the greatest of all by the _invisible_
rays _beyond_ the violet. When phosphorus is exposed to the action
of the invisible rays beyond the red, it emits white fumes; but the
invisible rays beyond the violet extinguish them. The influence of
these rays is daily seen in the change produced upon vegetable colours,
which fade, when frequently exposed to the direct influence of the sum.
What object they are destined to accomplish in the general economy of
nature, is not yet distinctly known; we cannot however doubt that
they are essentially requisite to various processes going forward in
the material system. And we know that, not only the comfort of all
the tribes of the living world, but the very existence of the animal
and vegetable creation depends upon the unremitting agency of the
_Calorific_ rays.

It has likewise been lately discovered that certain rays of the
spectrum, particularly the _violet, possesses the property of
communicating the magnetic power_. Dr. _Morichini_, of Rome, appears to
have been the first who found that the violet rays of the spectrum had
this property. The result of his experiments, however, was involved in
doubt, till it was established by a series of experiments instituted by
Mrs. _Somerville_, whose name is so well known in the scientific world.
This lady having covered half of a sewing-needle, about an inch long,
with paper, she exposed the other half for two hours, to the violet
rays. The needle had then acquired North polarity. The indigo rays
produced nearly the same effect; and the blue and green rays produced
it in a still less degree. In the yellow, orange, red and invisible
rays, no magnetic influence was exhibited, even though the experiment
was continued for three successive days. The same effects were produced
by enclosing the needle in blue or green glass, or wrapping it in blue
and green ribbands one half of the needle being always covered with
paper.

One of the most curious discoveries of modern times, in reference
to the solar spectrum, is that of Fraunhofer of Munich--one of the
most distinguished artists and opticians on the Continent.[13] He
discovered that the spectrum is covered with dark and  lines,
parallel to one another, and perpendicular to the length of the
spectrum; and he counted no less than 590 of these lines. In order
to observe these lines, it is necessary to use prisms of the most
perfect construction, of very pure glass, free of veins--to exclude
all extraneous light, and even to stop those rays which form the
 spaces, which we are not examining. It is necessary also to
use a magnifying instrument, and the light must enter and emerge from
the prism at equal angles. One of the important practical results of
this discovery is, that those lines are fixed points in the spectrum,
or rather, that they have always the same position in the 
spaces in which they are found. Fraunhofer likewise discovered in the
spectrum produced by the light of Venus, the same streaks, as in the
solar spectrum; in the spectrum of the light of Sirius, he perceived
three large streaks which, according to appearance, had no resemblance
to those of the light of the sun; one of them was in the green, two
in the blue. The stars appear to differ from one another in their
streaks. The electric light differs very much from the light of the sun
and that of a lamp, in regard to the streaks of the spectrum--‘This
experiment may also be made, though in an imperfect manner, by viewing
a narrow slit between two nearly closed window-shutters, through a very
excellent glass prism held close to the eye, with the refracting angle
parallel to the line of light. When the spectrum is formed by the sun’s
rays, either direct or indirect, as from the sky, clouds, rainbow,
moon, or planets, the black bands are always found to be in the same
parts of the spectrum, and under all circumstances to maintain the same
relative position, breadth and intensities.’

From what has been stated in reference to the solar spectrum it will
evidently appear, that white light is nothing else than a compound of
all the prismatic colours; and this may be still farther illustrated
by shewing, that the seven primary colours, when again put together,
recompose white light. This may be rudely proved for the purpose of
illustration, by mixing together seven different powders, having the
colours and proportion of the spectrum; but the best mode, on the
whole, is the following. Let two circles be drawn on a smooth round
board, covered with white paper, as in fig. 34: Let the outermost be
divided into 360 equal parts; then draw seven right lines as A,B,C,
&c., from the center to the outermost circle, making the lines A and
B include 80 degrees of that circle. The lines B and C, 40 degrees; C
and D, 60; D and E, 60; E and F, 48; F and G, 27; G and A, 45. Then
between these two circles paint the space AG red, inclining to orange
near G; GF orange, inclining to yellow near F; FE yellow, inclining to
green near E; ED green, inclining to blue near D; DC blue, inclining to
indigo near C; CB indigo, inclining to violet near B; and BA violet,
inclining to a soft red near A. This done, paint all that part of
the board black which lies within the inner circle; and putting an
axis through the centre of the board, let it be turned swiftly round
that axis, so that the rays proceeding from the above colours, may be
all blended and mixed together in coming to the eye. Then the whole
 part will appear like a white ring a little grayish--not
perfectly white, because no art can prepare or lay on perfect colours,
in all their delicate shades, as found in the real spectrum.

[Illustration: _figure 34._]

That all the colours of light, when blended together in their proper
proportions, produce a pure _white_ is rendered certain by the
following experiment. Take a large convex glass, and place it in the
room of the paper or screen on which the solar spectrum was depicted
(LM fig. 31), the glass will unite all the rays which come from the
prism, if a paper is placed to receive them, and you will see a
circular spot of a pure lively white. The rays will cross each other in
the focus of the glass, and, if the paper be removed a little further
from that point, you will see the prismatic colours again displayed,
but in an inverted order, owing to the crossing of the rays.


SECT. 2.--ON THE COLOURS OF NATURAL OBJECTS.

From what has been stated above we may learn the true cause of those
diversified hues exhibited by natural and artificial objects, and the
variegated colouring which appears on the face of nature. It is owing
to the surfaces of bodies being disposed to reflect one colour rather
than another. When this disposition is such that the body reflects
every kind of ray, in the mixed state in which it receives them, that
body appears _white_ to us--which, properly speaking, is no colour,
but rather the assemblage of all colours. If the body has a fitness
to reflect one sort of rays more abundantly than others, by absorbing
all the others, it will appear of the colour belonging to that species
of rays. Thus, the grass is _green_, because it absorbs all the rays
except the green. It is these green rays only which the grass, the
trees, the shrubs, and all the other verdant parts of the landscape
reflect to our sight, and which make them appear green. In the same
manner the different flowers reflect their respective colours; the
rose, the red rays; the violet, the blue; the jonquil, the yellow; the
marigold, the orange, and every object, whether natural or artificial,
appears of that colour which its peculiar texture is fitted to reflect.
A great number of bodies are fitted to reflect at once several kinds
of rays, and of consequence they appear under mixed colours. It may
even happen, that of two bodies which should be green, for example,
one may reflect the pure green of light, and the other the mixture of
yellow and blue. This quality, which varies to infinity, occasions the
different kinds of rays to unite in every possible manner, and every
possible proportion; and hence the inexhaustible variety of shades
and hues which nature has diffused over the landscape of the world.
When a body absorbs nearly all the light which reaches it, that body
appears _black_. It transmits to the eye so few reflected rays that
it is scarcely perceptible in itself, and its presence and form make
no impression upon us, unless as it interrupts the brightness of the
surrounding space. Black is, therefore, the absence of all the 
rays.

It is evident, then, that all the various assemblages of colours which
we see in the objects around us, _are not in the bodies themselves_,
but in the light which falls upon them. There is no colour _inherent_
in the grass, the trees, the fruits, and the flowers, nor even in the
most splendid and variegated dress that adorns a lady. All such objects
are as destitute of colour, in themselves, as bodies which are placed
in the centre of the earth, or as the chaotic materials out of which
our globe was formed, before light was created. For where there is no
light, there is no colour. Every object is black, or without colour,
in the dark, and it only appears  as soon as light renders it
visible. This is further evident from the following experiment. If we
place a <DW52> body in one of the colours of the spectrum which is
formed by the prism, it appears of the colour of the rays in which it
is placed. Take, for example, a red rose, and expose it first to the
red rays, and it will appear of a more brilliant ruddy hue. Hold it
in the blue rays, and it appears no longer red, but of a dingy blue
colour, and in like manner its colour will appear different, when
placed in all the other differently  rays. This is the reason
why the colours of objects are essentially altered by the nature of
the light in which they are seen. The colours of ribbons and various
pieces of silk or woollen stuff are not the same when viewed by
candle-light as in the day time. In the light of a candle or a lamp,
blue often appears green, and yellow objects assume a whitish aspect.
The reason is that the light of a candle is not so pure a white as that
of the sun, but has a yellowish tinge, and therefore, when refracted
by the prism, the yellowish rays are found to predominate, and the
superabundance of yellow rays gives to blue objects a greenish hue.

The doctrine we are now illustrating is one which a great many persons,
especially among the fair sex, find it difficult to admit. They cannot
conceive it possible that there is no colour really inherent in their
splendid attire, and no tints of beauty in their countenances. ‘What,’
said a certain lady, ‘are there no colours in my shawl, and in the
ribbons that adorn my head-dress--and, are we all as black as <DW64>s
in the dark; I should almost shudder to think of it.’ Such persons,
however, need be in no alarm at the idea; but may console themselves
with the reflection, that, when they are stripped of all their 
ornaments in the dark, they are certain that _they will never be seen
by any one_ in that state; and therefore, there is no reason to regret
the temporary loss of those beauties which light creates--when they
themselves and all surrounding objects are _invisible_. But, to give
a still more palpable proof of this position, the following popular
experiments may be stated.

Take a pint of common spirit, and pour it into a soup dish, and then
set it on fire; as it begins to blaze, throw a handful of salt into the
burning spirit, and keep stirring it with a spoon. Several handfuls
may thus be successively thrown in, and then the spectators, standing
around the flame, will see each other frightfully changed, their
colours being altered into a ghastly blackness, in consequence of the
nature of the light which falls upon them--which produces colours very
different from those of the solar light. The following experiment,
as described by Sir D. Brewster, illustrates the same principle.
‘Having obtained the means of illuminating any apartment with _yellow_
light, let the exhibition be made in a room with furniture of various
bright colours, and with oil or water  paintings on the wall.
The party which is to witness the experiment should be dressed in a
diversity of the gayest colours; and the brightest  flowers,
and highly  drawings should be placed on the tables. The room
being at first lighted with ordinary lights, the bright and gay colours
of every thing that it contains will be finely displayed. If the white
lights are now suddenly extinguished, and the yellow lamps lighted,
the most appalling metamorphosis will be exhibited. The astonished
individuals will no longer be able to recognise each other. All the
furniture of the room, and all the objects it contains, will exhibit
only _one_ colour. The flowers will lose their hues; the paintings
and drawings will appear as if they were executed in China ink, and
the gayest dresses, the brightest scarlets, the purest lilacs, the
richest blues and the most vivid greens, will all be converted into one
monotonous yellow. The complexions of the parties, too, will suffer a
corresponding change. One pallid deathlike yellow, will envelope the
young and the old, and the _sallow_ face will alone escape from the
metamorphosis. Each individual derives merriment from the cadaverous
appearance of his neighbour, without being sensible that he is one of
the ghastly assemblage.’

          ----Like the unnatural hue
    Which autumn paints upon the perished leaf,

From such experiments as these we might conclude, that were the solar
rays of a very different description from what they are now found to
be, the colours which embellish the face of nature, and the whole scene
of our sublunary creation would assume a new aspect, and appear very
different from what we now behold around us in every landscape. We find
that the stars display great diversity of colour; which is doubtless
owing to the different kinds of light which are emitted from those
bodies; and hence we may conclude, that the colouring thrown upon the
various objects of the universe is different in every different system,
and that thus, along with other arrangements, an infinite variety of
colouring and of scenery is distributed throughout the immensity of
creation.

The _atmosphere_, in consequence of its different refractive and
reflective powers, is the source of a variety of colours which
frequently embellish and diversify the aspect of our sky. The
air _reflects_ the blue rays most plentifully, and must therefore
_transmit_ the red, orange, and yellow, more copiously than the other
rays. When the sun and other heavenly bodies are at a high elevation,
their light is transmitted without any perceptible change, but when
they are near the horizon, their light must pass through a long and
dense track of air, and must therefore be considerably modified before
it reach the eye of the observer. The momentum of the red rays being
greater than that of the violet, will force their way through the
resisting medium, while the violet rays will be either reflected or
absorbed. If the light of the setting sun, by thus passing through
a long track of air, be divested of the green, blue, indigo, and
violet rays, the remaining rays which are transmitted through the
atmosphere, will illuminate the western clouds, first with an orange
colour; and then, as the sun gradually sinks into the horizon, the
track through which the rays must pass becoming longer, the yellow
and orange are reflected, and the clouds grow more deeply _red_, till
at length the disappearance of the sun leaves them of a leaden hue by
the reflection of the blue light through the air. Similar changes of
colour are sometimes seen on the eastern and western fronts of white
buildings. St. Paul’s Church, in London, is frequently seen at sun-set,
tinged with a very considerable degree of redness; and the same cause
occasions the moon to assume a ruddy colour, by the light transmitted
through the atmosphere. From such atmospherical refractions and
reflections are produced those rich and beautiful hues with which our
sky is gilded by the setting sun, and the glowing red which tinges the
morning and evening clouds, till their ruddy glare is tempered by the
purple of twilight, and the reflected azure of the sky.

When a direct spectrum is thrown on colours darker than itself, it
mixes with them: as the yellow spectrum of the setting sun, thrown on
the green grass, becomes a greener yellow. But when a direct spectrum
is thrown on colours brighter than itself, it becomes instantly changed
into the reverse spectrum, which mixes with those brighter colours.
Thus the yellow spectrum of the setting sun thrown on the luminous
sky, becomes blue, and changes with the colour or brightness of the
clouds on which it appears. The red part of light being capable of
struggling through thick and resisting mediums which intercept all
other colours--is likewise the cause why the sun appears red when seen
through a fog,--why distant light, though transmitted through blue
or green glass, appears red--why lamps at a distance, seen through
the smoke of a long street, are red, while those that are near, are
white. To the same cause it is owing that a diver at the bottom of the
sea is surrounded with the red light which has pierced through the
superincumbent fluid, and that the blue rays are reflected from the
_surface_ of the ocean. Hence, Dr. Halley informs us that, when he was
in a diving bell, at the bottom of the sea, his hand always appeared
red in the water.

The _blue_ rays, as already noticed, being unable to resist the
obstructions they meet with in their course through the atmosphere, are
either reflected or absorbed in their passage. It is to this cause,
that most philosophers ascribe _the blue colour of the sky_, the
faintness and obscurity of distant objects, and the bright azure which
tinges the mountains of a distant landscape.


SECT. 3.--PHENOMENA OF THE RAINBOW.

Since the rays of light are found to be decomposed by refracting
surfaces, and reflected in an infinite variety of modes and shades
of colour, we need not be surprised at the changes produced in any
scene or object by the intervention of another, and by the numerous
modifications of which the primary colours of nature are susceptible.
The vivid colours which gild the rising and the setting sun, must
necessarily differ from those which adorn its noon-day splendour.
Variety of atmospheric scenery will thus necessarily be produced,
greater than the most lively fancy can well imagine. The clouds will
sometimes assume the most fantastic forms, and at other times will be
irradiated with beams of light, or, covered with the darkest hues, will
assume a lowering aspect, prognostive of the thunder’s roar and the
lightning’s flash--all in accordance with the different rays that are
reflected to our eyes, or the quantity absorbed by the vapours which
float in the atmosphere.

Light, which embellishes with so much magnificence a pure and serene
sky, by means of innumerable bright starry orbs which are spread over
it, sometimes, in a dark and cloudy sky, exhibits an ornament which,
by its pomp, splendour and variety of colours, attracts the attention
of every eye that has an opportunity of beholding it. At certain
times, when there is a shower either around us, or at a distance from
us in an opposite quarter to that of the sun, a species of arch or
bow is seen in the sky, adorned with all the seven primary colours of
light. This phenomenon, which is one of the most beautiful meteors in
nature, has obtained the name of the RAINBOW. The rainbow was,
for ages, considered as an inexplicable mystery, and by some nations
it was adored as a deity. Even after the dawn of true philosophy,
it was a considerable time before any discovery of importance was
made, as to the true causes which operate in the production of this
phenomenon. About the year 1571, M. Fletcher of Breslau, made a certain
approximation to the discovery of the true cause, by endeavouring to
account for the colours of the rainbow by means of a double refraction
and one reflection. A nearer approximation was made by Antonio de
Dominis, bishop of Spalatro, about 1601. He maintained that the
double refraction of Fletcher, _with an intervening reflection_,
was sufficient to produce the colours of the bow, and also to bring
the rays that formed them to the eye of the spectator, without any
subsequent reflection. To verify this hypothesis, he procured a small
globe of solid glass, and viewing it when it was exposed to the rays
of the sun--with his back to that luminary--in the same manner as he
had supposed the drops of rain were situated with respect to them, he
observed the same colours which he had seen in the rainbow, and in
the same order. But he could give no good reason _why_ the bow should
be , and much less any satisfactory account of the _order_ in
which the colours appear. It was not till Sir I. Newton discovered
the different refrangibility of the rays of light, that a complete
and satisfactory explanation could be given of all the circumstances
connected with this phenomenon.

As the full elucidation of this subject involves a variety of optical
and mathematical investigations, I shall do little more than explain
the general principle on which the prominent phenomena of the rainbow
may be accounted for, and some of the facts and results which theory
and observation have deduced.

We have just now alluded to an experiment with a glass globe:--If,
then, we take either a solid glass globe, or a hollow globe filled with
water, and suspend it so high in the solar rays above the eye, that
the spectator, with his back to the sun, can see the globe _red_;--if
it be lowered slowly, he will see it orange, then yellow, then green,
then blue, then indigo, and then violet; so that the drop at different
heights, shall present to the eye the seven primitive colours in
succession. In this case, the globe, from its form, will act in some
measure like a prism, and the ray will be separated into its component
parts. The following figure will more particularly illustrate this
point. Suppose A (fig. 35.) to represent a drop of rain--which may be
considered as a globe of glass in miniature, and will produce the same
effect on the rays of light--and let SD represent a ray from
the sun falling upon the upper part of the drop at D. At the
point of entering the drop, it will suffer a refraction, and instead
of going forward to C, it will be bent to N. From
N a part of the light will be reflected to Q--some
part of it will, of course, pass through the drop. By the obliquity
with which it falls on the side of the drop at Q, that part
becomes a kind of prism, and separates the ray into its primitive
colours. It is found by computation that, after a ray has suffered
two refractions and one reflection, as here represented, the least
refrangible part of it, namely the _red_ ray, will make an angle with
the incident solar ray of 42° 2´, as SFQ; and the violet, or
greatest refrangible ray will make with the solar ray, an angle of 40°
17´, as SCQ; and thus all the particles of water within the
difference of those two angles, namely 1° 45´--(supposing the ray to
proceed merely from the centre of the sun)--will exhibit severally the
colours of the prism, and constitute the _interior_ bow of the cloud.
This holds good at whatever height the sun may chance to be in a shower
of rain. If he be at a high altitude, the rainbow will be low; if he be
at a low elevation, the rainbow must be high; and if a shower happen
in a vale, when the spectator is on a mountain, he will sometimes see
the bow in the form of a _complete circle_ below him. We have at
present described the phenomena only of a single drop; but it is to be
considered that in a shower of rain there are drops at all heights and
at all distances; and therefore the eye situated at G will see all the
different colours. All those drops that are in a certain position with
respect to the spectator will reflect the red rays, all those in the
next station the orange, those in the next the green, and so on with
regard to all the other colours.

[Illustration: _figure 35._]

It appears, then, that the first or primary bow is formed by two
refractions and one reflection; but there is frequently a second bow,
on the outside of the other, which is considerably fainter. This is
produced by drops of rain above the drop we have supposed at A. If B
(fig. 35.) represent one of these drops, the ray to be sent to the eye
enters the drop near the bottom, and suffers _two refractions_ and _two
reflections_, by which means the colours become reversed, that is, the
violet is lowest in the _exterior_ bow, and the red is lowest in the
_interior_ one, and the other colours are reversed accordingly. The ray
T is refracted at R: a part of it is reflected from S
to T, and at T it suffers another reflection from
T to U. At the points S and T part
of the ray _passes through_ the drop on account of its transparency,
towards W and X, and therefore we say that _part_
only of the ray is reflected. By these losses and reflections the
exterior bow becomes faint and ill-defined in comparison of the
interior or primary bow. In this case the upper part of the secondary
bow will not be seen when the sun is above 54° 10´ above the horizon;
and the lower part of the bow will not be seen when the sun is 60° 58´
above the horizon.

[Illustration: _figure 36._]

For the further illustrations of this subject, we may introduce the
following section of a bow, (fig. 36.) and, in order to prevent
confusion in attempting to represent all the different colours--let
us suppose only three drops of rain, and three different colours, as
shown in the figure. The spectator O being in the centre of the two
bows, here represented,--the planes of which must be considered as
perpendicular to his view--the drops A,B, and C produce part of the
interior bow by two refractions and one reflection as stated above,
and the drops D,E,F will produce the exterior bow by two refractions
and two reflections, the sun’s rays being represented by 3,3. It is
evident that the angle COP is less than the angle BOP, and that the
angle AOP is the greatest of the three. The largest angle, then, is
formed by the red rays, the middle one consists of the green, and the
smallest the purple or violet. All the drops of rain, therefore, that
happen to be in a certain position with respect to the spectator’s eye,
will reflect the red rays, and form a band or semicircle of red, and so
of the other colours from drops in other positions. If the spectator
alters his station, he will see a bow, but not the same as before; and
if there be many spectators, they will each see a different bow, though
it appears to be the same.

The rainbow assumes a _semicircular_ appearance, because it is only at
certain angles that the refracted rays are visible to our eyes, as is
evident from the experiment of the glass globe formerly alluded to,
which will refract the rays only in a certain position. We have already
stated that the red rays make an angle of 42° 2´, and the violet
an angle of 40° 17´. Now, if a line be drawn horizontally from the
spectator’s eye, it is evident that angles formed with this line, of a
certain dimension, in every direction, will produce a circle, as will
appear by attaching a cord of a given length to a certain point, round
which it may turn as round its axis; and, in every point will describe
an angle with the horizontal line of a certain and determinate extent.

Sometimes it happens that _three_ or more bows are visible, though
with different degrees of distinctness. I have more than once observed
this phenomenon, particularly in Edinburgh, in the month of August,
1825, when three rainbows were distinctly seen in the same quarter
of the sky; and, if I recollect right, a fragment of a fourth made
its appearance. This happens when the rays suffer a third or fourth
reflection; but, on account of the light lost by so many reflections,
such bows are, for the most part, altogether imperceptible.

If there were no ground to intercept the rain and the view of the
observer, the rainbow would form a _complete circle_, the centre
of which is diametrically opposite to the sun. Such circles are
sometimes seen in the spray of the sea or of a cascade, or from the
tops of lofty mountains, when the showers happen in the vales below.
Rainbows of various descriptions are frequently observed rising amidst
the spray and exhalations of waterfalls, and among the waves of the
sea whose tops are blown by the wind into small drops. There is one
regularly seen, when the sun is shining, and the spectator in a proper
position, at the fall of Staubbach, in the bosom of the Alps; one near
Schaffhausen; one at the cascade of Lauffen; and one at the cataract of
Niagara in North America. A still more beautiful one is said to be seen
at Terni, where the whole current of the river Velino, rushing from
a steep precipice of nearly 200 feet high, presents to the spectator
below, a variegated circle, over-arching the fall, and two other bows
suddenly reflected on the right and left. Don Ulloa, in the account
of his journeys in South America, relates that circular rainbows are
frequently seen on the mountains above Quito in Peru. It is said that
a rainbow was once seen near London, caused by the exhalations of
that city, after the sun had been below the horizon more than twenty
minutes.[14] A naval friend, says Mr. Bucke, informed me, that, as he
was one day watching the sun’s effect upon the exhalations near Juan
Fernandez, he saw upwards of five-and-twenty _ires marinæ_ animate
the sea at the same time. In these marine bows the concave sides were
turned upwards, the drops of water rising from below, and not falling
from above, as in the instances of the aerial arches. Rainbows are also
occasionally seen on the grass, in the morning dew, and likewise when
the hoar-frost is descending. Dr. Langwith once saw a bow lying on the
ground, the colours of which were almost as lively as those of a common
rainbow. It was not round but oblong, and was extended several hundred
yards. The colours took up less space, and were much more lively in
those parts of the bow which were near him than in those which were
at a distance. When M. Labillardiere was on Mount Teneriffe, he saw
the contours of his body traced on the clouds beneath him in all the
colours of the solar bow. He had previously witnessed this phenomenon
on the Kesrouan in Asia Minor. The rainbows of Greenland are said to
be frequently of a pale white, fringed with a brownish yellow, arising
from the rays of the sun being reflected from a frozen cloud.

The following is a summary view of the principal facts which have been
ascertained respecting the rainbow:--1. The rainbow can only be seen
when it rains, and in that point of the heavens which is opposite to
the sun. 2. Both the primary and secondary bows are variegated with all
the prismatic colours--the red being the highest colour in the primary,
or brightest bow, and the violet the highest in the exterior. 3. The
primary rainbow can never be a greater arc than a semicircle; and
when the sun is set, no bow, in ordinary circumstances, can be seen.
4. The breadth of the inner or primary bow--supposing the sun but a
point--is 1° 45´; and the breadth of the exterior bow 3° 12´, which is
nearly twice as great as that of the other; and the distance between
the bows is 8° 55´. But since the body of the sun subtends an angle of
about half a degree, by so much will each bow be increased, and their
distance diminished; and therefore the breadth of the interior bow
will be 2° 15´, and that of the exterior, 3° 42´, and their distance
8° 25´. The greatest semidiameter of the interior bow, on the same
grounds, will be 42° 17´, and the least of the exterior bow 50° 43´.
5. When the sun is in the horizon, either in the morning or evening,
the bows will appear complete semicircles. On the other hand, when the
sun’s altitude is equal to 42° 2´ or to 54° 10´, the summits of the
bows will be depressed below the horizon. Hence, during the days of
summer, within a certain interval each day, no visible rainbows can be
formed, on account of the sun’s high altitude above the horizon. 6.
The altitude of the bows above the horizon, or surface of the earth,
varies, according to the elevation of the sun. The altitude, at any
time, may be taken by a common quadrant, or other angular instrument;
but, if the sun’s altitude at any particular time be known, the height
of the summit of any of the bows may be found, by subtracting the sun’s
altitude from 42° 2´ for the inner bow, and from 54° 10´, for the
outer. Thus, if the sun’s altitude were 26°, the height of the primary
bow would 16° 2´, and of the secondary, 28° 10´. It follows, that the
height and the size of the bows diminish as the altitude of the sun
increases. 7. If the sun’s altitude is more than 42 degrees, and less
than 54°, the exterior bow may be seen though the interior bow is
invisible. 8. Sometimes only a portion of an arch will be visible while
all the other parts of the bow are invisible. This happens when the
rain does not occupy a space of sufficient extent to complete the bow;
and the appearance of this position, and even of the bow itself, will
be various, according to the nature of the situation, and the space
occupied by the rain.

The appearance of the rainbow may be produced by artificial means, at
any time when the sun is shining and not too highly elevated above the
horizon. This is effected by means of artificial fountains or _Jet
d’eaus_, which are intended to throw up streams of water to a great
height. These streams, when they spread very wide, and blend together
in their upper parts, form, when falling, a shower of artificial rain.
If, then, when the fountain is playing, we move between it and the sun,
at a proper distance from the fountain, till our shadow point directly
towards it, and look at the shower,--we shall observe the colours of
the rainbow, strong and vivid; and, what is particularly worthy of
notice, the bow appears, notwithstanding the nearness of the shower, to
be as large, and as far off, as the rainbow which we see in a natural
shower of rain. The same experiment may be made by candle-light, and
with any instrument that will form an artificial shower.

_Lunar Rainbows._--A lunar bow is sometimes formed at night by the rays
of the moon striking on a rain-cloud, especially when she is about
the full. But such a phenomenon is very rare. Aristotle is said to
have considered himself the first who had seen a lunar rainbow. For
more than a hundred years prior to the middle of the last century, we
find only two or three instances recorded in which such phenomena are
described with accuracy. In the philosophical transactions for 1783,
however, we have an account of three having been seen in one year,
and all in the same place, but they are by no means common phenomena.
I have had an opportunity within the last twenty years of witnessing
two phenomena of this description--one of which was seen at Perth, on
a sabbath evening, in the autumn of 1825, and the other at Edinburgh,
on Wednesday, the 9th of September 1840, about eight o’clock in the
evening--of both which I gave a detailed description in some of the
public journals. The Moon, in both cases, was within a day or two of
the full; the arches were seen in the northern quarter of the heavens,
and extended nearly from east to west, the moon being not far from the
southern meridian. The bows appeared distinct and well defined, but no
distinct traces of the prismatic colours could be perceived on any of
them. That which appeared in 1825 was the most distinctly formed, and
continued visible for more than an hour. The other was much fainter,
and lasted little more than half an hour, dark clouds having obscured
the face of the moon. These bows bore a certain resemblance to some
of the luminous arches which sometimes accompany the Aurora Borealis,
and this latter phenomenon has not unfrequently been mistaken for a
Lunar rainbow; but they may be always distinguished by attending to the
phases and position of the moon. If the moon be not visible above the
horizon, if she be in her first or last quarter, or if any observed
phenomenon be not in a direction opposite to the moon, we may conclude
with certainty that, whatever appearance is presented, there is no
lunar rainbow.

The rainbow is an object which has engaged universal attention, and
its beautiful colours and form have excited universal admiration. The
poets have embellished their writings with many beautiful allusions
to this splendid meteor; and the playful school-boy, while viewing
the ‘bright enchantment,’ has frequently run ‘to catch the falling
glory.’ When its arch rests on the opposite sides of a narrow valley,
or on the summits of two adjacent mountains, its appearance is both
beautiful and grand. In all probability, its figure first suggested the
idea of _arches_, which are now found of so much utility in forming
aqueducts and bridges, and for adorning the architecture of palaces and
temples. It is scarcely possible seriously to contemplate this splendid
phenomenon, without feeling admiration and gratitude towards that wise
and beneficent Being, whose hands have bent it into so graceful and
majestic a form, and decked it with all the pride of colours. “Look
upon the rainbow,” says the son of Sirach,[15] and praise Him that
made it: very beautiful it is in the brightness thereof. It compasseth
the heaven about with a glorious circle, and the hands of the Most
High have bended it." To this grand etherial bow, the inspired writers
frequently allude as one of the emblems of the majesty and splendour
of the Almighty. In the prophecies of Ezekiel, the throne of Deity is
represented as adorned with a brightness “like the appearance of the
bow that is in the cloud in the day of rain--the appearance of the
likeness of the glory of Jehovah.” And, in the visions recorded in the
Book of the Revelations, where the Most High is represented as sitting
upon a throne; “there was a rainbow round about the throne, in sight
like unto an emerald,” as an emblem of his propitious character and of
his faithfulness and mercy. After the deluge, this bow was appointed as
a sign and memorial of the covenant which God made with Noah and his
sons, that a flood of waters should never again be permitted to deluge
the earth and its inhabitants;--and as a pledge of inviolable fidelity
and Divine benignity. When, therefore, we at any time behold “the bow
in the cloud,” we have not only a beautiful and sublime phenomenon
presented to the eye of sense, but also a memorial exhibited to the
mental eye, assuring us, that, “While the earth remaineth, seed-time
and harvest, and cold and heat, and summer and winter, and day and
night, _shall not cease_.”[16]

                    ----On the broad sky is seen
    “A dewy cloud, and in the cloud a bow
    Conspicuous, with seven listed colours gay
    Betokening peace with God and covenant new.--
    He gives a promise never to destroy
    The earth again by flood, nor let the sea
    Surpass his bounds, nor rain to drown the world.”
                          _Milton. Par. Lost, Book XI._


SECT. 4.--REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS.

Colour is one of the properties of light which constitutes, chiefly,
the beauty and sublimity of the universe. It is colour, in all its
diversified shades, which presents to our view that almost infinite
variety of aspect which appears on the scene of nature, which gives
delight to the eye and the imagination, and which adds a fresh pleasure
to every new landscape we behold. Every flower which decks our fields
and gardens is compounded of different hues; every plain is covered
with shrubs and trees of different degrees of verdure; and almost every
mountain is clothed with herbs and grass of different shade from those
which appear on the hills and landscape with which it is surrounded.
In the country, during summer, nature is every day, and almost every
hour, varying her appearance, by the multitude and variety of her hues
and decorations, so that the eye wanders with pleasure over objects
continually diversified, and extending as far as the sight can reach.
In the flowers with which every landscape is adorned, what a lovely
assemblage of colours, and what a wonderful art in the disposition of
their shades! Here, a light pencil seems to have laid on the delicate
tints; there, they are blended according to the nicest rules of art.
Although green is the general colour which prevails over the scene
of sublunary nature, yet it is diversified by a thousand different
shades, so that every species of tree, shrub and herb, is clothed
with its own peculiar verdure. The dark green of the forests is thus
easily distinguished from the lighter shades of cornfields and the
verdure of the lawns. The system of animated nature likewise, displays
a diversified assemblage of beautiful colours. The plumage of birds,
the brilliant feathers of the peacock, the ruby and emerald hues which
adorn the little humming-bird, and the various embellishments of many
species of the insect tribe, present to the eye, in every region of
the globe, a scene of diversified beauty and embellishment. Nor is
the mineral kingdom destitute of such embellishments. For some of the
darkest and most unshapely stones and pebbles, when polished by the
hand of art, display a mixture of the most delicate and variegated
colours. All which beauties and varieties in the scene around us are
entirely owing to that property, in every ray of light, by which it is
capable of being separated into the primitive colours.

To the same cause, likewise, are to be ascribed those beautiful and
diversified appearances, which frequently adorn the face of the
sky,--the yellow, orange and ruby hues which embellish the firmament
at the rising of the sun, and when he is about to descend below the
western horizon; and those aerial landscapes, so frequently beheld in
tropical climes, where rivers, castles and mountains, are depicted
rolling over each other along the circle of the horizon. The clouds,
especially in some countries, reflect almost every colour in nature.
Sometimes they wear the modest blush of the rose; sometimes they appear
like stripes of deep vermillion, and sometimes as large brilliant
masses tinged with various hues; now they are white as ivory, and now
as yellow as native gold. In some tropical countries, according to St.
Pierre, the clouds roll themselves up into enormous masses as white as
snow, and are piled upon each other, like the Cordeliers of Peru, and
are moulded into the shape of mountains, of caverns and of rocks. When
the sun sets behind this magnificent aërial net-work, a multitude of
luminous rays are transmitted through each particular interstice, which
produce such an effect, that the two sides of the lozenge illuminated
by them, have the appearance of being begirt with a fillet of gold;
and the other two which are in the shade, seem tinged with a superb
ruddy orange. Four or five divergent streams of light, emanating from
the setting sun up to the zenith, clothe with fringes of gold the
undeterminate summits of this celestial barrier, and proceed to strike
with the reflexes of their fires the pyramids of the collateral aerial
mountains, which then appear to consist of silver and vermilion.--In
short, colour diversifies every sublunary scene, whether on the earth
or in the atmosphere, it imparts a beauty to the phenomena of falling
stars, of luminous arches, and the coruscations of the Aurora Borealis,
and gives a splendour and sublimity to the spacious vault of heaven.

Let us now consider for a moment, what would be the aspect of nature,
if, instead of the beautiful variety of embellishments which now
appear on every landscape, and on the concave of the sky,--_one_
uniform colour had been thrown over the scenery of the universe. Let
us conceive the whole of terrestrial nature to be covered with snow,
so that not an object on earth should appear with any other hue, and
that the vast expanse of the firmament presented precisely the same
uniform aspect. What would be the consequence? The light of the sun
would be strongly reflected from all the objects within the bounds of
our horizon, and would produce a lustre which would dazzle every eye.
The day would acquire a greater _brightness_ than it now exhibits,
and our eyes might, after some time, be enabled freely to expatiate
over the surrounding landscape; but every thing, though enlightened,
would appear _confused_, and particular objects would scarcely be
distinguishable. A tree, a house or a church, near at hand, might
possibly be distinguished, on account of its elevation above the
general surface of the ground, and the bed of a river by reason of its
being depressed below it. But we should be obliged rather to guess,
and to form a conjecture as to the particular object we wished to
distinguish, than to arrive at any certain conclusion respecting it;
and if it lay at a considerable distance, it would be impossible, with
any degree of probability, to discriminate any one object from another.
Notwithstanding the universal brightness of the scene, the uniformity
of colour thrown on every object, would most certainly prevent us from
distinguishing a church from a palace, a cottage from a knoll or a
heap of rubbish, a splendid mansion from rugged rocks, the trees from
the hills on which they grow, or a barren desert from rich and fertile
plains. In such a case, human beings would be confounded, and even
friends and neighbours be at a loss to recognize one another.

The vault of heaven, too, would wear a uniform aspect. Neither planets
nor comets would be visible to any eye, nor those millions of stars
which now shine forth with so much brilliancy, and diversify the
nocturnal sky. For, it is by the contrast produced by the deep azure
of the heavens and the white radiance of the stars, that those bodies
are rendered visible. Were they depicted on a pure white ground, they
would not be distinguished from that ground, and would consequently be
invisible, unless any of them occasionally assumed a different colour.
Of course, all that beautiful variety of aspect which now appears on
the face of sublunary nature--the rich verdure of the fields, the
stately port of the forest, the rivers meandering through the valleys,
the splendid hues that diversify and adorn our gardens and meadows, the
gay colouring of the morning and evening clouds, and all that variety
which distinguishes the different seasons, would entirely disappear. As
every landscape would exhibit nearly the same aspect, there would be no
inducement to the poet and the philosopher to visit distant countries
to investigate the scenes of nature, and journeyings from one region
to another would scarcely be productive of enjoyment. Were any other
single colour to prevail, nearly the same results would ensue. Were a
deep ruddy hue to be uniformly spread over the scene of creation, it
would not only be offensive to the eye, but would likewise prevent all
distinction of objects. Were a dark blue or a deep violet to prevail,
it would produce a similar effect, and at the same time, present the
scene of nature as covered with a dismal gloom. Even if creation were
arrayed in a robe of _green_, which is a more pleasant colour to the
eye--were it not diversified with the different shades it now exhibits,
every object would be equally undistinguishable.

Such would have been the aspect of creation, and the inconveniences
to which we should have been subjected, had the Creator afforded us
light without that intermixture of colours which now appears over all
nature, and which serves to discriminate one object from another. Even
our very apartments would have been tame and insipid, incapable of
the least degree of ornament, and the articles with which they are
furnished, almost undistinguishable, so that in discriminating one
object from another, we should have been as much indebted to the sense
of touch as to the sense of vision. Our friends and fellow men would
have presented no objects of interest in our daily associations. The
sparkling eye, the benignant smile, the modest blush, the blended
hues of white and vermillion in the human face, and the beauty of
the female countenance, would all have vanished, and we should have
appeared to one another as so many moving marble statues cast nearly
in the same mould. But, what would have been worst of all, the
numerous delays, uncertainties and perplexities to which we should
have been subjected, had we been under the necessity, every moment, of
distinguishing objects by trains of reasoning, and by circumstances
of time, place, and relative position? An artist, when commencing his
work in the morning, with a hundred tools of nearly the same size and
shape around him, would have spent a considerable portion of his time
before he could have selected those proper for his purpose, or the
objects to which they were to be applied; and in every department of
society, and in all our excursions from one place to another, similar
difficulties and perplexities would have occurred. The one half of our
time must thus have been employed in uncertain guesses, and perplexing
reasonings, respecting the real nature and individuality of objects,
rather than in a regular train of thinking and of employment; and after
all our perplexities and conjectures, we must have remained in the
utmost uncertainty, as to the thousands of scenes and objects, which
are now obvious to us, through the instrumentality of colours, as soon
as we open our eyes.

In short, without colour, we could have had no books nor writings:
we could neither have corresponded with our friends by letters, nor
have known any thing with certainty, of the events which happened in
former ages. No written revelation of the will of God, and of his
character, such as we now enjoy, could have been handed down to us
from remote periods and generations. The discoveries of science, and
the improvements of art, would have remained unrecorded. Universal
ignorance would have prevailed throughout the world, and the human mind
have remained in a state of demoralization and debasement. All these,
and many other inconveniences and evils would have inevitably followed,
had not God painted the rays of light with a diversity of colours,
And hence we may learn, that the most important scenes and events in
the universe, may depend upon the existence of a single principle in
nature, and even upon the most minute circumstances, which we may be
apt to overlook, in the arrangements of the material world.

In the existing state of things in the visible creation, we cannot but
admire the Wisdom and Beneficence of the Deity, in thus enabling us
to distinguish objects by so easy and expeditious a mode as _that of
colour_, which in a moment, discriminates every object and its several
relations. We rise in the morning to our respective employments,
and our food, our drink, our tools, our books, and whatever is
requisite for our comfort, are at once discriminated. Without the
least hesitation or uncertainty, and without any perplexing process
of reasoning, we can lay our hands on whatever articles we require.
Colour clothes every object with its peculiar livery, and infallibly
directs the hand in its movements, and the eye in its surveys and
contemplations. But, this is not the only end which the Divine
Being had in view, in impressing on the rays of light a diversity
of colours. It is evident, that he likewise intended to minister to
our _pleasures_, as well as to our wants. To every man of taste, and
almost to every human being, the combination of colours in flowers, the
delicate tints with which they are painted, the diversified shades of
green with which the hills and dales, the mountains and the vales are
arrayed; and that beautiful variety which appears in a bright summer
day, on all the objects of this lower creation--are sources of the
purest enjoyment and delight. It is colour, too, as well as magnitude,
that adds to the _sublimity_ of objects. Were the canopy of heaven of
one uniform hue, it would fail in producing those lofty conceptions,
and those delightful and transporting emotions, which a contemplation
of its august scenery is calculated to inspire. Colours are likewise of
considerable utility in the intercourse of general society. They serve
both for ornaments, and for distinguishing the different ranks and
conditions of the community: they add to the beauty and gracefulness
of our furniture and clothing. At a glance, they enable us at once to
distinguish the noble from the ignoble, the prince from his subjects,
the master from his servant, and the widow clothed with sable weeds
from the bride adorned with her nuptial ornaments.

Since colours, then, are of so much value and importance, they may
be reckoned as holding a rank among the noblest natural gifts of the
Creator. As they are of such essential service to the inhabitants of
our globe, there can be no doubt that they serve similar or analogous
purposes throughout all the worlds in the universe. The colours
displayed in the solar beams are common to all the globes which
compose the planetary system, and must necessarily be reflected, in
all their diversified hues, from objects on their surfaces. The light
which radiates from the fixed stars displays a similar diversity of
colours. Some of the double stars are found to emit light of different
hues;--the larger star exhibiting light of a ruddy or orange hue,
and the smaller one a radiance which approaches to blue or green.
There is therefore reason to conclude, that the objects connected
with the planets which revolve round such stars--being occasionally
enlightened by suns of different hues--will display a more variegated
and splendid scenery of colouring than is ever beheld in the world on
which we dwell; and that one of the distinguishing characteristics of
different worlds, in regard to their embellishments, may consist in
the splendour and variety of colours with which the objects on their
surfaces are adorned. In the metaphorical description of the glories
of the New Jerusalem, recorded in the Book of Revelation, one of the
chief characteristics of that city is said to consist in the splendour
and diversity of hues with which it is adorned. It is represented
as “coming down from heaven, _prepared as a bride adorned for her
husband_,” and as reflecting all the beautiful and variegated colours
which the finest gems on earth can exhibit; evidently indicating, that
splendour and variety of colouring are some of the grandest features of
celestial scenery.

On the whole, the subject of colours, when seriously considered,
is calculated to excite us to the adoration of the goodness and
intelligence of that Almighty Being whose wisdom planned all the
arrangements of the universe, and to inspire us with gratitude for the
numerous conveniences and pleasures we derive from those properties and
laws he has impressed on the material system. He might have afforded
us light, and even splendid illumination, without the pleasures
and advantages which diversified colours now produce, and man and
other animated beings might have existed in such a state. But, what
a very different scene would the world have presented from what it
now exhibits! Of how many thousands of pleasures should we have been
deprived! and to what numerous inconveniences and perplexities should
we have been subjected! The sublimity and glories of the firmament, and
the endless beauties and varieties which now embellish our terrestrial
system, would have been for ever unknown, and man could have had little
or no incitement to study and investigate the works of his Creator.
In this, as well as in many other arrangements in nature, we have a
sensible proof of the presence and agency of that Almighty Intelligence
“in whom we live, and move, and have our being.” None but an infinitely
Wise and Beneficent Being, intimately present in all places, could
thus so regularly create in us by means of colour, those exquisite
sensations which afford so much delight, and which unite us, as it
were, with every thing around us. In the diversity of hues spread
over the face of creation, we have as real a display of the Divine
presence as Moses enjoyed at the burning bush. The only difference
is, that the one was out of the common order of Divine procedure, and
the other in accordance with those permanent laws which regulate the
economy of the universe. In every colour, then, which we contemplate,
we have a sensible memorial of the presence of that Being “whose
Spirit garnished the heavens and laid the foundations of the earth,”
and whose “merciful visitation” sustains us every moment in existence.
But the revelation of God to our senses, through the various objects
of the material world, has become so familiar, that we are apt to
forget the Author of all our enjoyments, even at the moment when we
are investigating his works and participating of his benefits. “O that
men would praise Jehovah for his goodness, and for his wonderful works
towards the children of men.”




PART II.

ON TELESCOPES.




CHAPTER I.


HISTORY OF THE INVENTION OF TELESCOPES.

The telescope is an optical instrument for viewing objects at a
distance. Its name is compounded of two Greek words,--τηλε, which
signifies, _at a distance_, or _far off_, and σχοπειν, _to view_, or to
_contemplate_. By means of telescopes, remote objects are represented
as if they were near, small apparent magnitudes are enlarged, confused
objects are rendered distinct, and the invisible and obscure parts of
very distant scenes are rendered perceptible and clear to the organ
of vision. The telescope is justly considered as a grand and noble
instrument. It is not a little surprising that it should be in the
power of man to invent and construct an instrument by which objects,
too remote for the unassisted eye to distinguish, should be brought
within the range of distinct vision, as if they were only a few yards
from our eye, and that thousands of august objects in the heavens,
which had been concealed from mortals for numerous ages, should be
brought within the limits of our contemplation, and be as distinctly
perceived, as if we had been transported many millions of miles from
the space we occupy, through the celestial regions. The celebrated
Huygens remarks, in reference to this instrument, that, in his opinion,
‘the wit and industry of man has not produced any thing so noble and
so worthy of his faculties as this sort of knowledge; (namely of the
telescope) insomuch that if any particular person had been so diligent
and sagacious as to invent this instrument from the principles of
nature and geometry,--for my part, I should have thought his abilities
were more than human; but the case is so far from this, that the most
learned men have not yet been able sufficiently to explain the reason
of the effects of this casual invention.’

The persons who constructed the first telescopes, and the exact
period when they were first invented, are involved in some degree of
obscurity. It does not certainly appear that such instruments were
known to the ancients, although we ought not to be perfectly decisive
on this point. The cabinets of the curious contain some very ancient
gems, of admirable workmanship, the figures on which are so small,
that they appear beautiful through a magnifying glass, but altogether
confused and indistinct to the naked eye: and, therefore, it may
be asked, if they cannot be _viewed_, how could they _be wrought_,
without the assistance of glasses? And as some of the ancients have
declared that the moon has a form like that of the earth, and has
plains, hills, and valleys in it,--how could they know this--unless by
mere conjecture, without the use of a telescope? And how could they
have known that the _Milky Way_ is formed by the combined rays of an
infinite number of stars? For Ovid states, in reference to this zone,
‘its ground-work is of stars.’ But whatever knowledge the ancients may
have possessed of the telescope or other optical glasses, it is quite
evident that they never had telescopes of such size and power as those
which we now possess; and that no discoveries in the heavens, such as
are now brought to light, were made by any of the ancient astronomers;
otherwise some allusions to them must have been found in their writings.

Among the moderns, the illustrious Friar Bacon appears to have acquired
some rude ideas respecting the construction of telescopes. ‘Lenses and
specula’ says he, ‘may be so figured that one object may be multiplied
into many, that those which are situated at a great distance may be
made to appear very near, that those which are small may be made to
appear very large, and those which are obscure very plain; and we can
make stars to appear wherever we will.’ From these expressions, it
appears highly probable, that this philosopher was acquainted with
the general principle both of telescopes and microscopes, and that he
may have constructed telescopes of small magnifying power, for his
own observation and amusement, although they never came into general
use. He was a man of extensive learning, and made so rapid a progress
in the sciences, when attending the university of Paris, that he
was esteemed the glory of that seat of learning. He prosecuted his
favourite study of experimental philosophy with unremitting ardour; and
in this pursuit, in the course of twenty years, he expended no less
than £2000 in experiments, instruments, and in procuring scarce books.
In consequence of such extraordinary talents, and such astonishing
progress in the sciences, in that ignorant age, he was represented,
by the envy of his illiterate fraternity, as having dealings with
the devil; and, under this pretence, he was restrained from reading
lectures, and at length, in 1278, when sixty-four years of age, he was
imprisoned in his cell, where he remained in confinement for ten years.
He shone like a single bright star in a dark hemisphere--the glory of
our country--and died at Oxford, in the year 1294, in the eightieth
year of his age. ‘Friar Bacon,’ says the Rev. Mr. Jones, ‘may be
considered as the first of English philosophers; his profound skill in
mechanics, optics, astronomy, and chemistry, would make an honourable
figure in the present age. But he is entitled to further praise, as
he made all his studies subservient to theology, and directed all his
writings, as much as could be, to the glory of God. He had the highest
regard for the sacred scriptures, and was persuaded they contain the
principles of all true science.’

The next person who is supposed to have acquired a knowledge of
telescopes, was Joannes Baptista Porta, of Naples, who flourished
in the sixteenth century. He discovered the _Camera Obscura_--the
knowledge of which might naturally have led to the invention of the
telescope; but it does not appear that he ever constructed such an
instrument. Des Cartes considers James Metius, a Dutchman, as the
first constructor of a telescope, and says, that ‘as he was amusing
himself with making mirrors and burning-glasses, he casually thought
of looking through two of his lenses at a time, and found that distant
objects appeared very large and distinct.’ Others say that this great
discovery was first made by John Lippersheim, a maker of spectacles at
Middleburg, or rather by his children, who were diverting themselves
with looking through two glasses at a time, and placing them at
different distances from each other. But Borellus, who wrote a book ‘on
the invention of the telescope,’ gives this honour to Zacharias Jansen,
another spectacle-maker in the same town, who, he says, made the
first telescope in 1590. Jansen was a diligent inquirer into nature,
and, being engaged in such pursuits, he was trying what use could be
made of lenses for those purposes, when he fortunately hit upon the
construction. Having found the arrangement of glasses which produced
the effect desired, he enclosed them in a tube, and ran with his
instrument to prince Maurice, who, immediately conceiving that it might
be of use to him in his wars, desired the author to keep it a secret.
Such are the rude conceptions and selfish views of princely _warriors_,
who would apply every invention in their power for the destruction
of mankind. But the telescope was soon destined to more noble and
honourable achievements. Jansen, it is said, directed his instrument
towards celestial objects, and distinctly saw the spots on the surface
of the moon, and discovered many new stars, particularly seven pretty
considerable ones in the Great Bear. His son Joannes is said to have
noted the lucid circle near the lower limb of the moon, now named
_Tycho_, from whence several bright rays seem to dart in different
directions. In viewing Jupiter, he perceived two, sometimes three, and
at the most four small stars, a little above or below him, and thought
that they performed revolutions around him. This was, probably, the
first observation of the satellites of Jupiter, though the person who
made it was not aware of the importance of his discovery.[17]

It is not improbable that different persons about Middleburgh hit upon
the invention, in different modes, about the same time. Lippersheim
seems to have made his first rude telescope by adjusting two glasses
on a board, and supporting them on brass circles.[18] Other workmen,
particularly Metius and Jansen, in emulation of each other, seem to
have made use of that discovery, and by the new form they gave it, made
all the honour of it their own. One of them, considering the effects
of light as injurious to distinctness, placed the glasses in a tube
blackened within. The other, still more cautious, placed the same
glasses within tubes capable of sliding one in another, both to vary
the prospects, by lengthening the instrument, according to the pleasure
of the observer, and to render it portable and commodious. Thus, it
is probable that different persons had a share in the invention, and
jointly contributed to its improvement. At any rate, it is undoubtedly
to the Dutch that we owe the original invention. The first telescope
made by Jansen, did not exceed fifteen or sixteen inches in length, and
therefore its magnifying power could not have been very great.

The famous Galileo has frequently been supposed to have been the
inventor of the telescope, but he acknowledges that he had not the
honour of being the original inventor, having first learned from
a German, that such an instrument had already been made; although,
from his own account, it appears that he had actually re-invented
this instrument. The following is the account, in his own words, of
the circumstances which led him to construct a telescope. ‘Nearly
ten months ago (namely in April or May 1609) it was reported that a
certain Dutchman had made a perspective through which many distant
objects appeared distinct as if they were near: several effects of
this wonderful instrument were reported, which some believed and
others denied: but, having it confirmed to me a few days after by
a letter from the noble John Badoverie, at Paris, I applied myself
to consider the reason of it, and by what means I might contrive a
similar instrument, which I afterwards attained to by the doctrine of
refractions. And, first, I prepared a leaden tube, to whose extremities
I fitted two spectacle-glasses, both of them plain on one side, and
on the other side, one of them was spherically convex, and the other
concave. Then applying my eye to the concave, I saw objects appear
pretty large and pretty near me. They appeared three times nearer and
nine times larger in surface than to the naked eye: and soon after I
made another, which represented objects about sixty times larger, and
eight times nearer; and, at last, having spared no labour nor expense,
I made an instrument so excellent, as to show things almost a thousand
times larger, and above thirty times nearer, than to the naked eye.’
In another part of his writings, Galileo informs us that ‘he was at
Venice when he heard of Prince Maurice’s instrument, but nothing of
its construction; that the first night, after he returned to Padua,
he solved the problem, and made his instrument the next day; and soon
after, presented it to the Doge at Venice, who, to do him honour for
his grand invention, gave him the ducal letters which settled him for
life in his lectureship at Padua; and the Republic, on the twenty-fifth
of August in the same year (1610) more than tripled his salary as
professor.’

The following is the account which this philosopher gives of the
process of reasoning, which led him to the construction of a
telescope:--‘I argued in the following manner. The contrivance consists
either of one glass or more--one is not sufficient, since it must
be either convex, concave, or plane; the last does not produce any
sensible alteration in objects, the concave diminishes them; it is true
that the convex magnifies, but it renders them confused and indistinct;
consequently one glass is insufficient to produce the desired effect.
Proceeding to consider two glasses, and bearing in mind that the plane
glass causes no change, I determined that the instrument could not
consist of the combination of a plane glass with either of the other
two. I therefore applied myself to make experiments on combinations of
the two other kinds; and thus obtained that of which I was in search.’
If the true inventor is the person who makes the discovery by reasoning
and reflection, by tracing facts and principles to their consequences,
and by applying his invention to important purposes, then, Galileo may
be considered as the real inventor of the telescope. No sooner had he
constructed this instrument--before he had seen any similar one--than
he directed his tube to the celestial regions, and his unwearied
diligence and ardour were soon rewarded by a series of new and splendid
discoveries. He descried the four satellites of Jupiter, and marked the
periods of their revolutions; he discovered the phases of Venus, and
thus was enabled to adduce a new proof of the Copernican system, and
to remove an objection that had been brought against it. He traced on
the lunar orb, a resemblance to the structure of the earth, and plainly
perceived the outlines of mountains and vales, casting their shadows
over different parts of its surface. He observed, that when Mars was
in quadrature, his figure varied slightly from a perfect circle;
and that Saturn consisted of a triple body, having a small globe on
each side--which deception was owing to the imperfect power of his
telescope, which was insufficient to show him that the phenomenon was
in reality a ring. In viewing the sun, he discovered large dark spots
on the surface of that luminary, by which he ascertained that that
mighty orb performed a revolution round its axis. He brought to view
multitudes of stars imperceptible to the naked eye, and ascertained
that those nebulous appearances in the heavens which constitute the
Milky Way, consist of a vast collection of minute stars, too closely
compacted together to produce an impression on our unassisted vision.

The results of Galileo’s observations were given to the world in
a small work, entitled ‘_Nuncius Sidereus_,’ or, ‘News from the
starry regions,’ which produced an extraordinary sensation among
the learned. These discoveries soon spread throughout Europe, and
were incessantly talked of, and were the cause of much speculation
and debate among the circles of philosophers. Many doubted; many
positively refused to believe so novel and unlooked-for announcements,
because they ran counter to the philosophy of Aristotle, and all the
preconceived notions which then prevailed in the learned world. It
is curious, and may be instructive, to consider to what a length of
absurdity, ignorance and prejudice carried many of those who made
pretensions to learning and science. Some tried to reason against
the facts alleged to be discovered, others contented themselves, and
endeavoured to satisfy others, with the simple _assertion_ that such
things were not, and could not possibly be; and the manner in which
they supported themselves in their incredulity was truly ridiculous.
‘O my dear Kepler,’ says Galileo in a letter to that astronomer,
‘how I wish we could have one hearty laugh together. Here at Padua
is the principal professor of philosophy, whom I have repeatedly and
urgently requested to look at the moon and planets through my glass,
which _he pertinaciously refuses to do_, lest his opinions should be
overturned. Why are you not here? what shouts of laughter we should
have at this glorious folly! and to hear the professor of philosophy at
Pisa labouring with the Grand Duke with logical arguments, as if with
magical incantations, to charm the new planets out of the sky.’ Another
opponent of Galileo, one Christmann, says in a book he published, ‘We
are not to think that Jupiter has four satellites given him by nature,
in order, by revolving round him, to immortalize the Medici who first
had notice of the observation. These are the _dreams of idle men_,
who love ludicrous ideas better than our laborious and industrious
correction of the heavens. Nature abhors so horrible a chaos; and
to the truly wise, such vanity is detestable.’ One Martin Horky, a
would-be philosopher, declared to Kepler, ‘I will never concede his
four new planets to that Italian from Padua, _though I should die for
it_;’ and he followed up this declaration, by publishing a book against
Galileo, in which he examines four principal questions respecting the
alleged planets; 1. Whether they exist? 2. What they are? 3. What they
are like? 4. Why they are? The first question is soon disposed of by
declaring positively that he has examined the heavens with Galileo’s
own glass, and that no such thing as a satellite about Jupiter exists.
To the second, he declares solemnly that he does not more surely
know, that he has a soul in his body than that reflected rays are
the sole cause of Galileo’s erroneous observations. In regard to the
third question, he says, that these planets are like the smallest fly
compared to an elephant; and finally, concludes on the fourth, that
the only use of them is to gratify Galileo’s ‘thirst of gold,’ and to
afford himself a subject of discussion. Kepler, in a letter to Galileo,
when alluding to Horky, says, ‘He begged so hard to be forgiven, that I
have taken him again into favour upon this preliminary condition--that
I am to show him Jupiter’s satellites, AND HE IS TO SEE THEM,
and own that they are there.’

The following is a specimen of the reasoning of certain pretended
philosophers of that age against the discoveries of Galileo. Sizzi, a
Florentine astronomer, reasons in this strain: ‘There are seven windows
given to animals in the domicile of the head, through which the air
is admitted to the rest of the tabernacle of the body to enlighten,
to warm and to nourish it; two nostrils, two eyes, two ears, and a
mouth; so in the heavens, or the great world, there are two favourable
stars, two unpropitious, two luminaries, and Mercury alone undecided
and indifferent. From which and many other similar phenomena in nature,
such as the seven metals, &c., we gather that the number of planets
is _necessarily seven_. Moreover, the satellites are invisible to
the naked eye, and therefore can exert no influence on the earth, and
therefore would be useless, _and therefore do not exist_. Besides, as
well the Jews as other ancient nations have adopted the division of
the week into seven days, and have named them from the seven planets.
Now, if we increase the number of the planets, this whole system
falls to the ground.’ The opinions which then prevailed in regard to
Galileo’s observations on the moon, were such as the following:--Some
thought that the dark shades on the moon’s surface arose from the
interposition of opaque bodies floating between her and the sun, which
prevent his light from reaching those parts; others imagined that, on
account of her vicinity to the earth, she was partly tainted with the
imperfections of our terrestrial and elementary nature, and was not
of that entirely pure and refined substance of which the more remote
heavens consist: and a third party looked on her as a vast mirror, and
maintained that the dark parts of her surface were the reflected images
of our earthly forests and mountains.

Such learned nonsense is a disgrace to our species, and to the
rational faculties with which man is endowed, and exhibits, in a most
ludicrous manner, the imbecility and prejudice of those who made bold
pretensions to erudition and philosophy. The statement of such facts,
however, may be instructive, if they tend to guard us against those
prejudices and pre-conceived opinions, which prevent the mind from the
cordial reception of truth, and from the admission of improvements in
society which run counter to long-established customs. For the same
principles and prejudices, though in a different form, still operate
in society and <DW44> the improvement of the social state, the march
of science, and the progress of Christianity. How ridiculous is it for
a man, calling himself a philosopher, to be afraid to look through a
glass to an existing object in the heavens, lest it should endanger
his previous opinions! And how foolish is it to resist any improvement
or reformation in society, because it does not exactly accord with
existing opinions, and with ‘the wisdom of our ancestors.’

It is not a little surprising, that Galileo should have first hit on
that construction of a telescope which goes by his name, and which
was formed with a _concave_ glass next the eye. This construction of
a telescope is more difficult to be understood, in theory, than one
which is composed solely of convex glasses; and its field of view is
comparatively very small, so that it is almost useless when attempted
to be made of a great length. In the present day, we cannot help
wondering that Galileo and other astronomers, should have made such
discoveries as they did with such an instrument, the use of which must
have required a great degree of patience and address. Galileo’s best
telescope, which he constructed ‘with great trouble and expense,’
magnified the diameters of objects only thirty-three times; but its
length is not stated--which would depend upon the focal distance of the
concave eye-glass. If the eye-glass was two inches focus, the length
of the instrument would be five feet four inches; if it was only one
inch, the length would be two feet eight inches, which is the least we
can allow to it--the object-glass being thirty-three inches focus, and
the eye-glass placed an inch within this focus. With this telescope,
Galileo discovered the satellites of Jupiter, the crescent of Venus,
and the other celestial objects to which we have already alluded.
The telescopes made in Holland, are supposed to have been constructed
solely of _convex_ glasses, on the principle of the astronomical
telescope; and, if so, Galileo’s telescope was in reality a new
invention.

Certain other claimants of the invention of the telescope, have
appeared, besides those already mentioned. Francis Fontana, in his
‘celestial observations,’ says, that he was assured by a Mr. Hardy,
advocate of the parliament of Paris, a person of great learning and
undoubted integrity, that on the death of his father, there was found
among his things an old tube, by which distant objects were distinctly
seen, and that it was of a date long prior to the telescope lately
invented, and had been kept by him as a secret. Mr. Leonard Digges,
a gentleman who lived near Bristol, in the seventeenth century, and
was possessed of great and various knowledge, positively asserts in
his ‘_Stratoticos_,’ and in another work, that his father, a military
gentleman, had an instrument which he used in the field, by which he
could bring distant objects near, and could know a man at the distance
of three miles. Mr. Thomas Digges, in the preface to his ‘Pantometria,’
published in 1591, declares, “My father, by his continual painful
practices, assisted by demonstrations mathematical, was able, and
sundry times hath by proportional glasses, duly situate in convenient
angles, not only discovered things far off, read letters, numbered
pieces of money, with the very coin and superscription thereof, cast by
some of his friends of purpose, upon downs in open fields, but also,
seven miles off, declared what hath been done that instant, in private
places. He hath also, sundry times, by the sun-beams, fired powder
and discharged ordnance half a mile and more distant, and many other
matters far more strange and rare, of which there are yet living divers
witnesses.”

It is by no means unlikely, that persons accustomed to reflection,
and imbued with a certain degree of curiosity, when handling
spectacle-glasses, and amusing themselves with their magnifying powers
and other properties, might sometimes hit upon the construction
of a telescope; as it only requires two lenses of different focal
distances to be held at a certain distance from each other, in order
to show distant objects magnified. Nay, even one lens, of a long
focal distance, is sufficient to constitute a telescope of a moderate
magnifying power, as I shall show in the sequel. But such instruments,
when they happened to be constructed accidentally, appear to have been
kept as secrets, and confined to the cabinets of the curious, so that
they never came into general use; and as their magnifying power would
probably be comparatively small, the appearance of the heavenly bodies
would not be much enlarged by such instruments--nor is it likely that
they would be often directed to the heavens. On the whole, therefore,
we may conclude that the period when instruments of this description
came into general use, and were applied to useful purposes, was when
Galileo constructed his first telescopes.




CHAPTER II.


OF THE CAMERA OBSCURA.

Before proceeding to a particular description of the different kinds
of telescopes, I shall first give a brief description of the Camera
Obscura, as the phenomena exhibited by this instrument tend to
illustrate the principle of a refracting telescope.

The term _Camera Obscura_ literally signifies a darkened vault or roof;
and hence it came to denote a chamber, or box, or any other place
made dark for the purpose of optical experiments. The camera obscura,
though a simple, is yet a very curious and noble contrivance; as it
naturally and clearly explains the manner in which vision is performed,
and the principle of the telescope, and entertains the spectator with
a most exquisite picture of surrounding objects, painted in the most
accurate proportions and colours by the hand of nature. The manner of
exhibiting the pictures of objects in a dark room is as follows:--In
one of the window-shutters of a room which commands a good prospect
of objects not very distant, a circular hole should be cut of four or
five inches diameter. In this hole an instrument should be placed,
called a _Scioptric ball_, which has three parts, a frame, a ball, and
a lens. The ball has a circular hole cut through the middle, in which
the lens is fixed, and its use is, to turn every way so as to take in
a view of objects on every side. The chamber should be made perfectly
dark; and a white screen, or a large sheet of elephant paper, should be
placed opposite to the lens, and in its focus, to receive the image.
If then, the objects without be strongly enlightened by the sun, there
will be a beautiful living picture of the scene delineated on the white
screen, where every object is beheld in its proportions, and with its
colours even more vivid than life; green objects appear in the picture
more intensely green, and yellow, blue, red or white flowers appear
much more beautiful in the picture than in nature; if the lens be a
good one, and the room perfectly dark, the perspective is seen in
perfection. The lights and shadows are not only perfectly just, but
also greatly heightened; and, what is peculiar to this delineation,
and which no other picture or painting can exhibit--the _motions_ of
all the objects are exactly expressed in the picture; the boughs of
the trees wave, the leaves quiver, the smoke ascends in a waving form,
the people walk, the children at their sports leap and run, the horse
and cart move along, the ships sail, the clouds soar and shift their
aspects, and all as natural as in the real objects; the motions being
somewhat quicker, as they are performed in a more contracted scene.

These are the _inimitable_ perfections of a picture, drawn by the rays
of light as the only pencil in nature’s hand, and which are finished
in a moment; for no sensible interval elapses before the painting is
completed, when the ground on which it is painted is prepared and
adjusted. In comparison of such a picture, the finest productions of
the most celebrated artists, the proportions of Raphael, the natural
tints and colouring of Titian, and the shadowing of the Venetians,
are but coarse and sorry daubings, when set in competition with what
nature can exhibit by the rays of light passing through a single lens.
The Camera obscura is at the same time the painter’s assistant, and
the painter’s reproach. From the picture it forms he receives his best
instructions, and is shown what he should endeavour to attain; and
hence, too, he learns the imperfections of his art, and what it is
impossible for him to imitate. As a proof of this, the picture formed
in the dark chamber will bear to be magnified to a great extent,
without defacing its beauty, or injuring the fineness of its parts; but
the finest painted landscape, if viewed through a high magnifier will
appear only as a coarse daubing.

The following scheme will illustrate what has been now stated
respecting the dark chamber. EF represents a darkened room, in the side
of which, IK, is made the circular hole V, in which, on the inside,
is fixed the scioptric ball. At some considerable distance from this
hole is exhibited a landscape of houses, trees, and other objects,
ABCD, which are opposite to the window. The rays which flow from the
different objects which compose this landscape, to the lens at V, and
which pass through it, are converged to their respective foci, on the
opposite wall of the chamber HG or on a white moveable screen placed
in the focus of the lens, where they all combine to paint a lively and
beautiful picture of the range of objects directly opposite, and on
each side, so far as the lens can take in.

Though I have said, that a scioptric ball and socket are expedient to
be used in the above experiment, yet where such an instrument is not
at hand, the lens may be placed in a short tube made of pasteboard or
any other material, and fixed in the hole made in the window shutter.
The only imperfection attending this method is, that the lens can
exhibit those objects only which lie directly opposite the window.

[Illustration: _figure 37._]

Some may be disposed to consider it as an imperfection in this picture,
that all the objects appear in an _inverted_ position; as they must
necessarily do, according to what we formerly stated respecting the
properties of convex lenses, (p. 72). There are, however, different
modes of viewing the picture as if it were erect. For, if we stand
before the picture, and hold a common mirror against our breast at
an acute angle with the picture, and look down upon it, we shall see
all the images of the objects as if restored to their erect position;
and by the reflection of the mirror, the picture will receive such a
lustre as will make it still more delightful. Or, if a large concave
mirror were placed before the picture at such a distance, that its
image may appear before the mirror, it will then appear erect and
pendulous in the air in the front of the mirror. Or, if the image be
received on a frame of paper, we may stand behind the frame, with our
face towards the window, and look down upon the objects, when they will
appear as if erect.

The experiment of the Camera Obscura may serve to explain and
illustrate the nature of a common refracting telescope. Let us suppose,
that the lens in the window-shutter represents the object-glass of a
refracting telescope. This glass forms an image in its focus, which
is in every respect an exact picture or representation of the objects
before it; and consequently the same idea is formed in the mind, of the
nature, form, magnitude, and colour of the object--whether the eye at
the centre of the glass views the object itself, or the image formed in
its focus. For, as formerly stated, the object and its image are both
seen under the same angles by the eye placed at the centre of the lens.
Without such an image as is formed in the camera obscura--depicted
either in the tube of a telescope or in the eye itself--no telescope
could possibly be formed. If we now suppose that, behind the image
formed in the dark chamber, we apply a convex lens of a short focal
distance to view that image, then the image will be seen distinctly,
in the same manner as we view common objects, such as a leaf or a
flower, with a magnifying glass; consequently, the object itself will
be seen distinct and magnified. And, as the same image is nearer to one
lens than the other, it will subtend a larger angle at the nearest
lens, and of course, will appear larger than through the other, and
consequently the object will be seen magnified in proportion. For
example, let us suppose the lens in the camera obscura, or the object
lens of a telescope, to be five feet, or sixty inches focal distance,
at this distance from the glass, an image of the distant objects
opposite to it will be formed. If now, we place a small lens two inches
focal distance beyond this point, or five feet two inches from the
object-glass, the objects, when viewed through the small lens, will
appear considerably magnified, and apparently much nearer than to the
naked eye. The degree of magnifying power is in proportion to the
focal distances of the two glasses; that is, in the present case, in
the proportion of two inches, the focus of the small lens, to sixty
inches, the focus of the object lens. Divide sixty by two, the quotient
is thirty, which gives the magnifying power of such a telescope, that
is, it represents objects thirty times nearer, or under an angle thirty
times larger than to the naked eye. If the eye-glass, instead of being
two inches, were only one and a half inch focus, the magnifying power
would be in the proportion of one and a half to sixty, or forty times.
If the eye-glass were three inches focus, the magnifying power would
be twenty times; and so on, with regard to other proportions. In all
cases, where a telescope is composed of only two convex lenses, the
magnifying power is determined, _by dividing the focal distance of the
object-glass, by the focal distance of the eye-glass_, and the quotient
expresses the number of times the object is magnified, in length
and breadth. This and various other particulars, will be more fully
illustrated in the sequel.

In performing experiments with the camera obscura in a darkened
chamber, it is requisite that the following particulars be attended
to:--1. That the lens be well figured, and free from any veins or
blemishes that might distort the picture. 2. That it be placed
_directly against_ the object whose image we wish to see distinctly
delineated. 3. The lens should be of a proper size both as to its
breadth and focal distance. It should not be less than three or four
feet focal distance, otherwise the picture will be too small, and the
parts of objects too minute to be distinctly perceived; nor should
it exceed fifteen or eighteen feet, as in this case the picture will
be faint, and of course not so pleasing. The best medium as to focal
distance, is from five to eight or ten feet. The aperture, too, or
breadth of the glass, should not be too small, otherwise the image
will be obscure, and the minute parts of it invisible for want of a
sufficient quantity of light. A lens of six feet focal distance, for
example, will require an aperture of at least two inches. Lenses of a
shorter focal distance require less apertures, and those of a longer
focal distance larger. But if the aperture be too large, the image
will be confused, and indistinct, by the admission of too much light.
4. We should never attempt to exhibit the images of objects, unless
when the sun is shining and strongly illuminating the objects, except
in the case of very near objects placed in a good light. As one of the
greatest beauties, in the phenomena of the dark chamber, consists in
the exquisite appearance and contrast of light and shadows, nothing of
this kind can be perceived but from objects directly illuminated by the
sun. 5. A south window should never be used in the forenoon, as the sun
cannot then enlighten the north side of an object; and besides, his
rays would be apt to shine upon the lens, which would make the picture
appear with a confused lustre. An east window is best in the afternoon,
and a western in the morning; but a north window is in most cases to
be preferred, especially in the forenoon, when the sun is shining with
his greatest strength and splendour. In general, that window ought to
be used which looks to the quarter opposite to that in which the sun is
shining.

The picture should be received upon a very white surface, as the finest
and whitest paper, or a painted cloth, bordered with black; as white
bodies reflect most copiously the incident rays, while black surfaces
absorb them. If the screen could be bent into the concave segment of
a sphere, of which the focal distance of the double convex lens which
is used, is the radius, the parts of the picture adjacent to the
extremities would appear most distinct. Sir D. Brewster informs us
that, having tried a number of white substances of different degrees
of smoothness, and several metallic surfaces, on which to receive the
image, he happened to receive the picture on the silvered back of a
looking-glass, and was surprised at the brilliancy and distinctness
with which external objects were represented. To remove the spherical
protuberances of the tin foil, he ground the surface very carefully
with a bed of hones which he had used for working the plane specula of
Newtonian telescopes. By this operation, which may be performed without
injuring the other side of the mirror, he obtained a surface finely
adapted for the reception of images. The minute parts of the landscape
were formed with so much precision, and the brilliancy of colouring
was so uncommonly fine, as to equal, if not exceed the images that are
formed in the air by means of concave specula.

The following additional circumstances may be stated respecting the
phenomena exhibited in the dark chamber. A more critical idea may
be formed of any _movement_ in the picture here presented than from
observing the motion of the object itself. For instance, a man walking
in a picture appears to have an undulating motion, or to rise up and
down every step he takes, and the hands seem to move almost exactly
like a pendulum; whereas scarcely any thing of this kind is observed
in the man himself, as viewed by the naked eye. Again, if an object be
placed just twice the focal distance from the lens without the room,
the image will be formed at the same distance from the lens within the
room, and consequently will be equal in magnitude to the object itself.
The recognition of this principle may be of use to those concerned in
drawing, and who may wish, at any time, to form a picture of the exact
size of the object. If the object be placed further from the lens than
twice its focal length, the image will be less than the object. If it
be placed nearer, the image will be greater than the life. In regard to
immoveable objects, such as houses, gardens, trees, &c., we may form
the images of so many different sizes, by means of different lenses,
the shorter focus making the lesser picture, and the longer focal
distance the largest.

The experiments with the camera obscura, may likewise serve to
illustrate the nature of vision, and the functions of the human eye.
The frame or socket of the scioptric ball may represent the _orbit_
of the natural eye. The ball, which turns every way, resembles the
_globe_ of the eye, moveable in its orbit. The hole in the ball may
represent the _pupil_ of the eye; the convex lens corresponds to the
_crystalline humour_, which is shaped like a lens, and contributes
to form the images of objects on the inner part of the eye. The dark
chamber itself, is somewhat similar to the _internal part of the eye_,
which is lined all around, and under the retina, with a membrane,
over which is spread a mucous of a very black colour. The white wall
or frame of white paper to receive the picture of objects, is a fair
representation of the _retina_ of the eye, on which all the images of
external objects are depicted. Such are some of the general points of
resemblance between the apparatus connected with the dark chamber, and
the organ of vision; but the human eye is an organ of such exquisite
construction, and composed of such a number and variety of delicate
parts, that it cannot be adequately represented by any artificial
contrivance.

The darkened chamber is frequently exhibited in a manner somewhat
different from what we have above described, as in the following
scheme, (fig. 38) which is termed the _revolving camera obscura_. In
this construction, KH represents a plane mirror or metallic reflector,
placed at half a right angle to the convex lens HI, by which, rays
proceeding from objects situated in the direction O are reflected to
the lens, which forms an image of the objects on a round white table
at T, around which several spectators may stand, and view the picture,
as delineated on a horizontal plane. The reflector, along with its
case, is capable of being turned round, by means of a simple apparatus
connected with it, so as to take in, in succession, all the objects
which compose the surrounding scene. But as the image here is received
on a flat surface, the rays _fm_, _en_, will have to diverge farther
than the central rays _dc_; and hence the representation of the
object, near the sides, will be somewhat distorted; to remedy which,
the image should be received on a concave surface, as _ab_ or PS. This
is the general plan of those Camera Obscuras, fitted up in large wooden
tents, which are frequently exhibited in our large cities, and removed
occasionally from one town to another. Were an instrument of this
kind fitted up on a _small scale_, a hole might be made in one of the
sides, as at E, where the eye could be applied to view the picture. The
focal distances of the lenses used in large instruments of this kind,
are generally from eight to twelve feet, in which case they produce
a telescopic effect upon distant objects, so as to make them appear
nearer than when viewed with the naked eye.

[Illustration: _figure 38._]

[Illustration: _figure 39._]

The camera obscura is frequently constructed in a _portable form_,
so as to be carried about for the purpose of delineating landscapes.
The following is a brief description of the instrument in this form.
AC is a convex lens placed near the end of a tube or drawer, which is
moveable in the side of a square box, within which is a plane mirror
DE, reclining backward in an angle of forty-five degrees from the
perpendicular _pn_. The pencils of rays flowing from the object OB,
and passing through the convex lens--instead of proceeding forward and
forming the image HI, are reflected upward by the mirror, and meet in
points as FG, at the same distance at which they would have met at H
and I, if they had not been intercepted by the mirror. At FG, the image
of the object OB is received either on a piece of oiled paper, or more
frequently on a plane _unpolished_ glass, placed in the horizontal
situation FG, which receives the images of all objects, opposite to the
lens, and on which, or on an oiled paper placed upon it, their outlines
may be traced by a pencil. The moveable tube on which the lens is
fixed, serves to adjust the focus for near and distant objects, till
their images appear distinctly painted on the horizontal glass at FG.
Above is shown the most common form of the box of this kind of Camera
Obscura. A is the position of the lens, BC, the position of the mirror,
D, the plane unpolished glass on which the images are depicted, GH a
moveable top or screen to prevent the light from injuring the picture,
and EF, the moveable tube.

[Illustration: _figure 40._]

_The Daguerreotype._--An important, and somewhat surprising discovery
has lately been made, in relation to the picture formed by the Camera
Obscura. It is found, that the images formed by this instrument are
capable of being indelibly fixed on certain surfaces previously
prepared for the purpose, so that the picture is rendered permanent.
When a Camera is presented to any object or landscape strongly
illuminated by the sun, and the prepared ground for receiving the
image is adjusted, and a certain time allowed to elapse till the
rays of light produce their due effect, in a few minutes or even
seconds, a picture of the objects opposite to the lens is indelibly
impressed upon the prepared plate, in all the accurate proportions
and perspective, which distinguish the images formed in a dark
chamber--which representations may be hung up in apartments, along
with other paintings and engravings; and will likely retain their
beauty and lustre for many years. These are pictures of nature’s own
workmanship finished in an extremely short space of time, and with the
most exquisite delicacy and accuracy. The effect is evidently owing to
certain chemical properties in the rays of light; and opens a new field
for experiment and investigation to the philosopher. The only defect
in the picture is, that it is not ; but, in the progress of
experiments on this subject, it is not unlikely that even this object
may be accomplished, in which case, we should be able to obtain the
most accurate landscapes and representations of all objects, which
can possibly be formed. This art or discovery goes by the name of the
_Daguerreotype_ from M. Daguerre, a Frenchman, who is supposed to have
been the first discoverer, and who received a large premium from the
French government for disclosing the process, and making the discovery
public. Several improvements and modifications, in reference to the
preparation of the plates, have been made since the discovery was first
announced, about the beginning of 1839; and the pictures formed on this
principle, are frequently distinguished by the name of _Photogenic_
drawings; and are now exhibited at most of our public scientific
institutions.

This new science or art, has been distinguished by different names.
It was first called _Photography_, from two Greek words, signifying
_writing by light_: it was afterwards called the art of _Photogenic
Drawing_, or drawing produced by light. M. Daguerre gave it the name
of _Heliography_, or _writing by the sun_, all which appellatives
are derived from the Greek, and are expressive, in some degree, of
the nature of the process. We shall, however, make use of the term
Daguerreotype, derived from the name of the inventor.

As it does not fall within our plan to give any minute descriptions of
the Daguerreotype process, we shall just give a few general hints in
reference to it, referring those who wish for particular details, to
the separate treatises which have been published respecting it. The
first thing necessary to be attended to in this art is, the preparation
of the plate on which the drawing is to be made. The plate consists
of a thin leaf of copper, plated with silver; both metals together,
not being thicker than a card. The object of the copper is simply to
support the silver, which must be the purest that can be procured. But
though the copper should be no thicker than to serve the purpose of
support, it is necessary that it should be so thick as to prevent the
plate from being warped, which would produce a distortion of the images
traced upon it. This plate must be polished;--and for this purpose, the
following articles are required--a phial of olive oil--some very fine
cotton--pumice-powder, ground till it is almost impalpable, and tied up
in a piece of fine muslin, thin enough to let the powder pass through
without touching the plate when the bag is shaken. A little nitric acid
diluted with sixteen times, by measure, its own quantity of water--a
frame of wire on which to place the plate, when being heated--a spirit
lamp to make the plate hot--a small box with inclined sides within, and
having a lid to shut it up close--and a square board large enough to
hold the drawing, and having catches at the side to keep it steady.

To the above prerequisites, a good _Camera Obscura_ is, of course,
essentially necessary. This instrument should be large enough to admit
the plate of the largest drawing intended to be taken. The lens which
forms the image of the object, should, if possible, be _achromatic_,
and of a considerable diameter. In an excellent instrument of this
description, now before me, the lens is an achromatic, about 3 inches
diameter, but capable of being contracted to a smaller aperture. Its
focal distance is about 17 inches; and the box, exclusive of the tube
which contains the lens, is 15 inches long, 13-1/2 inches broad, and
11 inches deep. It forms a beautiful and well-defined picture of every
well-enlightened object to which it is directed.

Before the plate is placed in the camera, there are certain operations
to be performed. 1. The surface of the plate should be made perfectly
smooth, or highly polished. For this purpose, it must be laid flat,
with the silver side upwards, upon several folds of paper for a
bedding; and having been well polished in the usual way, the surface
must be powdered equally and carefully with fine pumice enclosed in
the muslin bag. Then taking a little cotton wool, dipped in olive
oil, it must be rubbed over the plate with rounding strokes, and then
crossing them by others which commence at right angles with the first.
This process must be repeated frequently, changing the cotton, and
renewing the pumice powder every time. A small portion of cotton must
now be moistened with the diluted nitric acid, and applied equally
to the whole surface. The next thing to be done is to make the plate
thoroughly and equally hot, by holding the plate with a pair of
pincers, by the corner, over a charcoal fire, and when the plate is
sufficiently hot, a white coating will be observed on the silver, which
indicates that that part of the operation is finished. An even cold
surface is next wanted, such as a metallic plate cooled almost to the
freezing point by muriate of soda, and to this the heated plate must be
suddenly transferred.

2. The next operation is to give the plate a coating of _Iodine_. This
is accomplished by fixing the plate upon a board, and then putting it
into a box containing a little dish with iodine divided into small
pieces, with its face downward, and supported with small brackets at
the corners. In this position, the plate must remain till it assume
_a full gold colour_, through the condensation of the iodine on its
surface--which process should be conducted in a darkened apartment.
The requisite time for the condensation of the iodine varies from
five minutes to half an hour. When this process is satisfactorily
accomplished, the plate should be immediately fixed in a frame with
catches and bands, and placed in the Camera; and the transference from
one receptacle to another should be made as quickly as possible, and
with only so much light as will enable the operator to see what he is
doing.

3. The next operation is to obtain the drawing. Having placed the
Camera in front of the scene to be represented, and the lens being
adjusted to the proper focus, the ground-glass of the Camera is
withdrawn, and the prepared plate is substituted for it; and the
whole is left till the natural images are drawn by the natural light
from the object. The time necessary to leave the plate for a complete
delineation of the objects, depends upon the intensity of the light.
Objects in the shade will require more time for their delineation
than those in the broad light. The full clear light of the south of
Europe, Spain, Italy, and particularly, the more glowing brilliancy
of tropical countries, will effect the object much more speedily than
the duller luminosity of a northern clime. Some hours of the day are
likewise more favourable than others. Daguerre states, that ‘the most
favourable, is from 7 A.M. to 3 o’clock P.M., and
that a drawing could be effected in Paris in 3 or 4 minutes, in June
and July, which would require 5 or 6, in May and August, and 7 or 8 in
April and September.’ In the progress of this art, at the present time,
portraits and other objects are frequently delineated in the course of
a few seconds.

4. Immediately after removing the plate from the Camera, it is next
placed over the vapour of mercury, which is placed in a cup at the
bottom of a box, and a spirit lamp applied to its bottom, till the
temperature rise to 140 of Fahrenheit. This process is intended to
bring out the image, which is not visible when withdrawn from the
Camera; but in the course of a few minutes a faint tracery will
begin to appear, and in a very short time the figure will be clearly
developed.

5. The next operation is _to fix the impression_. In order to this, the
coating on which the design was impressed must be removed, to preserve
it from being decomposed by the rays of light. For this purpose, the
plate is placed in a trough containing common water, plunging, and
withdrawing it immediately, and then plunging it into a solution of
salt and water, till the yellow coating has disappeared.

Such is a very brief sketch of the _photogenic_ processes of Daguerre.
Other substances, however, more easily prepared, have been recommended
by Mr. Talbot, F.R.S., who appears, about the same time, to have
invented a process somewhat similar to that of Daguerre. The following
are his directions for the preparation of _Photogenic Paper_.

The paper is to be dipped into a solution of salt in water, in the
proportion of half an ounce of salt to half a pint of water. Let
the superfluous moisture drain off, and then, laying the paper upon
a clean cloth, _dab_ it gently with a napkin, so as to prevent the
salt collecting in one spot more than another. The paper is then to
be pinned down by two of its corners on a drawing board, by means
of common pins, and one side washed or wetted with the Photogenic
fluid, using the brush prepared for that purpose, and taking care to
distribute it equally. Next dry the paper as rapidly as you can at
the fire, and it will be fit for use for most purposes. If, when the
paper is exposed to the sun’s rays, it should assume an irregular tint,
a very thin extra wash of the fluid will render the colour uniform,
and at the same time somewhat darker. Should it be required to make a
more sensitive description of paper, after the first application of
the fluid, the solution of salt should be applied, and the paper dried
at the fire. Apply a second wash of the fluid, and dry it at the fire
again: employ the salt a third time, dry it,--and one application more
of the fluid will, when dried, have made the paper extremely sensitive.
When slips of such papers, differently prepared, are exposed to the
action of day light, those which are soonest affected by the light, by
becoming dark, are the best prepared.

When photogenic drawings are finished in a perfect way, the designs
then taken on the plate or paper are exceedingly beautiful and
correct, and will bear to be inspected with a considerable magnifying
power, so that the most minute portions of the objects delineated may
be distinctly perceived. We have seen portraits, finished in this way
by a London artist, with an accuracy which the best miniature painter
could never attempt--every feature being so distinct, as to bear being
viewed with a deep magnifier. And in landscapes and buildings, such
is the delicacy and accuracy of such representations, that the marks
of the chisel and the crevices in the stones may frequently be seen
by applying a magnifying lens to the picture; so that we may justly
exclaim, in the words of the Poet: ‘Who can paint like nature!.’ That
LIGHT--which is the first-born of Deity, which pervades all
space, and illuminates all worlds--in the twinkling of an eye, and with
an accuracy which no art can imitate, depicts every object in its exact
form and proportions, superior to every thing that human genius can
produce.

The Photogenic art, in its progress, will doubtless be productive of
many highly interesting and beneficial effects. It affords us the power
of representing, by an accurate and rapid process, all the grand and
beautiful objects connected with our globe--the landscapes peculiar
to every country--the lofty ranges of mountains which distinguish
Alpine regions--the noble edifices which art has reared--the monumental
remains of antiquity--and every other object which it would be
interesting for human beings to contemplate; so that in the course
of time, the general scenery of our world, in its prominent parts,
might be exhibited to almost every eye. The commission of the French
Chambers, when referring to this art, has the following remark, ‘To
copy the millions upon millions of hieroglyphics which cover even
the exterior of the great monuments of Thebes and Memphis, of Carnac,
&c., would require scores of years and legions of designers. By the
assistance of the Daguerreotype, a single man could finish that immense
work.’--This instrument lays down objects, which the visual organs
of man would overlook, or might be unable to perceive, with the same
minuteness and nicety, that it delineates the most prominent features
of a landscape. The time-stained excrescences on a tree, the blades
of grass, the leaf of a rose, the neglected weed, the moss on the
summit of a lofty tower, and similar objects, are traced with the same
accuracy as the larger objects in the surrounding scene.

It is not improbable, likewise, that this art (still in its
infancy) when it approximates to perfection, may enable us to take
representations of the sublime objects in the heavens. The sun affords
sufficient light for this purpose; and there appears no insurmountable
obstacle in taking, in this way, a highly magnified picture of that
luminary, which shall be capable of being again magnified by a powerful
microscope. It is by no means improbable, from experiments that have
hitherto been made, that we may obtain an accurate delineation of
the lunar world from the moon herself. The plated disks prepared by
Daguerre receive impressions from the action of the lunar rays to such
an extent as permits the hope that photographic charts of the moon
may soon be obtained; and, if so, they will excel in accuracy all the
delineations of this orb that have hitherto been obtained; and if they
should bear a microscopic power, objects may be perceived on the lunar
surface which have hitherto been invisible. Nor is it impossible that
the planets Venus, Mars, Jupiter and Saturn, may be delineated in
this way, and objects discovered which cannot be descried by means of
the telescope. It might perhaps be considered as beyond the bounds of
probability to expect that even distant _Nebulæ_, might thus be fixed,
and a delineation of their objects produced which shall be capable of
being magnified by microscopes. But we ought to consider that the art
is yet only in its infancy--that plates of a more delicate nature than
those hitherto used, may yet be prepared, and that other properties of
light may yet be discovered, which shall facilitate such designs. For,
we ought now to set no boundaries to the discoveries of science, and to
the practical applications of scientific discovery which genius and art
may accomplish.

In short, this invention leads to the conclusion, that we have
not yet discovered all the wonderful properties of that Luminous
Agent which pervades the universe, and which unveils to us its
beauties and sublimities--and that thousands of admirable objects
and agencies may yet be disclosed to our view through the medium of
light, as philosophical investigators advance in their researches
and discoveries. In the present instance, as well as in many others,
it evidently appears, that the Creator intends, in the course of his
providence, by means of scientific researches, gradually to open to the
view of the inhabitants of our world the wonders, the beauties and the
sublimities of his vast creation, to manifest his infinite wisdom, and
his superabundant goodness, and to raise our souls to the contemplation
and the love of Him who is the original source of all that is glorious
and beneficent in the scene of nature.




CHAPTER III.


ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS.

In order to understand the principle on which telescopes represent
distant objects as magnified, it may be expedient to explain what is
meant by the angle of vision, and the apparent magnitudes under which
different objects appear, and the same object, when placed at different
distances.

[Illustration: _figure 40_*.]

The optical angle is the angle contained under two right lines drawn
from the extreme points of an object to the eye. Thus AEB or CED (fig.
40.) is the optical or visual angle, or the angle under which the
object AB or CD, appears to the eye at E. These two objects, being at
different distances, are seen under the same angle, although CD is
evidently larger than AB. On the retina of the eye, their images are
exactly of the same size, and so is the still larger object FG.

[Illustration: _figure 41._]

The _apparent magnitude_ of objects denotes their magnitude as they
appear to us, in contradistinction from their real or true magnitude,
and it is measured by the visual angle; for whatever objects are seen
under the same or equal angles _appear_ equal, however different their
real magnitudes. If a half-crown or half-dollar be placed at about 120
yards from the eye, it is just perceptible as a visible point, and its
apparent magnitude, or the angle under which it is seen, is very small.
At the distance of thirty or forty yards, its bulk appears sensibly
increased, and we perceive it to be a round body; at the distance
of six or eight yards, we can see the king or queen’s head engraved
upon it; and at the distance of eight or ten inches from the eye it
will appear so large, that it will seem to cover a large building
placed within the distance of a quarter of a mile, in other words, the
apparent magnitude of the half-crown held at such a distance, will more
than equal that of such a building, in the picture on the retina, owing
to the increase of the optical angle. If we suppose A (fig. 41.) to
represent the apparent size of the half-crown at nine yards distance,
then we say it is seen under the small angle FED. B will represent its
apparent magnitude at 4-1/2 yards distant under the angle HEG, and the
circle C, its apparent magnitude at 3 yards distant, under the large
angle KEI.

[Illustration: _figure 42._]

This may be otherwise illustrated by the following figure. Let AB (fig.
42.) be an object viewed directly by the eye QR. From each extremity A
and B draw the lines AN,BM, intersecting each other in the crystalline
humour in I: then is AIB the optical angle which is the measure of the
apparent magnitude or length of the object AB. From an inspection of
this figure, it will evidently appear that the apparent magnitudes of
objects will vary according to their distances. Thus AB, CD, EF, the
real magnitudes of which are unequal, may be situated at such distances
from the eye, as to have their _apparent_ magnitudes all equal, and
occupying the same space on the retina MN, as here represented. In like
manner, objects of equal magnitude, placed at unequal distances, will
appear unequal. The objects AB and GH which are equal, being situated
at different distances from the eye, GH will appear under the large
angle TIV, or as large as an object TV, situated at the same place as
the object AB, while AB appears under the smaller angle AIB. Therefore
the object GH is _apparently_ greater than the object AB, though it is
only equal to it. Hence it appears that we have no certain standard of
the _true magnitude_ of objects, by our visual perception abstractly
considered, but only of the _proportions_ of magnitude.

In reference to apparent magnitudes, we scarcely ever judge any object
to be so great or so small as it appears to be, or that there is so
great a disparity in the visible magnitude of two equal bodies at
different distances from the eye. Thus, for example, suppose two men,
each six feet 3 inches high, to stand directly before us, one at the
distance of a pole, or 5-1/2 yards, and the other at the distance of
100 poles, or 550 yards--we should observe a considerable difference in
their apparent size, but we should scarcely suppose, at first sight,
that the one nearest the eye appeared a hundred times greater than the
other, or that, while the nearest one appeared 6 feet 3 inches high,
the remote one appeared only about _three fourths of an inch_. Yet
such is in reality the case; and not only so, but the visible bulk
or area of the one is to that of the other, as the square of these
numbers, namely as 10,000 to 1; the man nearest us presenting to the
eye a magnitude or surface ten thousand times greater than that of the
other. Again, suppose two chairs standing in a large room, the one
21 feet distance from us, and the other 3 feet--the one nearest us
will appear 7 times larger both in length and breadth, than the more
distant one, and consequently, its visible area 49 times greater. If
I hold up my finger at 9 inches distant from my eye, it seems to cover
a large town a mile and a half in extent, situated at 3 miles distant;
consequently, the apparent magnitude of my finger, at 9 inches distant
from the organ of vision, is greater than that of the large town at 3
miles distance, and forms a larger picture on the retina of the eye.
When I stand at the distance of a foot from my window, and look through
one of the panes to a village less than a quarter of a mile distant,
I see, through that pane, nearly the whole extent of the village,
comprehending two or three hundred houses; consequently, the apparent
magnitude of the pane is equal to nearly the extent of the village,
and all the buildings it contains do not appear larger than the pane
of glass in the window, otherwise, the houses and other objects which
compose the village could not be seen through that single pane. For, if
we suppose a line drawn from one end of the village, passing through
the one side of the pane, and another line drawn from the other end,
and passing through the other side of the pane to the eye, these lines
would form the optical angle under which the pane of glass and the
village appears. If the pane of glass be fourteen inches broad, and
the length of the village 2640 yards, or half a mile--this last lineal
extent is 6,788 times greater than the other, and yet they have the
same _apparent_ magnitude in the case supposed.

Hence we may learn the absurdity and futility of attempting to
describe the extent of spaces in the heavens, by saying, that a
certain phenomenon was two or three feet or yards distant from
another, or that the tail of a comet appeared several yards in length.
Such representations can convey no definite ideas in relation to
such magnitudes, unless it be specified at what distance from the
eye, the foot or yard is supposed to be placed. If a rod, a yard
in length, be held at nine inches from the eye, it will subtend an
angle, or cover a space in the heavens, equal to more than one fourth
of the circumference of the sky, or about one hundred degrees. If
it be eighteen inches from the eye, it will cover a space equal to
fifty degrees; if at three feet, twenty-five degrees, and so on in
proportion to the distance from the eye; so that we can form no correct
conceptions of apparent spaces or distances in the heavens, when we
are merely told that two stars, for example, appear to be three yards
distant from each other. The only definite measure we can use, in such
cases, is that of degrees. The sun and moon are about half a degree
in apparent diameter, and the distance between the extreme stars in
_Orion’s belt_, three degrees, which measures being made familiar
to the eye, may be applied to other spaces of the heavens, and an
approximate idea conveyed of the relative distances of objects in the
sky.

From what has been stated above, it is evident that the magnitude
of objects may be considered in different points of view. The true
dimensions of an object, considered in itself, give what is called its
_real_ or _absolute magnitude_; and the opening of the visual angle
determines the _apparent magnitude_. The real magnitude, therefore, is
a constant quantity; but the apparent magnitude varies continually with
the distance, real or imaginary; and therefore, if we always judged
of the dimensions of an object from its apparent magnitude, every
thing around us would, in this respect, be undergoing very sensible
variations, which might lead us into strange and serious mistakes. A
fly, near enough to the eye, might appear under an angle as great as
an elephant at the distance of twenty feet, and the one be mistaken for
the other. A giant eight feet high, seen at the distance of twenty-four
feet, would not appear taller than a child two feet in height, at the
distance of six feet; for both would be seen nearly under the same
angle. But our experience generally prevents us from being deceived by
such illusions. By the help of touch, and by making allowance for the
different distances at which we see particular objects, we learn to
correct the ideas we might otherwise form from attending to the optical
angle alone, especially in the case of objects that are near us. By the
sense of touch we acquire an impression of the distance of an object;
this impression combines itself with that of the apparent magnitude,
so that the impression which represents to us the real magnitude is
the product of these two elements. When the objects, however, are at
a great distance, it is more difficult to form a correct estimate
of their true magnitudes. The visual angles are so small, that they
prevent comparison; and the estimated bulks of the objects depend in
a great measure upon the _apparent_ magnitudes; and thus an object
situated at a great distance, appears to us much smaller than it is
in reality. We also estimate objects to be nearer or farther distant
according as they are more or less clear, and our perception of them
more or less distinct and well defined; and likewise, when several
objects intervene between us and the object we are particularly
observing. We make a sort of addition of all the estimated distances
of intermediate objects, in order to form a total distance of the
remote object, which in this case appears to be farther off than if the
intervening space were unoccupied. It is generally estimated that no
terrestrial object can be distinctly perceived, if the visual angle it
subtends be less than _one minute of a degree_; and that most objects
become indistinct, when the angle they subtend at the pupil of the eye
is less than six minutes.

We have deemed it expedient to introduce the above remarks on the
apparent magnitude of objects, because the principal use of a telescope
is to increase the angle of vision, or to represent objects under a
larger angle than that under which they appear to the naked eye, so as
to render the view of distant objects more distinct, and to exhibit to
the organ of vision those objects which would otherwise be invisible.
A telescope may be said to enlarge an object just as many times as the
angle under which the instrument represents it, is greater than that
under which it appears to the unassisted eye. Thus the moon appears to
the naked eye under an angle of about half a degree; consequently a
telescope magnifies 60 times if it represents that orb under an angle
of 30 degrees; and if it magnified 180 times, it would exhibit the moon
under an angle of 90 degrees, which would make her appear to fill half
of the visible heavens, or the space which intervenes from the horizon
to the zenith.




CHAPTER IV.


ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES.

There are two kinds of telescopes, corresponding to two modes of
vision, namely, those which perform their office by _refraction_
through lenses, and those which magnify distant objects by _reflection_
from mirrors. The telescope which is constructed with lenses, produces
its effects solely by refracted light, and is called a Dioptric, _or
refracting telescope_. The other kind of telescope produces its effects
partly by reflection, and partly by refraction, and is composed both
of mirrors and lenses; but the mirrors form the principal part of the
telescope; and therefore such instruments are denominated _reflecting
telescopes_. In this chapter I shall describe the various kinds of
_refracting_ telescopes.


SECT 1.--THE GALILEAN TELESCOPE.

This telescope is named after the celebrated Galileo, who first
constructed, and probably _invented_ it in the year 1609. It consists
of only two glasses, a _convex_ glass next the object, and a _concave_
next the eye. The convex is called the _object-glass_, and the concave
to which the eye is applied, is called the _eye-glass_. Let C (fig.
43.) represent the convex object-glass, presented to any object in the
direction DEI, so that the rays fall parallel upon it;--if these rays,
after passing through it, were not intercepted by the concave lens
K, they would pass on, and cross each other in the focus F, where an
inverted image of the object would be formed. But the concave lens K,
the virtual focus of which is at F, being interposed, the rays are not
suffered to converge to that point, but are made less convergent,[19]
and enter the pupil almost parallel, as GH, and are converged by the
humours of the eye to their proper foci on the retina. The object,
through this telescope, is seen upright, or in its natural position,
because the rays are not suffered to come to a focus, so as to form an
inverted picture. The concave eye-glass is placed as far within the
focus of the object-glass, as is equal to its own virtual focus; and
the magnifying power is as the focal length of the object-glass to
that of the eye-glass, that is, as CF to BF. Thus, suppose the focus of
the object-glass to be 10 inches, and the focus of the eye-glass to be
1 inch, the magnifying power will be 10 times--which is always found by
dividing the focal length of the object-glass by that of the eye-glass.
The interval between the two glasses, in this case, will be 9 inches,
which is the length of the telescope, and the objects seen through it
will appear under an angle ten times greater than they do to the naked
eye. These propositions might be proved mathematically; but the process
is somewhat tedious and intricate, and might not fully be understood
by general readers. I shall therefore only mention some of the general
properties of this telescope, which is now seldom used, except for the
purpose of _opera-glasses_.

[Illustration: _figure 43_]

1. The focal distance of the object-glass must be greater than that of
the eye-glass, otherwise it would not magnify an object: if the focal
distance of the eye-glass were greater than that of the object-glass,
it would diminish objects, instead of magnifying them. 2. The visible
area of the object is greater, the nearer the eye is to the glass; and
it depends on the diameter of the pupil of the eye, and on the breadth
of the object-glass; consequently the field of view in this telescope
is very small. 3. The distinctness of vision in this construction
of a telescope exceeds that of almost any other. This arises from
the rays of light proceeding from the object directly through the
lenses, _without crossing_ or intersecting each other; whereas in the
combination of convex lenses, they intersect one another to form an
image in the focus of the object-glass, and this image is magnified by
the eye-glass with all its imperfections and distortions. The thinness
of the centre of the concave lens also contributes to distinctness.
4. Although the field of view in this telescope is very small, yet
where no other telescope can be procured, it might be made of such a
length as to show the spots on the Sun, the crescent of Venus, the
satellites of Jupiter, and the ring of Saturn; and, requiring only two
glasses, it is the cheapest of all telescopes. It has been found that
an object-lens 5 feet focal distance, will bear a concave eye-glass of
only 1 inch focal distance, and will consequently magnify the diameters
of the planets 60 times, and their surfaces 3600 times, which is
sufficient to show the phenomena now stated. And, although only a small
portion of the sun and moon can be seen at once, yet Jupiter and all
his satellites may sometimes be seen at one view; but there is some
difficulty in finding objects with such telescopes. 5. Opera-glasses,
which are always of this construction, have the object-lens generally
about 6 inches focus and 1 inch diameter, with a concave eye-glass of
about 2 inches focus. These glasses magnify about 3 times in diameter,
have a pretty large field, and produce very distinct vision. When
adjusted to the eye, they are about 4 inches in length. To the object
end of an opera-glass there is sometimes attached a plane mirror,
placed at an angle of 45 degrees, for the purpose of viewing objects on
either side of us. By this means, in a theatre or assembly, we can take
a view of any person without his having the least suspicion of it, as
the glass is directed in quite a different direction. The instrument
with this appendage is sometimes called a _Polemoscope_.


SECT. 2.--THE COMMON ASTRONOMICAL REFRACTING TELESCOPE.

The astronomical telescope is the most simple construction of a
telescope, composed of convex lenses only, of which there are but
two essentially necessary, though a third is sometimes added to
the eye-piece for the purpose of enlarging the field of view. Its
construction will be easily understood from a description of the
following figure. Its two essential parts are, an object-glass AD,
and an eye-glass EY, so combined in a tube that the focus F of the
object-glass is exactly coincident with the focus of the eye-glass.
Let OB (fig. 44.) represent a distant object, from which rays nearly
parallel proceed to the object-lens AD. The rays passing through this
lens will cross at F, and form an image of the object at IM. This image
forms as it were an object to the eye-glass EY, which is of a short
focal distance, and the eye is thus enabled to contemplate the object
as if it were brought much nearer than it is in reality. For the rays,
which after crossing proceed in a divergent state, fall upon the lens
EY, as if they proceeded from a real object situated at F. All that
is effected therefore, by such a telescope is, to form an image of
a distant object by means of the object-lens, and then to give the
eye such assistance as is necessary for viewing that image as near as
possible, so that the angle it shall subtend at the eye shall be very
large compared with the angle which the object itself would subtend in
the same situation.

[Illustration: _figure 44._]

Here it may be expedient to explain, 1. how this arrangement of glasses
shows distant objects distinctly, and 2. the reason why objects appear
magnified when seen through it. As to the first particular, it may be
proved as follows:--The rays OA and BD, which are parallel before they
fall upon the object-glass, are by this glass refracted and united
at its focus: In order, then, to distinct vision, the eye-glass must
re-establish the parallelism of the rays,--which is effected by placing
the eye-glass so that its focus may be at F, and consequently the rays
will proceed from it parallel to each other and fall upon the eye in
that direction. For distinct vision is produced by _parallel_ rays. 2.
The reason why the object appears magnified will appear, if we consider
that, if the eye viewed the object from the centre of the object-glass,
it would see it under the angle OCB; let OC and BC then be produced
to the focus of the glass, they will then limit the image IM formed
in the focus. If then, two parallel rays are supposed to proceed to
the eye-glass EY, they will be converged to its focus H, and the eye
will see the image under the angle EHY. The apparent magnitude of the
object, therefore, as seen by the naked eye, is to the magnitude of the
image as seen through the telescope, as OCB to EHY, or as the distance
CF to the distance FG, in other words, _as the focal length of the
object-glass to that of the eye-glass_.

It is obvious from the figure, that, through this telescope, all
objects will appear _inverted_; since the object OB is depicted by the
object-glass in an inverted position at IM, and in this position is
viewed by the eye-glass EY; and, therefore this kind of telescope is
not well adapted for viewing terrestrial objects, since it exhibits
the tops of trees, houses, and other objects as undermost, and the
heads of people as pointing downwards. But this circumstance is of
no consequence with respect to the heavenly bodies, since they are
round, and it can make little difference to an observer which side
of a globular body appears uppermost or undermost. All astronomical
refracting telescopes invert objects; but they are preferred to any
other telescopes, because they have few glasses, and consequently more
light. This telescope however, can be transformed into a common day
telescope for land objects, by the addition of two other eye-glasses,
as we shall afterwards explain; but in this case a quantity of light is
lost by refraction at each lens; for there is scarcely any transparent
substance that transmits all the rays of light that fall upon it.

The _magnifying power_ of this telescope is found _by dividing the
focal distance of the object-glass by the focal distance of the
eye-glass_: the quotient gives the magnifying power, or the number of
times that the object seen through the telescope, appears larger or
nearer than to the naked eye. Thus, for example, if the focal distance
of the object-glass be 28 inches, and the focal distance of the
eye-glass 1 inch, the magnifying power will be 28 times. If we would
enlarge the telescope and select an object-glass 10 feet, or 120 inches
focus, an eye-glass of 2 inches focal length might be applied, and
then the diameter of objects would be magnified 60 times, and their
surfaces 3600 times. If we would use an object-glass of 100 feet, it
would be necessary to select an eye-glass about 6 inches focus, and
the magnifying power would be 200 times, equal to 1200 inches divided
by 6. Since, then, the power of magnifying depends on the proportion
of the focal length of the object and eye-glasses, and this proportion
may be varied to any degree, it may seem strange to some that a short
telescope of this kind will not answer that purpose as well as a long
one. For instance, it may be asked why an object-glass of 10 feet
focus, may not be made to magnify as much, as one of 100 feet focal
length, by using an eye-glass of half an inch focus, in which case,
the magnifying power would be 240 times? But it is to be considered,
that if the power of magnifying be increased, while the length of the
telescope remains the same, it is necessary to diminish the focal
length of the eye-glass in the same proportion, and this cannot be
done on account of the great distortion and colouring which would then
appear in the image, arising both from the deep convexity of the lens
and the different refrangibility of the rays of light. It is found
that the length of common refracting telescopes must be increased in
proportion to the square of the increase of their magnifying power; so
that in order to magnify twice as much as before, with the same light
and distinctness, the telescope must be lengthened four times; to
magnify 3 times as much, 9 times; and to magnify four times as much,
sixteen times; that is--suppose a telescope of 3 feet to magnify 33
times,--in order to procure a power four times as great, or 132 times,
we must extend the telescope to the length of 48 feet, or 16 times the
length of the other. Much likewise depends upon the breadth or aperture
of the object-glass. If it be too small, there will not be sufficient
light to illuminate the object; and if it be too large, the redundance
of light will produce confusion in the image.

The following table, constructed originally by Huygens, and which I
have re-calculated and corrected, shows the linear aperture, the focal
distance of the eye-glass, and the magnifying power of astronomical
telescopes of different lengths, which may serve as a guide to those
who wish to construct telescopes of this description.

  ---------------+----------------------------------+-----------
  Focal distance | Linear aperture   Focal distance | Magnifying
     of the      |    of the            of the      |   power.
  object-glass.  | object-glass.       eye-glass.   |
  ---------------+-----------------+----------------+-----------
      Feet.      |   Inch.  Dec.   |  Inch.  Dec.   |
       1         |      0.  545    |     0.  605    |    20
       2         |      0.  76     |     0.  84     |    28.5
       3         |      0.  94     |     1.  04     |    34.6
       4         |      1.  08     |     1.  18     |    40
       5         |      1.  21     |     1.  33     |    45
       6         |      1.  32     |     1.  45     |    50
       7         |      1.  43     |     1.  58     |    53
       8         |      1.  53     |     1.  69     |    56.8
       9         |      1.  62     |     1.  78     |    60.6
      10         |      1.  71     |     1.  88     |    63.8
      15         |      2.  10     |     2.  30     |    78
      20         |      2.  43     |     2.  68     |    89.5
      30         |      3.  00     |     3.  28     |   109
      40         |      3.  43     |     3.  76     |   127
      50         |      3.  84     |     4.  20     |   142
      60         |      4.  20     |     4.  60     |   156
      70         |      4.  55     |     5.  00     |   168
      80         |      4.  83     |     5.  35     |   179
      90         |      5.  15     |     5.  65     |   190
     100         |      5.  40     |     5.  95     |   200
     120         |      5.  90     |     6.  52     |   220

In the above table, the first column expresses the focal length of
the object-glass in feet; the second column, the diameter of the
aperture[20] of the object-glass, the third column, the focal distance
of the eye-glass, and the fourth, the magnifying power, which is found
by reducing the feet in the first column to inches, and dividing by the
numbers in the third column. From this table it appears that, in order
to obtain a magnifying power of 168 times, by this kind of telescope,
it is requisite to have an object-glass of 70 feet focal distance,
and an eye-glass five inches focus, and that the aperture of the
object-glass ought not to be more than about 4-1/2 inches diameter. To
obtain a power of 220 times requires a length of 120 feet.

The following is a summary view of the properties of this telescope. 1.
The object is always inverted. 2. The magnifying power is always in the
proportion of the focal distance of the object-glass to the eye-glass.
3. As the rays emerging from the eye-glass, should be rendered parallel
for every eye, there is a small sliding tube next the eye, which should
be pushed out or in till the object appears distinct. When objects
are pretty near, this tube requires to be pulled out a little. These
circumstances require to be attended to in all telescopes. 4. The
apparent magnitude of an object is the same wherever the eye be placed,
but the visible area, or field of view, is the greatest when the eye
is nearly at the focal distance of the eye-glass. 5. The visual angle
depends on the breadth of the eye-glass; for it is equal to the angle
which the eye-glass subtends at the object-glass; but the breadth of
the eye-glass cannot be increased beyond a certain limit, without
producing colouring and distortion.

If the general principles on which this telescope is constructed
be thoroughly understood, it will be quite easy for the reader to
understand the construction of all the other kinds of telescopes,
whether refracting or reflecting. A small astronomical telescope can be
constructed in a few moments, provided one has at hand the following
lenses:--1. A common reading-glass, eight or ten inches focal distance;
2. A common magnifying lens, such as watchmakers or botanists use, of
about 1-1/2 or 2 inches focus. Hold the reading-glass--suppose of ten
inches focus--in the left hand opposite any object, and the magnifying
lens of two inches focus, in the right hand near the eye, at twelve
inches distance from the other in a direct line, and a telescope is
formed which magnifies five times. I have frequently used this plan,
when travelling, when no other telescope was at hand.


SECT. 3.--THE AERIAL TELESCOPE.

The Aerial is a refracting telescope of the kind we have now described,
intended to be used without a tube in a dark night; for the use of a
tube is not only to direct the glasses, but to make the place dark
where the images are formed. It appears from the preceding table
inserted above, that we cannot obtain a high magnifying power, with
the common astronomical telescope, without making it of an extreme
length, in which case the glasses are not manageable in tubes--which
are either too slight and apt to bend, or too heavy and unwieldy if
made of wood, iron or other strong materials. The astronomers of the
seventeenth century, feeling such inconveniences in making celestial
observations with long tubes, contrived a method of using the glasses
without tubes. Hartsocker, an eminent optician, contrived to fix
them at the top of a tree, a high wall, or the roof of a house; but
the celebrated Huygens, who was not only an astronomer, but also an
excellent mechanic, made considerable improvements in the method of
using an object-glass without a tube. He placed it at the top of a
very long pole, having previously enclosed it in a short tube, which
was made to turn in all directions by means of a ball and socket. The
axis of this tube he could command with a fine silken string, so as
to bring it into a line with the axis of another short tube which he
held in his hand, and which contained the eye-glass. The following is
a more particular description of one of these telescopes. On the top
of a long pole or mast _ab_ (fig. 45), is fixed a board moveable up
and down in the channel _cd_: _e_ is a perpendicular arm fixed to it,
and _ff_ is a transverse board that supports the object glass enclosed
in the tube _i_, which is raised or lowered by means of the silk cord
_rl_; _gg_ is an endless rope with a weight _h_, by which the apparatus
of the object-glass is counterpoised; _kl_ is a stick fastened to the
tube _i_; _m_ the ball and socket, by means of which the object-glass
is moveable every way: and to keep it steady, there is a weight _n_
suspended by a wire; _l_ is a short wire to which the thread _rl_ is
tied; _o_ is the tube which holds the eye-glass; _q_ the stick fixed
to this tube, _s_ a leaden bullet, and _t_ a spool to wind the thread
on; _u_ is pins for the thread to pass through; _x_ the rest for the
observer to lean upon, and _y_ the lantern. Fig. 46 is an apparatus
contrived by M. de la Hire for managing the object-glass; but which it
would be too tedious particularly to describe. To keep off the dew from
the object-glass, it was sometimes included in a pasteboard tube, made
of spongy paper, to absorb the humidity of the air. And to find an
object more readily, a broad annulus of white pasteboard was put over
the tube that carried the eye-glass; upon which the image of the object
being painted, an assistant who perceived it, might direct the tube of
the eye-glass into its place.

[Illustration: _figure 45._]

[Illustration: _fig 46._]

Such was the construction of the telescopes with which Hevelius,
Huygens, Cassini, and other eminent astronomers of the seventeenth
century made their principal discoveries. With such telescopes,
Huygens discovered the fourth satellite of Saturn, and determined
that this planet was surrounded with a ring; and with the same kind
of instrument Cassini detected the first, second, third, and fifth,
satellites of Saturn, and made his other discoveries. When the night
was very dark, they were obliged to make the object-glass visible,
by means of a lantern so constructed as to throw the rays of light
up to it in a parallel direction. In making such observations, they
must have taken incredible pains, endured much cold and fatigue,
and subjected themselves to very great labour and expense--which
almost makes us wonder at the discoveries they were instrumental in
bringing to light--and should make modern philosophers sensible of the
obligations they are under to such men as Newton and Dollond, through
whose inventions such unwieldy instruments are no longer necessary.
Telescopes of the description now stated were made of all sizes,
from 30 to above 120 feet in length. Divini at Rome, and Campani at
Bologna, were famed as makers of the object-glasses of the long focal
distance to which we have alluded, who sold them for a great price,
and took every method to keep the art of making them a secret. It was
with telescopes made by Campani, that Cassini made his discoveries.
They were made by the express order of Louis XIV, and were of 86, 100,
and 136 Paris feet in focal length. M. Auzout made one object-glass
of 600 feet focus; but he was never able to manage it, so as to make
any practical observations with it. Hartsocker is said to have made
some of a still greater focal length. The famous aerial telescope of
Huygens was 123 feet in focal length, with six inches of aperture. At
his death, he bequeathed it to the Royal Society of London, in whose
possession it still remains. It required a pole of more than a hundred
feet high, on which to place the object-glass for general observations.
It was with this glass, that Dr. Derham made the observations to which
he alludes in his preface to his ‘Astro-Theology.’ When this glass
was in the possession of Mr. Cavendish, it was compared with one of
Mr. Dollond’s forty-six inch treble object-glass Achromatics, and the
gentlemen who were present at the trial, said that ‘the Dwarf was
fairly a match for the Giant.’ It magnified 218 times, and the trouble
of managing it, was said to be extremely tiresome and laborious.


SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS.

[Illustration: _figure 47._]

This telescope is constructed on the same principle as the astronomical
telescope already described, with the addition of two or three glasses.
In fig. 47, OB represents a distant object, LN, the object glass, which
forms the image IM in its focus, which is, of course, in an inverted
position, and, if the eye were applied at the lens EE, the object would
appear, exactly as through the astronomical telescope, every object
being apparently turned upside down. To remedy this inconvenience,
there are added two other glasses FF and GG, by which a second image is
formed from the first, in the same position as the object. In order to
effect this, the first of these two glasses, namely FF, is placed at
twice its focal distance from the former glass EE, and the other lens
GG, next the eye, is placed at the same distance from FF. For all the
three glasses are supposed to be of the same focal distance. Now, the
lens FF, being placed at twice the focal distance for parallel rays
from EE, receives the pencils of parallel rays after they have crossed
each other at X, and forms an image at _i m_ similar to that at IM and
equal to it, but contrary in position, and consequently erect; which
last image is viewed by the lens GG, in the same manner as the first
image IM would be viewed by the lens EE. In this case, the image IM is
considered as an object to the lens FF of which it forms a picture in
its focus, in a reverse position from that of the first image, and of
course, in the same position as the object.

The magnifying power of this telescope is determined precisely in
the same way as that of the astronomical telescope. Suppose the
object-glass to be thirty inches focal distance, and each of the
eye-glasses 1-1/2 inch focal distance, the magnifying power is in the
proportion of 30 to 1-1/2, or 20 times, and the instrument is, of
course, considerably longer than an astronomical telescope of the same
power. The distance, in this case, between the object-glass and the
first eye-glass EE is 31-1/2 inches; the distance between EE, and the
second glass FF, is 3 inches, and the distance between FF and the glass
GG next the eye, 3 inches; in all 37-1/2 inches, the whole length of
the telescope. Although it is usual to make use of three eye-glasses in
this telescope, yet two will cause the object to appear erect, and of
the same magnitude. For suppose the middle lens FF taken away, if the
first lens EE be placed at X, which is double its focal distance from
the image IM, it will at the same distance X _m_, on the other side,
form a secondary image _i m_ equal to the primary image IM, and also in
a contrary position. But such a combination of eye-glasses produces a
great degree of colouring in the image, and therefore is seldom used.
Even the combination now described, consisting of three lenses of equal
focal distances, is now almost obsolete, and has given place to a much
better arrangement consisting of _four_ glasses, of different focal
distances--which shall be afterwards described.

The following figures, 48, 49, 50 represent the manner in which the
rays of light are refracted through the glasses of the telescopes we
have now described. Fig. 48 represents the rays of light as they pass
from the object to the eye in the Galilean telescope. After passing in
a parallel direction to the object-glass, they are refracted by that
glass, and undergo a slight convergence in passing towards the concave
eye-glass, where they enter the eye in a parallel direction, but no
image is formed previous to their entering the eye, till they arrive
at the retina. Fig. 49 represents the rays as they pass through the
glasses of the astronomical telescope. The rays, after entering the
object-glass, proceed in a converging direction, till they arrive at
its focus, about A, where an image of the object is formed; they then
proceed diverging to the eye-glass, where they are rendered parallel,
and enter the eye in that direction. Fig. 50 represents the rays as
they converge and diverge in passing through the four glasses of
the common day-telescope described above. After passing through the
object-glass, they converge towards B, where the first image is formed.
They then diverge towards the first eye-glass where they are rendered
parallel; and passing through the second eye-glass, they again converge
and form a second image at C; from which point they again diverge,
and passing through the first eye-glass enter the eye in a parallel
direction. If the glasses of these telescopes were fixed on long pieces
of wood, at their proper distances from each other, and placed in a
darkened room, when the sun is shining, the beam of the sun’s light
would pass through them in the same manner as here represented.

[Illustration: _fig. 48._]

[Illustration: _fig. 49._]

[Illustration: _fig. 50._]


SECT. 5.--TELESCOPE FORMED BY A SINGLE LENS.

This is a species of telescope altogether unnoticed by optical writers,
so far as I know; nor has the property of a single lens in magnifying
distant objects been generally adverted to or recognised. It may not
therefore be inexpedient to state a few experiments which I have made
in relation to this point. When we hold a spectacle-glass of a pretty
long focal distance--say, from 20 to 24 inches--close to the eye, and
direct it to distant objects, they do not appear sensibly magnified.
But if we hold the glass about 12 or 16 inches from our eye, we shall
perceive a sensible degree of magnifying power, as if distant objects
were seen at less than half the distance at which they are placed. This
property of a spectacle-glass I happened to notice when a boy, and, on
different occasions since that period have made several experiments on
the subject, some of which I shall here relate.

With the object-glass of a common refracting telescope 4-1/2 feet focal
distance, and 2-1/2 inches diameter, I looked at distant objects--my
eye being at about 3-1/2 feet from the lens, or about 10 or 12 inches
within its focus--and it produced nearly the same effect as a telescope
which magnifies the diameters of objects 5 or 6 times. With another
lens 11 feet focal distance and 4 inches diameter--standing from it at
the distance of about 10 feet, I obtain a magnifying power of about 12
or 14 times, which enables me to read the letters on the sign-posts
of a village half a mile distant. Having some time ago procured a
very large lens 26 feet focal distance, and 11-1/2 inches diameter,
I have tried with it various experiments of this kind upon different
objects. Standing at the distance of about 25 feet from it, I can see
distant objects through it magnified about 26 times in diameter, and
consequently 676 times in surface, and remarkably clear and distinct,
so that I can distinguish the hour and minute hands of a public clock
in a village two miles distant. This single lens, therefore answers the
purpose of an ordinary telescope with a power of 26 times. In making
such experiments our eye must always be _within_ the focus of the
lens, at least 8 or 10 inches. The object will, indeed, be seen at any
distance from the glass within this limit; but the magnifying power is
diminished in proportion as we approach nearer to the glass. Different
eyes, too, will require to place themselves at different distances, so
as to obtain the greatest degree of magnifying power with distinctness,
according as individuals are long or short-sighted.

This kind of telescope stands in no need of a tube, but only of a small
pedestal on which it may be placed on a table, nearly at the height
of the eye, and that it be capable of a motion in a perpendicular
or parallel direction, to bring it in a line with the eye and the
object. The principle on which the magnifying power, in this case, is
produced, is materially the same as that on which the performance of
the Galilean telescope depends. The eye of the observer serves instead
of the concave lens in that instrument; and as the concave lens is
placed as much within the focus of the object-glass, as is equal to
its own focal distance, so the eye, in these experiments, must be
placed at least its focal distance within the focus of the lens with
which we are experimenting; and the magnifying power will be nearly
in the proportion of the focal distance of the lens to the focal
distance of the eye. If, for example, the focal distance of the eye,
or the distance at which we see to read distinctly, be 10 inches, and
the focal distance of the lens, 11 feet, the magnifying power will
be as 11 feet, or 132 inches to 10, that is, about 13 times. Let A
(fig. 51.) represent the lens placed on a pedestal; the rays of light
passing through this lens from distant objects will converge towards a
focus at F. If a person then, place his eye at E, a certain distance
within the focal point, he will see distant objects magnified nearly in
the proportion of the focal distance of the lens to that of the eye;
and when the lens is very broad--such as the 26 feet lens mentioned
above--two or three persons may look through it at once, though they
will not all see the same object. I have alluded above to a lens made
by M. Azout of 600 feet focal distance. Were it possible to use such a
lens for distant objects, it might represent them as magnified 5 or 600
times, without the application of any eye-glass. In this way the aerial
telescope of Huygens would magnify objects above 100 times, which is
about half the magnifying power it produced with its eye-piece. Suppose
Azout’s lens had been fitted up as a telescope, it would not have
magnified above 480 times, as it would have required an eye-glass of 14
or 15 inches focal distance, whereas, without an eye-glass, it would
have magnified objects considerably above 500 times. It is not unlikely
that the species of telescope to which I have now adverted, constituted
one of those instruments for magnifying distant objects which were said
to have been in the possession of certain persons long before their
invention in Holland, and by Galileo in Italy--to which I have referred
in p. 182. Were this kind of telescope to be applied to the celestial
bodies, it would require to be elevated upon a pole in the manner
represented, fig. 45, p. 226.

[Illustration: _figure 51._]


SECT. 6.--THE ACHROMATIC TELESCOPE.

This telescope constitutes the most important and useful improvement
ever made upon telescopic instruments; and, it is probable, it will,
ere long, supersede the use of all other telescopes. Its importance and
utility will at once appear when we consider, that a good achromatic
telescope of only 4 or 5 feet in length will bear a magnifying power
as great, as that of a common astronomical telescope 100 feet long,
and even with a greater degree of distinctness, so that they are now
come into general use both for terrestrial and celestial observations.
There are, indeed, certain obstructions which prevent their being made
of a very large size; but from the improvement in the manufacture
of achromatic glass which is now going forward, it is to be hoped
that the difficulties which have hitherto impeded the progress of
opticians will soon be removed. In order to understand the nature
of this telescope, it will be necessary to advert a little to the
_imperfections_ connected with common refracting telescopes.

[Illustration: _figure 52._]

The first imperfection to which I allude is this, that _spherical
surfaces do not refract the rays of light accurately to a point_;
and hence the image formed by a single convex lens is not perfectly
accurate and distinct. The rays which pass near the extremities of such
a lens meet in foci nearer to the lens than those which pass nearly
through the centre, which may be illustrated by the following figure.
Let PP (fig. 52) be a convex lens and E_e_ an object, the point E of
which corresponds with the axis, and sends forth the rays EM, EN, EA,
&c., all of which reach the surface of the glass, but in different
parts. It is manifest that the ray EA which passes through the middle
of the glass, suffers no refraction. The rays EM, EM, likewise, which
pass through near to EA, will be converged to a focus at F, which we
generally consider as the focus of the lens. But the rays EN, EN, which
are nearer to the edge of the glass will be differently refracted, and
will meet about G, nearer to the lens, where they will form another
image G_g_. Hence, it is evident, that the first image F_f_, is formed
only by the union of those rays which pass very near the centre of the
lens; but as the rays of light proceeding from every point of an object
are very numerous, there is a succession of images formed, according
to the parts of the lens where they penetrate, which necessarily
produces indistinctness and confusion. This is the imperfection which
is distinguished by the name of _spherical aberration_, or the error
arising from the spherical form of lenses.

The second and most important imperfection of single lenses, when used
for the object-glasses of telescopes, is, that the rays of compounded
light being differently refrangible, come to their respective foci at
different distances from the glass; the more refrangible rays, as the
_violet_, converging sooner than those which are less refrangible, as
the _red_. I have had occasion to illustrate this circumstance, when
treating on the colours produced by the prism, (see p. 128, and figures
32 and 33,) and it is confirmed by the experiment of a paper painted
red, throwing its image, by means of a lens, at a greater distance
than another paper painted blue. From such facts and experiments, it
appears, that the image of a white object consists of an indefinite
number of  images, the violet being nearest, and the red
farthest from the lens, and the images of intermediate colours at
intermediate distances. The aggregate, or image itself, must therefore
be in some degree confused; and this confusion being much increased
by the magnifying power, it is found necessary to use an eye glass of
a certain limited convexity to a given object glass. Thus, an object
glass of 34 inches focal length will bear an eye-glass of only 1 inch
focus, and will magnify the diameters of objects 34 times; one of 50
feet focal distance will require an eye-glass of 4-1/2 inches focus,
and will magnify only 142 times; whereas, could we apply to it an
eye-glass of only 1 inch focus, as in the former case, it would magnify
no less than 600 times. And were we to construct an object-glass of 100
feet focal length, we should require to apply an eye-glass, not less
than 6 inches focus, which would produce a power of about 200 times;
so that there is no possibility of producing a great power by single
lenses, without extending the telescope to an immoderate length.

Sir Isaac Newton, after having made his discoveries respecting the
colours of light, considered the circumstance we have now stated as
an insuperable barrier to the improvement of refracting telescopes;
and therefore turned his attention to the improvement of telescopes
by _reflection_. In the telescopes which he constructed and partly
invented, the images of objects are formed by reflection from speculums
or mirrors; and being free from the irregular convergency of the
various  rays of light, will admit of a much larger aperture
and the application of a much greater degree of magnifying power. The
reflector which Newton constructed was only 6 inches long, but it was
capable of bearing a power equal to that of a 6 feet refractor. It was
a long time, however, after the invention of these telescopes before
they were made of a size fitted for making celestial observations.
After reflecting telescopes had been some time in use, Dollond made his
famous discovery of the principle which led him to the construction
of the _achromatic_ telescope. This invention consists of a compound
object glass formed of two different kinds of glass, by which both
the spherical aberration and the errors arising from the different
refrangibility of the rays of light are, in a great measure corrected.
For the explanation of the nature of this compound object glass and of
the effects it produces; it may be expedient to offer the following
remarks respecting the dispersion of light and its refraction by
different substances.

The _dispersion_ of light is estimated by the variable angle formed by
the red and violet rays which bound the solar spectrum;--or rather,
it is the excess of the refraction of the most refrangible ray above
that of the least refrangible ray. The dispersion is not proportional
to the refraction--that is, the substances which have an equal mean
refraction, do not _disperse_ light in the same ratio. For example,
if we make a prism with plates of glass, and fill it with oil of
Cassia, and adjust its refracting angle ACB, (fig. 31, p. 127,) so
that the middle of the spectrum which it forms falls exactly at the
same place where the green rays of a spectrum formed by a glass prism
would fall--then we shall find that the spectrum formed by the _oil
of Cassia_ prism will be two or three times _longer_ than that of the
_glass_ prism. The oil of Cassia, therefore, is said to _disperse_ the
rays of light more than the glass, that is, to separate the extreme
red and violet rays at O and P more than the mean ray at _green_, and
to have a greater _dispersive power_. Sir I. Newton appears to have
made use of prisms composed of different substances, yet, strange to
tell, he never observed that they formed spectrums, whose lengths were
different, when the refraction of the green ray was the same; but
thought that the dispersion was proportional to the refraction. This
error continued to be overlooked by philosophers for a considerable
time, and was the cause of retarding the invention of the achromatic
telescope for more than 50 years.

Dollond was among the first who detected this error. By his experiments
it appears, that the different kinds of glass differ extremely with
respect to the divergency of colours produced by equal refractions.
He found that two prisms, one of white flint glass, whose refracting
angle was about 25 degrees, and another of crown glass whose refracting
angle was about 29 degrees, refracted the beam of light nearly alike;
but that the divergency of colour in the white flint was considerably
more than in the crown glass; so that when they were applied together,
to refract contrary ways, and a beam of light transmitted through
them, though the emergent continued parallel to the incident part,
it was, notwithstanding, separated into component colours. From this
he inferred, that, in order to render the emergent beam white, it is
necessary that the refracting angle of the prism of crown glass should
be _increased_, and by repeated experiments he discovered the exact
quantity. By these means he obtained a theory in which refraction was
performed without any separation or divergency of colour; and thus the
way was prepared for applying the principle he had ascertained to the
construction of the object glasses of refracting telescopes. For the
edges of a convex and concave lens, when placed in contact with each
other, may be considered as two prisms which refract contrary ways;
and if the excess of refraction in the one be such as precisely to
destroy the divergency of colour in the other, a colourless image will
be formed. Thus, if two lenses are made of the same focal length, the
one of flint glass and the other of crown, the length or diameter of
the  image in the first will be to that produced by the crown
glass, as 3 to 2 nearly. Now, if we make the focal lengths of the
lenses in this proportion, that is, as 3 to 2, the  spectrum
produced by each will be equal. But if the flint lens be concave, and
the crown convex--when placed in contact--they will mutually correct
each other, and a pencil of white light refracted by the compound lens
will remain colourless.

[Illustration: _figure 53._]

The following figure may perhaps illustrate what has been now stated.
Let LL (fig. 53.) represent a convex lens of _crown glass_, and _ll_
a concave lens of _flint glass_. A ray of the sun S, falls at F on
the convex lens which will refract it exactly as the prism ABC, whose
faces touch the two surfaces of the lens at the points where the ray
enters and quits it. The solar ray, SF, thus refracted by the lens LL,
or prism ABC, would have formed a spectrum PT on the wall, had there
been no other lens, the violet ray F crossing the axis of the lens
at V, and going to the upper end P of the spectrum; and the red ray
FR, going to the lower end T. But as the flint-glass lens _ll_, or the
prism A_a_C which receives the rays FV, FR, at the same points, is
interposed, these rays will be united at _f_, and form a small circle
of white light; the ray SF of the sun being now refracted without
colour from its primitive direction SFY into the new direction F_f_.
In like manner the corresponding ray SM will be refracted to _f_,
and a white and colourless image of the sun will be there formed by
the two lenses. In this combination of lenses it is obvious that the
spherical aberration of the flint lens corrects to a considerable
degree that of the crown-glass, and by a proper adjustment of the radii
of the surfaces, it may be almost wholly removed. This error is still
more completely corrected in the _triple_ achromatic object-glass,
which consists of three lenses--a concave flint lens placed between
convexes of crown glass. Fig. 54 shows the _double_ achromatic lens,
and fig. 55, the _triple_ object-glass, as they are fitted up in their
cells, and placed at the object end of the telescope. In consequence
of their producing a focal image free of colour they will bear a
much larger aperture and a much greater magnifying power than common
refracting telescopes of the same length. While a common telescope
whose object-glass is 3-1/2 feet focal distance will bear an aperture
of scarcely 1 inch, the 3-1/2 feet Achromatic will bear an aperture
of 3-1/4 inches, and consequently transmits 10-1/2 times the quantity
of light. While the one can bear a magnifying power of only about 36
times, the other will bear a magnifying power for celestial objects of
more than 200 times.

[Illustration: _figure 54._]

[Illustration: _figure 55._]

The theory of the achromatic telescope is somewhat complicated and
abstruse, and would require a more lengthened investigation than my
limits will permit. But what has been already stated may serve to
give the reader a general idea of the _principle_ on which it is
constructed, which is all I intended. The term _achromatic_ by which
such instruments are now distinguished was first given to them by Dr.
Bevis. It is compounded of two Greek words which signify, ‘free of
colour.’ And, were it not that even philosophers are not altogether
free of that pedantry which induces us to select Greek words which
are unintelligible to the mass of mankind, they might have been
contented with selecting the plain English word _colourless_, which
is as significant and expressive as the Greek word _achromatic_. The
_crown-glass_, of which the convex lenses of this telescope are made,
is the same as good common window-glass; and the _flint-glass_ is that
species of glass of which wine-glasses, tumblers, decanters and similar
articles are formed, and is sometimes distinguished by the name of
crystal-glass. Some opticians have occasionally formed the concave
lens of an achromatic object-glass from the bottom of a broken tumbler.

This telescope was invented and constructed by Mr. John Dollond, about
the year 1758. When he began his researches into this subject, he was
a silk weaver in Spitalfields, London. The attempt of the celebrated
Euler to form a colourless telescope, by including water between two
meniscus glasses, attracted his attention, and, in the year 1753, he
addressed a letter to Mr. Short, the optician, which was published in
the Philosophical Transactions of London, ‘concerning a mistake in
Euler’s theorem for correcting the aberrations in the object glasses
of refracting telescopes.’ After a great variety of experiments on
the refractive and dispersive powers of different substances, he
at last constructed a telescope in which an exact balance of the
opposite dispersive powers of the crown and flint lenses made the
colours disappear, while the predominating refraction of the crown
lens disposed the achromatic rays to meet at a distant focus. In
constructing such object glasses, however, he had several difficulties
to encounter. In the first place, the focal distance as well as the
particular surfaces must be very nicely proportioned to the densities
or refractive powers of the glasses, which are very apt to vary in the
same sort of glass made at different times. In the next place, the
centers of the two glasses must be placed truly in the common axis of
the telescope, otherwise the desired effect will be in a great measure
destroyed. To these difficulties is to be added--that there are four
surfaces (even in double achromatic object glasses) to be wrought
perfectly spherical; and every person practised in optical operations
will allow, that there must be the greatest accuracy throughout the
whole work. But these and other difficulties were at length overcome by
the judgment and perseverance of this ingenious artist.

It appears, however, that Dollond was not the only person who had
the merit of making this discovery--a private gentleman, Mr. Chest,
of Chest-hall, a considerable number of years before, having made a
similar discovery, and applied it to the same purpose. This fact was
ascertained in the course of a process raised against Dollond at the
instance of Watkins, optician at Charing-cross, when applying for a
patent. But as the other gentleman had kept his invention a secret,
and Dollond had brought it forth for the benefit of the public, the
decision was given in his favour. There was no evidence that Dollond
borrowed the idea from his competitor, and both were, to a certain
extent, entitled to the merits of the invention.

One of the greatest obstructions to the construction of large
achromatic telescopes is, the difficulty of procuring large discs of
flint glass of an uniform refractive density--of good colour, and free
from veins. It is said that, fortunately for Mr. Dollond, this kind
of glass was procurable when he began to make achromatic telescopes,
though the attempts of ingenious chemists have since been exerted to
make it without much success. It is also said, that the glass employed
by Dollond in the fabrication of his best telescopes, was of the same
melting, or made at the same time, and that, excepting this particular
treasure, casually obtained, good dense glass for achromatic purposes,
was always as difficult to be procured as it is now. The dispersion of
the flint glass, too, is so variable, that, in forming an achromatic
lens, trials on each specimen require to be made before the absolute
proportional dispersion of the substances can be ascertained. It is
owing, in a great measure, to these circumstances, that a large and
good achromatic telescope cannot be procured unless at a very high
price. Mr. Tulley of Islington--who has been long distinguished as a
maker of excellent achromatic instruments--showed me, about six years
ago, a rude piece of flint glass about five inches diameter, intended
for the concave lens of an achromatic object glass, for which he paid
eight guineas. This was before the piece of glass was either figured
or polished, and, consequently, he had still to perform the delicate
operation of figuring, polishing, and adjusting this concave to the
convex lenses with which it was to be combined; and during the process
some veins or irregularities might be detected in the flint glass which
did not then appear. Some years before, he procured a disc of glass
from the continent about seven or eight inches diameter, for which he
paid about thirty guineas, with which an excellent telescope, twelve
feet focal length, was constructed for the Astronomical Society of
London. It is obvious therefore, that large achromatic telescopes must
be charged at a pretty high price.

In order to stimulate ingenious chemists and opticians to make
experiments on this subject, the Board of Longitude, more than half
a century ago, offered a considerable reward for bringing the art
of making good flint glass for optical purposes to the requisite
perfection. But considerable difficulties arise in attempting
improvements of this kind; as the experiments must all be tried on a
very large scale, and are necessarily attended with a heavy expence.
And although government has been extremely liberal in voting money for
warlike purposes, and in bestowing pensions on those who stood in no
need of them, it has hitherto thrown an obstruction in the way of such
experiments, by the heavy duty of excise, which is rigorously exacted,
whether the glass be manufactured into saleable articles or not; and
has thus been instrumental in retarding the progress of improvement
and discovery. It would appear that experiments of this kind have
been attended with more success in France, Germany, and other places
on the continent, than in Britain; as several very large achromatic
telescopes have been constructed in those countries by means of flint
glass which was cast for the purpose in different manufactories, and
to which British artists have been considerably indebted; as the
London opticians frequently purchase their largest discs of flint
glass from Parisian agents. Guinaud, a continental experimenter, and
who was originally a cabinet maker, appears to have had his labours
in this department of art crowned with great success. Many years were
employed in his experiments, and he too frequently, notwithstanding all
his attention, discovered his metal to be vitiated by striæ, spects
or grains, with cometic tails. He constructed a furnace capable of
melting two _cwt_ of glass in one mass, which he sawed vertically,
and polished one of the sections, in order to observe what had taken
place during the fusion. From time to time, as he obtained blocks,
including portions of good glass, his practice was to separate them by
sawing the blocks into horizontal sections, or perpendicular to their
axes. A fortunate accident conducted him to a better process. While
his men were one day carrying a block of this glass, on a hand-barrow,
to a saw mill which he had erected at the Fall of the Doubs, the
mass slipped from its bearers, and, rolling to the bottom of a steep
and rocky declivity, was broken to pieces. Guinaud having selected
those fragments which appeared perfectly homogeneous, softened them
in circular moulds, in such a manner, that on cooling, he obtained
discs that were afterwards fit for working. To this method he adhered,
and contrived a way for clearing his glass while cooling, so that the
fractures should follow the most faulty parts. When flaws occurred in
the large masses, they were removed by cleaving the pieces with wedges;
then smelting them again in moulds, which give them the form of discs.
The Astronomical Society of London have made trial of discs made by
Guinaud, and have found them entirely homogeneous and free from fault.
Of this ingenious artist’s flint glass, some of the largest achromatic
telescopes on the continent have been constructed. But, it is more
than twenty years since this experimenter took his flight from this
terrestrial scene, and it is uncertain whether his process be still
carried on with equal success.


_Notices of some large Achromatic telescopes on the Continent and in
Great Britain._

1. _The Dorpat Telescope._--This is one of the largest and most
expensive Refracting telescopes ever constructed. It was made by
the celebrated Fraunhofer of Munich for the observatory of the
Imperial University of Dorpat, and was received into the observatory
by Professor Struve in the year 1825. The aperture of the object
glass of this telescope is 9-1/2 English inches, and its solar focal
length about fourteen feet, the main tube being thirteen French feet
exclusive of the tube which holds the eye pieces. The smallest of the
four magnifying powers it possesses, is 175, and the largest 700,
which, in favourable weather, is said to present the object with the
utmost precision. ‘This instrument,’ says Struve, ‘was sold to us by
Privy-Counsellor VON UTZCHNEIDER, the chief of the optical
establishment at Munich, for 10,500 florins, (about £950 sterling), a
price which only covers the expenses which the establishment incurred
in making it.’ The frame work of the stand of this telescope is of oak
inlaid with pieces of mahogany in an ornamental manner, and the tube is
of deal veneered with mahogany and highly polished. The whole weight
of the telescope and its counterpoises is supported at one point, at
the common center of gravity of all its parts; and though these weigh
3000 Russian pounds, yet, we are told that this enormous telescope may
be turned in every direction towards the heavens with more ease and
certainty than any other hitherto in use. When the object end of the
telescope is elevated to the zenith, it is sixteen feet four inches,
Paris measure, above the floor, and its eye end in this position is
two feet nine inches high. This instrument is mounted on an Equatorial
stand, and clock work is applied to the Equatorial axis, which gives it
a smooth and regular sidereal motion, which, it is said, keeps a star
in the exact center of the field of view, and produces the appearance
of a state of rest in the starry regions, which motion can be made
solar, or even lunar, by a little change given to the place of a
pointer, that is placed as an index on the dial plate. Professor Struve
considers the optical powers of this telescope superior to those of
Schröeter’s twenty-five feet reflector, from having observed σ Orionis
with fifteen companions, though Schröeter observed only twelve, that
he could count with certainty. Nay, he seems disposed to place it in
competition with the late Sir W. Herschel’s forty feet reflector. The
_finder_ of this telescope has a focal distance of 30 French inches,
and 2-42 aperture.

2. _Sir James South’s Telescope._--About the year 1829, Sir J.
South, President of the London Astronomical Society, procured of M.
Cauchoix of Paris, an achromatic object glass of 11-2/10 inches, clear
aperture, and of 19 feet focal length. The flint glass employed in its
construction was the manufacture of the late Guinaud le Pere, and was
found to be absolutely perfect. The first observation was made with
this telescope, while on a temporary stand, on Feb. 13, 1830, when Sir
J. Herschel discovered with it a _sixth_ star in the trapezium in the
nebula of Orion, whose brightness was about one third of that of the
fifth star discovered by Struve, which is as distinctly seen as the
companion to Polaris is in a five feet achromatic. Sir James gives the
following notices of the performance of this instrument on the morning
of May 14, 1830. ‘At half past two, placed the 20 feet achromatic on
the Georgium Sidus, saw it with a power of 346, a beautiful planetary
disc; not the slightest suspicion of any ring, either perpendicular or
horizontal; but the planet three hours east of the meridian, and the
moon within three degrees of the planet.’ At a quarter before three,
viewed _Jupiter_ with 252 and 346, literally covered with belts, and
the diameters of his satellites might have been as easily measured
as himself. One came from behind the body, and the contrast of the
colour with that of the planet’s limb was striking. At three o’clock
viewed _Mars_. The contrast of light in the vicinity of the poles very
decided. Several spots on his body well and strongly marked--that about
the south pole seems to overtake the body of the planet, and gives an
appearance not unlike that afforded by the new moon, familiarly known
as ‘the old moon in the new moon’s arms.’ _Saturn_ has been repeatedly
seen with powers from 130 to 928 under circumstances the most
favourable; but not any thing anomalous about the planet or its ring
could even be suspected. This telescope is erected on an Equatorial
stand at Sir J. South’s observatory, Kensington.

3. _Captain Smyth’s Telescope in his private observatory at
Bedford._--This Achromatic telescope is 8-1/2 feet focal length, with a
clear aperture of 5-9/10 inches worked by the late Mr. Tulley, Senior,
from a disk purchased by Sir James South at Paris. It is considered
by Captain Smyth to be the finest specimen of that eminent optician’s
skill, and, it is said, will bear with distinctness, a magnifying
power of 1200. Its distinctness has been proved by the clear vision
it gives of the obscure nebulæ, and of the companions of Polaris,
Rigel, α Lyræ, and the most minute double stars---the lunar mountains,
cavities and shadows under all powers--the lucid polar regions of
Mars--the sharpness of the double ring of Saturn--the gibbous aspect
of Venus--the shadows of Jupiter’s satellites across his body, and
the splendid contrast of colours in α Hercules, γ Andromedæ and other
superb double stars.

_Other large Achromatics._--Besides the above, the following, belonging
to public observatories and private individuals, may be mentioned. In
the Royal observatory at Greenwich, there is an Achromatic of 10 feet
focal distance, having a double object glass 5 inches diameter, which
was made by Mr. Peter Dollond, and the only one of that size he ever
constructed. There is also a 46 inch achromatic, with a triple object
glass 3-3/4 inches aperture, which is said to be the most perfect
instrument of the kind ever produced. It was the favourite instrument
of Dr. Maskelyne, late Astronomer Royal, who had a small room fitted
up in the observatory for this telescope. The observatory, some years
ago erected near Cambridge, is perhaps the most splendid structure of
the kind in Great Britain. It is furnished with several very large
achromatic telescopes on Equatorial machinery: but the Achromatic
telescope, lately presented to it by the Duke of Northumberland, is
undoubtedly the largest instrument of this description which is to be
found in this country. The object glass is said to be 25 feet focal
distance, and of a corresponding diameter, but as there was no access
to this instrument at the time I visited this observatory, nearly six
years ago, I am unable to give a particular description of it. In
the Royal Observatory at Paris, which I visited in 1837, I noticed,
among other instruments, two very large Achromatic telescopes which,
measuring them rudely by the eye--I estimated to be from 15 to 18
feet long, and the aperture at the object end, from 12 to 15 inches
diameter. They were the largest achromatics I had previously seen; but
I could find no person in the observatory at that time, who could give
me any information as to their history, or to their exact dimensions,
or powers of magnifying.[21]

The Rev. Dr. Pearson, Treasurer to the Astronomical Society of London,
is in possession of the telescope formerly alluded to, made by Mr.
Tulley, of twelve feet focal distance and seven inches aperture, which
is said to be a very fine one. The small star which accompanies the
pole star, with a power of a 100, appears through this telescope, as
distinct and steady as one of Jupiter’s satellites. With a single lens
of 6 inches focus, which produced a power of 24 times, according to
the testimony of an observer who noticed it--the small star appeared
as it does in an achromatic of 3 inches aperture, which shows the
great effect of illuminating power in such instruments. Mr. Lawson, a
diligent astronomical observer in Hereford, possesses a most beautiful
achromatic telescope of about 7 inches aperture, and 12 feet focal
distance, which was made by one of the Dollonds, who considered it
as his _chief d’oeuvre_. It is said to bear powers as high as 1100
or 1400; and has been fitted up with mechanism devised by Mr. Lawson
himself, so as to be perfectly easy and manageable to the observer,
and which displays this gentleman’s inventive talent. In several of
his observations with this instrument, he is said to have had a view
of some of the more minute subdivisions of the ring of Saturn. A very
excellent achromatic telescope was fitted up some years ago by my
worthy friend William Bridges, Esq., Blackheath. Its object glass is
5-1/2 inches diameter, and about 5-1/2 feet focal length. It is erected
upon Equatorial machinery, and placed in a circular observatory which
moves round with a slight touch of the hand. The object glass of this
instrument cost about 200 Guineas, the equatorial machinery on which it
is mounted cost 150 Guineas, and the circular observatory in which it
is placed about 100 Guineas; in all 450 Guineas. Its powers vary from
50 to 300 times.[22]


_Achromatic telescopes of a moderate size._

Such telescopes as I have alluded to above, are among the largest which
have yet been made on the achromatic principle; they are, of course,
comparatively rare, and can be afforded only at a very high price. Few
of the _object glasses_ in the telescopes to which I have referred,
would be valued at less than 200 Guineas, independently of the tubes,
eye pieces and other apparatus with which they are fitted up. It is so
difficult to procure large discs of flint glass for optical purposes,
to produce the requisite curves of the different lenses, and to combine
them together with that extreme accuracy which is requisite, that when
a good compound lens of this description is found perfectly achromatic,
the optician must necessarily set a high value upon it; since it may
happen that he may have finished half a dozen before he has got one
that is nearly perfect. The more common sizes of achromatic telescopes
for astronomical purposes, which are regularly sold by the London
opticians, are the following:--

1. _The 2-1/2 feet Achromatic._--This telescope has an object glass 30
inches in focal length, and 2 inches clear aperture. It is generally
furnished with two eye pieces, one for terrestrial objects, magnifying
about 30 or 35 times, and one for celestial objects with a power of 70
or 75 times. It might be furnished with an additional astronomical
eye-piece--if the object glass be a good one, so as to produce a power
of 90 or 95 times. With such a telescope, the belts and satellites of
Jupiter, the phases of Venus and the ring of Saturn may be perceived;
but not to so much advantage as with larger telescopes. It is generally
fitted up either with a mahogany or a brass tube, and is placed upon a
tripod brass stand, with a universal joint which produces a horizontal
and vertical motion. It is packed, along with the eye-pieces, and
whatever else belongs to it, in a neat mahogany box. Its price varies,
according as it is furnished with an elevating rack or other apparatus.

The following are the prices of this instrument as marked in the
catalogue of Mr. Tulley, Terrett’s Court, Islington, London.

                                                                £  s. d.
  2-1/2 feet telescopes, brass mounted on plain pillar and claw
  stand, with one eye piece for astronomical purposes, and
  one for land objects, to vary the magnifying power,
  packed in a mahogany box                                       10 10 0

  Ditto, ditto, brass mounted on pillar and claw stand, with
  elevating rack, 1 eye piece for astronomical purposes, and
  1 for land objects, to vary the magnifying power, packed
  in a mahogany box                                              12 12 0

The following prices of the same kind of telescope are from the
catalogue of Messrs. W. and, S. Jones, 30, Lower Holborn, London.

                                                                £  s. d.
  The improved 2-1/2 feet achromatic refractor, on a brass
  stand, mahogany tube, with three eye pieces, two magnifying
  about 40 and 50 times for terrestrial objects, and the
  other about 75 times for astronomical purposes, in a
  mahogany case                                                  10 10 0

  Ditto, ditto, the tube all brass, with three eye pieces        11 11 0

  Ditto, ditto, with vertical and horizontal rack work
  motions                                                        15 15 0

2. _The 3-1/2 feet Achromatic Telescope._--The object glass of this
telescope is from 44 to 46 inches focal length, and 2-3/4 inches
diameter. It is generally furnished with four eye-pieces, two for
terrestrial and two for celestial objects. The lowest power for land
objects is generally about 45, which affords a large field of view, and
exhibits the objects with great brilliance. The other terrestrial power
is usually from 65 to 70. The astronomical powers are about 80 and 130;
but such a telescope should always have another eye-piece, to produce
a power of 180 or 200 times, which it will bear with distinctness,
in a serene state of the atmosphere, if the object glass be truly
achromatic. The _illuminating power_ in this telescope is nearly double
that of the 2-1/2 feet telescope, or in the proportion of 7, 56 to
4; and therefore it will bear about double the magnifying power with
nearly equal distinctness. This telescope is fitted up in a manner
somewhat similar to the former, with a tripod stand which is placed
upon a table. Sometimes, however, it is mounted on a long mahogany
stand which rests upon the floor, (as in fig. 58.), and is fitted with
an equatorial motion; and has generally a small telescope fixed near
the eye end of the large tube, called a _finder_, which serves to
direct the telescope to a particular object in the heavens when the
higher powers are applied. It is likewise eligible that it should have
an elevating rack and sliding tubes, for supporting the eye end of the
instrument, to keep it steady during astronomical observations, and it
would be an advantage, for various purposes which shall be afterwards
described, to have fitted to it a _Diagonal Eye Piece_ magnifying 40
times or upwards.

The prices of this instrument, as marked in Mr. Tulley’s Catalogue, are
as follows:--

                                                                £  s. d.
  The 3-1/2 feet achromatic telescope 2-3/4 inches aperture, on
  plain pillar and claw stand, 2 eye pieces for astronomical
  purposes, and 1 for land objects to vary the magnifying
  power, packed in a mahogany box                               21  0  0

  Ditto, ditto, with elevating rack and achromatic finder,
  2 eye pieces for astronomical purposes, and 1 for day objects
  to vary the magnifying power, packed in a mahogany
  box                                                           26  5  0

The following are the prices as marked in Messrs. W. and S. Jones’
Catalogue.

                                                                £   s.  d.
  The 3-1/2 feet achromatic, plain mahogany tube                 18  18  0

  Ditto, ditto, brass tube                                       21   0  0

  Ditto, all in brass, with rack work motions, &c.               26   5  0

  Ditto, the object glass of the largest aperture, and the
  rack motions on an improved principle      _from_ 37l. 16s. to 42   0  0

  Ditto, fitted up with Equatorial motion, framed mahogany
  stand, divided altitude, and azimuth arches, or declination
  and right ascension circles, &c. &c.             _from_ 60l to 80   0  0

This is the telescope which I would particularly recommend to
astronomical amateurs, whose pecuniary resources do not permit them
to purchase more expensive instruments. When fitted up with the eye
pieces and powers already mentioned, and with a finder and elevating
rack,--price 25 guineas--it will serve all the purposes of general
observation. By this telescope, satisfactory views may be obtained of
most of the interesting phenomena of the heavens, such as the spots of
the sun--the mountains, vales, and caverns on the lunar surface--the
phases of Mercury and Venus--the spots on Mars--the satellites and
belts of Jupiter--the ring of Saturn--many of the more interesting
nebulæ, and most of the double stars of the second and third classes.
When the object glass of this telescope is accurately figured and
perfectly achromatic, a power of from 200 to 230 maybe put upon it, by
which the division of Saturn’s ring might occasionally be perceived. It
is more easily managed and represents objects considerably brighter
than reflecting telescopes of the same price and magnifying power,
and it is not so apt to be deranged as reflectors generally are. A
telescope of a less size would not in general be found satisfactory for
viewing the objects I have now specified, and for general astronomical
purposes. It may not be improper for the information of some readers,
to explain what is meant in Mr. Tulley’s catalogue, when it is stated
that this instrument has ‘one eye piece for day objects, _to vary the
magnifying power_.’ The eye piece alluded to is so constructed, that
by drawing out a tube next the eye, you may increase the power at
pleasure, and make it to vary, say from 40 to 80 or 100 times; so that
such a construction of the terrestrial eye piece (to be afterwards
explained) serves in a great measure, the purpose of separate
eye-pieces. The whole length of the 3-1/2 feet telescope, when the
terrestrial eye piece is applied, is about 4-1/2 feet from the object
glass to the first eye glass.

When the aperture of the object glass of this telescope exceeds 2-3/4
inches its price rapidly advances.

The following is Mr. Tulley’s scale of prices, proportionate to the
increase of aperture:--

                                                                  £  s. d.
  3-1/2 feet telescopes 3-1/4 inches aperture, with vertical and
  horizontal rack work motions, achromatic finder, 3 eye
  pieces for astronomical purposes, and one for day objects
  to vary the magnifying power, packed in a mahogany box         42  0  0

  Ditto, ditto, 3-3/4 inches diameter, mounted as above          68  5  0

  Ditto, with universal Equatorial, instead of pillar and
  claw stand                                                     84  0  0

Here, in the one case, the increase of half an inch in the diameter
of the object-glass, adds about £16. to the expense; and in the
other case no less than £26. 5s. The proportion of light in those
two telescopes, compared with that of 2-3/4 inches aperture, is as
follows:--The square of the 2-3/4 object-glass is 7.56; that of 3-1/4,
10.56, and that of the 3-3/4, 14.06; so that the light admitted by
the 3-1/4 compared with the 2-3/4 aperture, is nearly as 10 to 7; and
the light admitted by the 3-3/4 object-glass is nearly double that of
the 2-3/4 aperture, and will bear nearly a proportional increase of
magnifying power.

3. _The 5 feet Achromatic telescope._ The focal length of the
object-glass of this telescope is 5 feet 3 inches, and the diameter
of its aperture 3-8/10 inches. The usual magnifying powers applied
to it are, for land objects 65 times; and for celestial objects,
110, 190, 250, and sometimes one or two higher powers. The quantity
of light it possesses is not much larger than that of the 3-1/2 feet
telescope, with 3-3/4 inches aperture; but the larger focal length of
this telescope is considered to be an advantage; since the longer the
focus of the object-glass, the less will be its chromatic and spherical
aberrations, and the larger may be the eye-glasses, and the flatter the
field of view.

The following are the prices of these telescopes as marked in Mr.
Tulley’s catalogue.

                                                               £   s. d.
  5 feet telescopes 3-3/4 inches aperture, on an universal
  equatorial stand, with achromatic finder, 4 eye pieces for
  astronomical purposes, and 1 for day objects to vary the
  power, packed in a mahogany box               100 guineas to 157 10  0

  7 feet ditto, 5 inches aperture, on a newly improved
  universal equatorial stand, 6 eye pieces for astronomical
  purposes, and 1 for day objects to vary the magnifying
  power, with achromatic finder, and Troughton’s Micrometer     207 5  0

The above are all the kinds of achromatic telescopes _generally_
made by the London opticians. Those of the larger kind, as 5 and 7
feet telescopes, and the 3-1/2 feet with 3-3/4 inches aperture,
are generally made to order, and are not always to be procured. But
the 2-1/2 and 3-1/2 feet achromatics of 2-3/4 inches aperture, are
generally to be found ready-made at most of the optician’s shops in the
metropolis. The prices of these instruments are nearly the same in most
of the optician’s shops in London. Some of them demand a higher price,
but few of them are ever sold lower than what has been stated above,
unless in certain cases, where a discount is allowed.

[Illustration: _figure 57._]

The stands for these telescopes, and the manner in which they are
fitted up for observation are represented in figures 57, 58, and 59.
Fig. 57 represents either the 2-1/2 or the 3-1/2 feet telescopes
mounted on a plain brass stand, to be placed on a table. A is the long
eye-piece for land objects, and B the small eye-piece for astronomical
observation, which is composed of two lenses, and represents the object
in an inverted position. These eye-pieces are screwed on, as occasion
requires, at E, the eye-end of the telescope. The shorter of the two
astronomical eye-tubes which accompany this telescope, produces the
highest magnifying power. For adjusting the telescope to distinct
vision, there is a brass knob or button at _a_, which moves a piece of
rack-work connected with the eye-tube, which must be turned either one
way or the other till the object appears distinctly; and different eyes
frequently require a different adjustment.

Fig. 58, represents a 5 feet telescope fitted up for astronomical
observations. It is mounted on a mahogany stand, the three legs of
which are made to close up together by means of the brass frame _aaa_,
which is composed of three bars, connected with three joints in the
centre, and three other joints, connected with the three mahogany bars.
It is furnished with an apparatus for equatorial motions. The brass pin
is made to move round in the brass socket _b_, and may be tightened by
means of the finger screw _d_, when the telescope is directed nearly to
the object intended to be viewed. This socket may be set perpendicular
to the horizon, or to any other required angle; and the quantity of
the angle is ascertained by the divided arc, and the instrument made
fast in that position by the screw _e_. If this socket be set to the
latitude of the place of observation, and the plane of this arc be
turned so as to be in the plane of the meridian, the socket _b_ being
fixed to the inclination of the pole of the earth, the telescope
when turned in this socket, will have an equatorial motion, so that
celestial objects may be always kept in view, when this equatorial
motion is performed. The two handles at _k_ are connected with
rack-work, intended to move the telescope in any required direction.
The two sets of brass sliding rods _ii_ are intended to render the
telescope as steady as possible, and to elevate and depress it at
pleasure, and are so constructed as to slide into each other with the
utmost ease.

[Illustration: _figure 58._]

The _Finder_ is placed at AE, either on the top or the left side of
the tube of the telescope. When high magnifying powers are applied to
any telescope, it is sometimes difficult, on account of the smallness
of the field of view, to direct the main tube of the telescope to the
object. But the Finder, which is a telescope with a small power, and
consequently has a large field of view--when directed to any object, it
is easily found, and being brought to the centre of the field, where
two cross hairs intersect each other, it will then be seen in the
larger telescope. B is the eye-tube for terrestrial objects, containing
4 glasses, and C, one of the astronomical eye-pieces. A socket is
represented at _g_, containing a stained glass, which is screwed to any
of the eye-pieces, to protect the eye from the glare of light, when
viewing the spots of the sun. The brass nut above _f_, is intended for
the adjustment of the eye-piece to distinct vision. The 3-1/2 feet
telescope is sometimes mounted in this form.

Fig. 59, represents a 5 or 6 feet telescope, mounted on a stand of a
new construction by Dollond. It possesses the advantage of supporting
the telescope in two places, which renders it extremely steady--a
property of great importance when viewing celestial objects with high
magnifying powers. It possesses likewise, the advantage of enabling the
observer to continue seated at the same height from the floor, although
the telescope be raised to any altitude--_the elevation being entirely
at the object end_, although it may be changed from the horizon to
the zenith. The frame-work is composed of bars of mahogany, and rests
on three castors, two of which are made fast to their respective legs
in the usual way, and the third stands under the middle of the lower
horizontal bar that connects the two opposite legs, so that the frame
has all the advantages of a tripod. As it becomes very inconvenient to
stoop to the eye end of a telescope, when the altitude of an object is
considerable, and the centre of motion at the middle of the tube, this
construction of a stand serves to remedy such inconvenience.

[Illustration: _figure 59._]


_Proportions of curvature of the lenses which form an achromatic
object-glass._

As some ingenious mechanics may feel a desire to attempt the
construction of a compound achromatic object-glass, I shall here state
some of the proportions of curvature of the concave and convex lenses,
which serve to guide opticians in their construction of achromatic
instruments. These proportions are various; and even when demonstrated
to be mathematically correct, it is sometimes difficult to reduce them
to practice, on account of the different powers of refraction and
dispersion possessed by different discs of crown and flint-glass, and
of the difficulty of producing by mechanical means, the exact curves
which theory requires. The following table shows the radii of curvature
of the different surfaces of the lenses necessary to form _a double
achromatic object-glass_--it being supposed that the sine of refraction
in the crown-glass is as 1.528 to 1, and in the flint as 1.5735 to 1;
the ratio of their dispersive powers being as 1 to 1.524. It is also
assumed that the curvatures of the concave lens are as 1 to 2, that
is, that the one side of this lens is ground on a tool, the radius
of which is double that of the other. The 1st column expresses the
compound focus of the object-glass in _inches_; the 2nd column states
the radius of the _anterior_ surface of the _crown_, and column 3rd,
its _posterior_ side. Column 4th expresses the radius of the anterior
surface of the _concave_ lens, and column 5th its posterior surface,
which, it will be observed, is exactly double that of the other.

  -------+-----------+-----------+-----------+-----------
  Focus  |Radius of  |Radius of  |Radius of  |Radius of
  in     |anterior   |posterior  |anterior   |posterior
  inches.|surface,   |surface.   |surface,   |surface.
         |convex.    |           |concave.   |
  -------+-----------+-----------+-----------+-----------
         |Inc.  Dec. |Inc.  Dec. |Inc.  Dec. |Inc.  Dec.
    12   |  3        |  4.   652 |  4.   171 |  8.   342
    24   |  6        |  9.   304 |  8.   342 | 16.   684
    30   |  7.    5  | 11.   063 | 10.   428 | 20.   856
    36   |  9        | 13.   956 | 12.   513 | 25.   027
    48   | 12        | 18.   608 | 16.   684 | 33.   369
    60   | 15        | 23.   260 | 20.   856 | 41.   712
   120   | 30        | 46.   520 | 41.   712 | 83.   424

From the above table it will be seen, that to construct, for example,
a 30 inch compound object-glass, the radius of the anterior side of
the crown must be 7-1/2 inches, and that of the posterior side 11.63
inches; the radius of the anterior surface of the concave 10.428, and
that of the posterior 20.856 inches. It may be proper to observe,
that in these computations, the radius of the anterior surface of the
concave is less than the posterior side of the convex, and consequently
admits of its approach, without touching in the centre--a circumstance
which always requires to be guarded against in the combination of
achromatic glasses. The following table shows the radii of curvature of
the lenses of a _triple_ object-glass, calculated from formula deduced
by Dr. Robison of Edinburgh.

  -------+--------------------+---------------------+-------------------
  Focal  |  Convex lens of    |  Concave lens of    | Convex lens of
  length.|   crown glass.     |    flint glass.     |   crown glass.
  -------+--------------------+---------------------+-------------------
  Inches |Inc. Dec. Inc. Dec. |Inc.  Dec. Inc. Dec. |Inc.  Dec Inc. Dec.
    6    |  4. 54     3.  03  |  3.  03     6.  36  |  6.  36    0.  64
    9    |  6. 83     4.  56  |  4.  56     9.  54  |  9.  54    0.  92
   12    |  9. 25     6.  17  |  6.  17    12.  75  | 12.  75    1.  28
   18    | 13. 67     9.  12  |  9.  12    19.  08  | 19.  08    1.  92
   24    | 18. 33     12. 25  | 12.  25    25.  50  | 25.  50    2.  56
   30    | 22. 71     15. 16  | 15.  16    31.  79  | 31.  79    3.  20
   36    | 27. 33     18. 25  | 18.  25    38.  17  | 38.  17    3.  84
   42    | 31. 87     21. 28  | 21.  28    44.  53  | 44.  53    4.  48
   48    | 36. 42     24. 33  | 24.  33    50.  92  | 50.  92    5.  12
   54    | 40. 96     27. 36  | 27.  36    57.  28  | 57.  28    5.  76
   60    | 45. 42     30. 33  | 30.  33    63.  58  | 63.  58    6.  40

The following table contains the proportions of curvature, said to be
employed by the London opticians.

  --------+-----------------------+-------------+------------------------
          |                       |             |
          |                       |  Radius of  |
          |                       |  both the   |
   Focal  |   Convex of crown     | surfaces of |    Convex lens of
  length. |         glass.        | the concave |      crown glass.
          |                       |  of flint   |
          |                       |    glass    |
  --------+-----------------------+-------------+------------------------
          |           |           |             |            |
  Inches. | Inc. Dec. | Inc. Dec. | Inc. Dec.   | Inc.  Dec. | Inc.  Dec.
      6   |   3.  77  |  4.   49  |  3.   47    |  3.    77  |  4.    49
      9   |   5.  65  |  6.   74  |  5.   21    |  5.    65  |  6.    74
     12   |   7.  54  |  8.   99  |  6.   95    |  7.    54  |  8.    99
     18   |  11.  30  | 13.   48  | 10.   42    | 11.    30  | 13.    48
     24   |  15.  08  | 17.   98  | 13.   90    | 15.    08  | 17.    98
     36   |  22.  61  | 26.   96  | 20.   84    | 22.    61  | 26.    96
     42   |  26.  38  | 31.   45  | 24.   31    | 26.    38  | 31.    45
     48   |  30.  16  | 35.   96  | 27.   80    | 30.    16  | 35.    96
     54   |  33.  91  | 40.   45  | 31.   27    | 33.    91  | 40.    45
     60   |  37.  68  | 44.   94  | 34.   74    | 37.    68  | 44.    94

From this table it appears, that the two convex lenses, have the same
radii of their respective sides and that the concave flint lens has
its two surfaces equally concave, so that a triple object-glass formed
according to these proportions, would require only three pair of
grinding tools. The following are the curves of the lenses of one of
the best of Dollond’s achromatic telescopes, the focal length of the
compound object-glass being 46 inches. Reckoning from the surface next
the object--the radii of the crown-glass were 28 and 40 inches: the
concave lens 20.9 inches, and the inner crown-glass lens, 28.4 and 28.4
inches. This telescope carried magnifying powers of from 100 to 200
times.

Although I have inserted the above tables, which might in some measure
guide an ingenious artist, yet on the whole, a private amateur has
little chance in succeeding in such attempts. The diversity of glasses,
and the uncertainty of an unpractised workman’s producing the precise
curvatures he intends, is so great, that the object-glass, for the
most part, turns out different from his expectations. The great
difficulty in the construction is to find the exact proportion of the
dispersive powers of the crown and flint glass. The crown is pretty
constant, but there are hardly two pots of flint glass which have the
same dispersive power. Even if constant, it is difficult to measure
it accurately; and an error in this greatly affects the instrument;
because the focal distances of the lenses must be nearly as their
dispersive powers. In the two preceding tables, the sine of incidence,
in the crown glass, is supposed to be to the sine of refraction as
1.526 to 1; and in the flint glass, as 1.604 to 1. Opticians who make
great numbers of lenses both of flint and crown glass, acquire, in
time, a pretty good guess of the nature of the errors which may remain
after they have finished an object-glass; and having many lenses
intended to be of the same form, but unavoidably differing a little
from it, they try several of the concaves with the two convexes, and
finding one better than the rest, they make use of it to complete the
set. In this way some of the best achromatic telescopes are frequently
formed. I have sometimes found, when supplying a concave flint glass
to a telescope where it happened to be wanting, that, of four or five
concave lenses which appeared to be the same as to curvature and other
properties, only one was found to produce a distinct and colourless
image. Should any one, however, wish to attempt the construction of
an achromatic lens, the best way for preventing disappointments in
the result is, to procure a _variety_ of tables of the respective
curvatures founded on _different conditions_, and which, of course,
require the surfaces of the several lenses to be of different curves.
Having lenses of different radii at his command, and having glass of
different refractive or dispersive powers, when one combination does
not exactly suit, he may try another, and ultimately may succeed in
constructing a good achromatic telescope; for, in many cases, it has
been found that chance, or a happy combination of lenses by trial, has
led to the formation of an excellent object-glass.


_Achromatic telescopes composed of fluid lenses._

The best achromatic telescopes, when minutely examined, are found to be
in some respects defective, on account of that slight degree of colour
which, by the aberration of the rays, they give to objects, unless
the object-glass be of small diameter. When we examine with attention
a good achromatic telescope we find that it does not show white or
luminous objects perfectly free from colour, their edges being tinged
on one side with a claret- fringe, and on the other with a
green fringe. This telescope, therefore, required farther improvement,
to get rid of these secondary colours, and Father Boscovich, to whom
every branch of optics is much indebted, displayed much ingenuity in
his attempts to attain this object. But it is to Dr. Blair, professor
of astronomy in Edinburgh, that we are chiefly indebted for the first
successful experiments by which this end was accomplished. By a
judicious set of experiments, he proved that the quality of dispersing
the rays in a greater degree than crown-glass, is not confined to a few
mediums; but is possessed by a great variety of fluids, and by some of
these in a most extraordinary degree. Having observed that when the
extreme red and violet rays were perfectly united, the green were left
out, he conceived the idea of making an achromatic concave lens which
should refract the green less than the united red and violet, and an
achromatic convex lens which should do the same, and as the concave
lens refracted the outstanding green _to_ the axis, while the concave
one refracted them _from_ the axis, it followed, that, by a combination
of these two opposite effects, the green would be united with the red
and violet.

By means of an ingenious prismatic apparatus, he examined the optical
properties of a great variety of fluids. The solutions of metals and
semi-metals proved in all cases more dispersive than crown glass. Some
of the salts, such as sal-ammoniac, greatly increased the dispersive
power of water. The marine acid disperses very considerably, and this
quality increases with its strength. The most dispersive fluids were
accordingly found to be those in which this acid and the metals were
combined. The chemical preparation called _causticum antimoniale_, or
butter of antimony, in its most concentrated state, when it has just
attracted sufficient humidity to render it fluid, possesses the quality
of dispersing the rays in an astonishing degree. The great quantity
of the semi-metal retained in solution, and the highly concentrated
state of the marine acid, are considered as the cause of this striking
effect. Corrosive sublimate of mercury, added to a solution of
_sal-ammoniacum_ in water, possesses the next place to the butter of
antimony among the dispersive fluids, which Dr. Blair examined. The
essential oils were found to hold the next rank to metallic solutions,
among fluids which possess the dispersive quality, particularly those
obtained from bituminous minerals, as native petrolea, pit coal, and
amber. The dispersive power of the essential oil of sassafras, and
the essential oil of lemons, when genuine, were found to be not much
inferior to any of these. But of all the fluids fitted for optical
purposes, Dr. Blair found that _the muriatic acid mixed with a metallic
solution_, or, in other words, a fluid in which the marine acid and
metalline particles, hold a due proportion, most accurately suited his
purpose. In a spectrum formed by this fluid the green were among the
most refrangible rays, and when its dispersion was corrected by that
of glass, there was produced an inverted secondary spectrum, that is,
one in which the green was above, when it would have been below with a
common medium. He therefore placed a concave lens of muriatic acid with
a metallic solution between the two lenses, as in fig. 60, where AB is
the concave fluid lens, CF a plano-convex lens, with its plane side
next the object, and ED, a meniscus. With this object-glass the rays of
different colours were bent from their rectilineal course with the same
equality and regularity as in reflection.

[Illustration: _figure 60._]

Telescopes constructed with such object-glasses were examined by the
late Dr. Robison and professor Playfair. The focal distance of the
object-glass of one of these did not exceed 17 inches, and yet it
bore an aperture of 3-1/2 inches. They viewed some single and double
stars and some common objects with this telescope; and found, that,
in magnifying power, brightness, and distinctness, it was manifestly
superior to one of Mr. Dollond of 42 inches focal length. They had
most distinct vision of a star, _when using an erecting eye-piece_,
which made this telescope magnify more than a 100 times; and they found
the field of vision as uniformly distinct as with Dollond’s 42 inch
telescope magnifying 46 times; and were led to admire the nice figuring
and centering of the very deep eye-glasses which were necessary for
this amplification. They saw double stars with a degree of perfection
which astonished them. These telescopes, however, have never yet come
into general use; and one reason perhaps, is, that they are much more
apt to be deranged, than telescopes constructed of object-glasses
which are solid. If any species of glass, or other solid transparent
substance could be found with the same optical properties, instruments
might perhaps be constructed of a larger size, and considerably
superior to our best achromatic telescopes.[23] It is said that Mr.
Blair, the son of Dr. Blair, some years ago, was engaged in prosecuting
his father’s views, but I have not heard any thing respecting the
result of his investigations.


_Barlow’s refracting telescope with a fluid concave lens._

Professor Barlow, not many years ago, suggested a new fluid telescope,
which is deserving of attention; and, about the year 1829 constructed
one of pretty large dimensions. The fluid he employs for this purpose
is the _sulphuret of Carbon_, which he found to be a substance which
possessed every requisite he could desire. Its index is nearly the
same as that of the best flint glass, with a dispersive power more
than double. It is perfectly colourless, beautifully transparent,
and although very expansible, possesses the same, or very nearly
the same optical properties under all circumstances to which it is
likely to be exposed in astronomical observations--except perhaps,
direct observations on the solar disc, which will probably be found
inadmissible. Mr. Barlow first constructed an object-glass with this
fluid of 3 inches aperture, with which he could see the small star in
Polaris with a power of 46, and with the higher powers several stars
which are considered to require a good telescope, for example 70, ρ
Ophinchi, 39 Bootis, the quadruple star ε Lyræ, ζ Aquarii, α Herculis,
&c. He next constructed a 6 inch object-glass. With this instrument
the small star in Polaris is so distinct and brilliant, with a power
of 143, that its transit might be taken with the utmost certainty. As
the mode of constructing these telescopes is somewhat novel, it may be
expedient to enter somewhat into detail.

In the usual construction of achromatic telescopes, the two or three
lenses composing the object-glass are brought into immediate contact;
and in the fluid telescope of Dr. Blair, the construction was the same,
the fluid having been enclosed in the object-glass itself. But in Mr.
Barlow’s telescope, the fluid correcting lens is placed at a distance
from the plate lens equal to half its focal length; and it might be
carried still farther back, and yet possess dispersive power to render
the object-glass achromatic. By this means the fluid lens--which is the
most difficult part of the construction--is reduced to one half or
to less than one half of the size of the plate lens; consequently, to
construct a telescope of 10 or 12 inches aperture involves no greater
difficulty in the manipulation, than in making a telescope of the usual
description of 5 or 6 inches aperture, except in the simple plate lens
itself; and, hence, a telescope of this kind, of 10 or 12 feet length,
will be equivalent in its focal power to one of 16 or 20 feet. By this
means, the tube may be shortened several feet and yet possess a focal
power more considerable than could be conveniently given to it on the
usual principle of construction. This will be better understood from
the annexed diagram. (fig. 61.)

[Illustration: _figure 61._]

In this figure ABCD represent the tube of the 6 inch telescope, CD, the
plate object-glass, F the first focus of rays, _de_ the fluid concave
lens, distant from the former 24 inches. The focal length MF being
48, and consequently, as 48 : 6 :: 24 : 3 inches, the diameter of the
fluid lens. The resulting compound focus is 62.5 inches. It is obvious,
therefore, that the rays _df_, _ef_, arrive at the focus under the same
convergency, and with the same light as if they proceeded from a lens
of 6 inches diameter, placed at a distance beyond the object-glass
CD (as GH,) determined by producing those rays till they meet the
sides of the tube in GH, namely at 62.5 inches beyond the fluid lens.
Hence, it is obvious, the rays will converge as they would do from an
object-glass GH of the usual kind with a focus of 10 feet 5 inches. We
have thus, therefore, shortened the tube 38.5 inches, or have at least
the advantage of a focus 38.5 inches longer than our tube; and the
same principle may be carried much farther, so as to reduce the usual
length of refracting telescopes nearly one half without increasing
the aberration in the first glass beyond the least that can possibly
belong to a telescope of the usual kind of the whole length. It should
likewise be observed that the adjustment for focus may be made either
in the usual way, or by a slight movement of the fluid lens, as in the
Gregorian Reflectors, by means of the small speculum.

Mr. Barlow afterwards constructed another and a larger telescope on the
same principle, the clear aperture of which is 7.8 inches. Its tube is
11 feet, which, together with the eye-piece, makes the whole length
12 feet, but its effective focus is on the principle stated above, 18
feet. It carries a power of 700 on the closest double stars in South’s
and Herschel’s catalogue, and the stars are, with that power, round
and defined, although the field is not then so bright as could be
desired. The telescope is mounted on a revolving stand, which works
with considerable accuracy as an azimuth and altitude instrument. To
give steadiness to the stand it has been made substantial and heavy;
its weight by estimation being 400 pounds, and that of the telescope
130 pounds, yet its motions are so smooth, and the power so arranged,
that it may be managed by one person with the greatest ease, the star
being followed by a slight touch, scarcely exceeding that of the keys
of a piano-forte. The focal length of the plate lens is 78 inches,
and of the fluid lens 59.8 inches--which at the distance of 40 inches
produce a focal length of 104 inches, a total length of 12 feet, and
an equivalent focus of 18 feet. The curves of the parallel meniscus
checks for containing the fluid are--30 inches, and 144 inches, the
latter towards the eye. The curves for the plate lens are 56.4 and
144. There is an interior tube 5 inches diameter, and 3 feet 6 inches
long, which carries the cell in which the fluid is enclosed, and an
apparatus by which it may be moved backwards and forwards, so that
the proper adjustment may be made for colour, in the first instance,
and afterwards the focus is obtained by the usual rack-work motion.
The following is the mode by which the fluid was enclosed. After the
best position has been determined practically for the checks forming
the fluid lens, these, with the ring between them ground and polished
accurately to the same curves, are applied together, and taken into
an artificial high temperature, exceeding the greatest at which the
telescope is ever expected to be used. After remaining here with the
fluid some time, the space between the glasses is completely filled,
immediately closed, cooled down by evaporation, and removed into a
lower temperature. By this means a sudden condensation takes place, an
external pressure is brought on the checks, and a bubble formed inside,
which is of course filled with the vapour of the fluid; the excess of
the atmospheric pressure beyond that of the vapour being afterwards
always acting externally to prevent contact. The extreme edges are then
sealed with the serum of human blood, or by strong fish-glue, and some
thin pliable metal surface. By this process, Mr. Barlow says, ‘I have
every reason to believe the lens becomes as durable as any lens of
solid glass. At all events I have the satisfaction of stating, that my
first 3 inch telescope has now been completed more than fifteen months,
and that no change whatever has taken place in its performance, nor the
least perceptible alteration either in the quantity or the quality of
the fluid.’

The following are some of the observations which have been made with
this telescope, and the tests to which it has been subjected. The very
small star which accompanies the pole-star is generally one of the
first tests applied to telescopes. This small point of light appeared
brilliant and distinct; it was best seen with a power of 120, but was
visible with a power of 700. The small star in Aldebaran was very
distinct with a power of 120. The small star α Lyræ was distinctly
visible with the same power. The small star called by Sir J. Herschel
_Debilissima_, between 4 ε and 5 Lyræ, whose existence, he says, could
not be suspected in either the 5 or 7 feet equatorial, and invisible
also with the 7 and 10 feet reflectors of six and 9 inches aperture,
but seen double with the 20 feet reflector, is seen very satisfactorily
double with this telescope. η Persei, marked as double in South and
Herschel’s catalogue, at the distance of 28´´, with another small star
at the distance of 3´ 67´´, is seen distinctly sixfold, four of the
small stars being within a considerably less distance than the remote
one of η marked in the catalogue. And, rejecting the remote star, the
principal, and the four other stars, form a miniature representation
of Jupiter and his satellites, three of them being nearly in a line
on one side, and the other on the opposite. _Castor_, is distinctly
double with 120, and well opened and stars perfectly round with 360
and 700: γ Leonis and α Piscium are seen with the same powers equally
round and distinct. In ε Bootis, the small star is well separated
from the larger, and its blue colour well marked with a power of 360.
η Coronæ Borealis is seen double with a power of 360 and 700. 52
Orionis, ζ Orionis, and others of the same class are also well defined
with the same powers. In regard to the planets which happened to be
visible--Venus appeared beautifully white and well defined with a power
of 120, but showed some colour with 360. Saturn with the 120 power,
is a very brilliant object, the double ring and belts being well and
satisfactorily defined, and with the 360 power, it is still very fine.
The moon also is remarkably beautiful, the edges and the shadows being
well marked, while the quantity of light is such as to bring to view
every minute distinction of figure and shade.

The principal objections that may be made to this construction of a
telescope are such as these:--Can the fluid be permanently secured?
Will it preserve its transparency and other optical properties? Will
it not act upon the surface of the glass and partially destroy it? &c.
To such enquiries Mr. Barlow replies, that experience is the only test
we have; our spirit levels, spirit thermometers, &c., show that some
fluids at least may be preserved for many years, without experiencing
any change, and without producing any in the appearance of the glass
tubes containing them. But should any of these happen, except the last,
nothing can be more simple than to supply the means of replacing the
fluid at any time, and by any person, without disturbing the adjustment
of the telescope. He expresses his hope that, should these experiments
be prosecuted, an achromatic telescope may ultimately be produced
which shall exceed in aperture and power, any instruments of the kind
hitherto attempted. If the prejudice against the use of fluids could be
removed, he feels convinced that well-directed practice would soon lead
to the construction of the most perfect instruments, on this principle,
at a comparatively small expense. ‘I am convinced,’ he says, ‘judging
from what has been paid for large object-glasses, that my telescope,
telescope stand, and the building for observation, with every other
requisite convenience, have been constructed for a less sum than would
be demanded for the object-glass only, if one could be produced of the
same diameter of plate and flint-glass; and this is a consideration
which should have some weight, and encourage a perseverance in the
principle of construction.’[24]


ROGERS’ ACHROMATIC TELESCOPE ON A NEW PLAN.

The object of this construction is to render a small disc of
flint-glass available to perform the office of compensation to a much
larger one of crown-glass, and thus to render possible the construction
of telescopes of much larger aperture than are now common, without
hindrance from the difficulty at present experienced in procuring large
discs of flint-glass. It is well known to those who are acquainted
with telescopes, that in the construction of an ordinary achromatic
object-glass, in which a single crown lens is compensated by a single
one of flint, the two lenses admit of being separated only by an
interval too small to afford any material advantage, _in diminishing
the diameter of the flint lens_, by placing it in a narrower part
of the cone of rays--the actual amount of their difference in point
of dispersive power being such as to render the correction of the
chromatic aberration impossible, when their mutual distance exceeds
a certain limit. This inconvenience Mr. Rogers proposes to obviate,
by employing, as a correcting lens--not a single lens of flint, but a
compound one consisting of a convex crown and concave flint, whose foci
are such as to cause their combination to act as a plain glass on the
mean refrangible rays. Then it is evident, that by means of the greater
dispersive power of flint than of crown glass, this will act as a
concave on the violet, and as a convex on the red rays, and _that_ the
more powerfully, according as the lenses separately have greater powers
or curvature. If then, such a compound lens be interposed between the
object-glass of a telescope--supposed to be a single lens of plate or
crown-glass--and its focus, it will cause no alteration in the focus
for mean rays, while it will lengthen the focus for violet, and shorten
it for red rays. Now this is precisely what is wanted to produce an
achromatic union of all the rays in the focus; and as nothing in this
construction limits the powers of the individual correcting lenses,
they may therefore be applied any where that convenience may dictate;
and thus, theoretically speaking, a disc of flint-glass, however small,
may be made to correct the colour of one of crown however large.

This construction, likewise, possesses other and very remarkable
advantages. For, first, when the correcting lens is approximately
constructed on a calculation founded on its intended aperture, and on
the refractive and dispersive indices of its materials, the final and
complete dispersion of colour may be effected, not by altering the
lenses by grinding them anew, but by shifting the combination nearer
to, or farther from, the object-glass, as occasion may require, along
the tube of a telescope, by a screw motion, till the condition of
achromaticity is satisfied in the best manner possible. And secondly,
the spherical aberration may in like manner be finally corrected, by
slightly separating the lenses of the correcting glass, whose surfaces
should for this purpose be figured to curvatures previously determined
by calculation, to admit of this mode of correction--a condition which
Mr. Rogers finds to be always possible. The following is the rule
he lays down for the determination of the foci of the lenses of the
correcting glass:--‘The focal length of either lens of the correcting
lens is to that of the object-glass, in a ratio compounded of the ratio
of the square of the aperture of the correcting lens to that of the
object-glass, and of the ratio of the difference of the dispersive
indices of the crown and flint glass, to the dispersive index of
crown.’ For example, to correct the colour of a lens of crown or plate
glass of 9 inches aperture, and 14 feet focal length (the dimensions
of the telescope of Fraunhofer at Dorpat) by a disc of flint glass 3
inches in diameter, the focus of either lens of the correcting lens
will require to be about 9 inches. To correct it by a 4 inch disc will
require a focus of about 16 inches each.

Mr. Rogers remarks, that it is not indispensable to make the
correcting glass act as a plane lens. It is sufficient if it be so
adjusted as to have a shorter focus for red rays than for violet.
If, preserving this condition, it be made to act as a concave lens,
the advantage procured by Mr. Barlow’s construction of reducing the
length of the telescope with the same focal power, is secured, and he
considers, moreover, that by a proper adaptation of the distances,
foci, &c., of the lenses, we might hope to combine with all these
advantages that of the destruction of the secondary spectrum, and thus
obtain a perfect telescope.

The above is an abstract of a paper read to the ‘Astronomical Society
of London’ in April 1828, by A. Rogers, Esq.

The reader will easily perceive that the principle on which Mr.
Rogers proposes to construct his telescope is very nearly similar
to that of professor Barlow, described above, with this difference,
that the correcting lens of the Professor’s telescope is composed of
a transparent _fluid_, while that of Mr. Rogers is a _solid_ lens
consisting of a convex crown and concave flint. The general object
intended to be accomplished by both is the same, namely, to make a
correcting lens of a comparatively small diameter serve the purpose of
a large disc of flint glass, which has hitherto been very expensive,
and very difficult to be procured; and likewise to reduce the length of
the telescope while the advantage of a long focal power is secured.--A
telescope, on this principle, was constructed 7 or 8 years ago by
Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he
was aware that Mr. Rogers had proposed a similar plan. I have had an
opportunity of particularly inspecting Mr. Wilson’s telescope, and
trying its effects on terrestrial objects with high powers, and was on
the whole highly pleased with its performance. It appeared to be almost
perfectly achromatic, and produced a distinct and _well-defined_ image
of minute distant objects, such as small letters on sign-posts, at 2,
3 and 4 miles distant. But I had no opportunity of trying its effects
on double stars or any other celestial objects. The instrument is above
6 feet long; the object lens is a plano-convex of crown glass 4 feet
focal distance, and 4 inches diameter, the plain side next the object.

At 26 inches distant from the object lens is the compound lens of 2
inches in diameter; and the two lenses of which it is composed are
both ground to a radius of 3-3/4 inches. That made of crown glass is
_plano-convex_, the other, made of flint glass, is plano-concave, and
are placed close together, the convex side being next the object, and
the concave side next the eye. The greater refractive power of the
flint glass renders the compound one slightly concave in its effect
(although the radius of curvature is similar in both), and lengthens
the focus to 6 feet from the object-glass; and this is consequently the
length of the instrument. The compound corrector so placed intercepts
all those rays which go to form the image in the field of view,
producing there an achromatic image. The concave power of the corrector
renders the image larger than if directly produced by a convex lens
of the same focus. The concavity of the corrector is valuable also in
this respect, that a very slight alteration in its distance from the
object-glass, changes the focal distance much more than if it were
plain, and enables us to adjust the instrument to perfect achromatism
with great precision.




CHAPTER V.


ON REFLECTING TELESCOPES.

SECT. 1.--HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE
CONSTRUCTION OF THESE INSTRUMENTS.

Reflecting telescopes are those which represent the images of distant
objects by reflection, chiefly from concave mirrors.

Before the achromatic telescope was invented, there were two glaring
imperfections in refracting telescopes, which the astronomers of
the 17th century were anxious to correct. The first was its very
great length when a high power was to be applied, which rendered it
very unwieldy and difficult to use. The second imperfection was the
incorrectness of the image as formed by a single lens. Mathematicians
had demonstrated that a pencil of rays could not be collected in a
single point by a spherical lens, and also that the image transmitted
by such a lens would be in some degree incurvated. After several
attempts had been made to correct this imperfection by grinding lenses
to the figure of one of the conic sections, Sir I. Newton happened to
commence an examination of the colours formed by a prism; and having,
by the means of this simple instrument, discovered the different
refrangibility of the rays of light--to which we have several times
adverted in the preceding descriptions--he then perceived that the
errors of telescopes, arising from that cause alone, were some hundred
times greater than such as were occasioned by the spherical figure
of lenses; which induced this illustrious philosopher to turn his
attention to the improvement of telescopes by reflection.

It is generally supposed that Mr. James Gregory--a son of the Rev. John
Gregory, minister of Drumoak in the county of Aberdeen--was the first
who suggested the construction of a reflecting telescope. He was a
young man of uncommon genius, and an eminent mathematician; and in the
year 1663, at the age of only 24, he published in London, his treatise
entitled ‘Optica Promota,’ in which he explained the theory of that
species of reflecting telescope which still bears his name, and which
he stated as being his own invention. But as Gregory, according to his
own account, was endowed with no mechanical dexterity, and could find
no workman capable of realizing his invention--after some fruitless
attempts to form proper specula, he was obliged to give up the pursuit;
so that this telescope remained for a considerable time neglected.
It was several years after Gregory suggested the construction of
reflecting telescopes, till Newton directed his attention fully to the
subject. In a letter addressed to the secretary of the Royal Society,
dated in February, 1672, he says, ‘Finding reflections to be regular,
so that the angle of reflection of all sorts of rays was equal to
the angle of incidence, I understood that, by their mediation, optic
instruments might be brought to any degree of perfection imaginable,
providing a reflecting substance could be found which would polish as
finely as glass, and reflect as much light as glass transmits, and the
art of communicating to it a parabolic figure be also obtained. Amidst
these thoughts I was forced from Cambridge by the intervening plague,
and it was more than two years before I proceeded further.’

It was towards the end of 1668, or in the beginning of the following
year, when Newton, being obliged to have recourse to reflectors, and
not relying on any artificer for making the specula, set about the work
himself, and early in the year 1672, completed two small reflecting
telescopes. In these he ground the great speculum into a spherical
concave, although he approved of the parabolic form, but found himself
unable to accomplish it. These telescopes were of a construction
somewhat different from what Gregory had suggested, and though only 6
inches long, were considered as equal to a 6 feet common refracting
telescope. It is not a little singular, however, that we hear no more
about the construction of reflectors till more than half a century
afterwards. It was not till the year 1723, that any reflectors were
known to have been made, adapted to celestial observations. In that
year, Mr. Hadley, the inventor of the reflecting quadrant, which goes
by his name, published in No. 376 of the Philosophical Transactions, an
account of a large reflector on Newton’s plan, which he had just then
constructed, the performance of which left no room to doubt that this
invention would remain any longer in obscurity. The large speculum of
this instrument was 62-5/8 inches focal distance and 5 inches diameter,
was furnished with magnifying powers of from 190 to 230 times, and
equalled in performance the famous aerial telescope of Huygens of 123
feet in length.[25] Since this period, the reflecting telescope has
been in general use among astronomers in most countries of Europe,
and has received numerous improvements, under the direction of Short,
Mudge, Edwards and Herschel--the last of whom constructed reflectors
of 7, 10, 20, and even 40 feet in focal length, which far surpassed,
in brightness and magnifying power, all the instruments of this
description, which had previously been attempted.

I shall now proceed to give a brief sketch of the nature of a
reflecting telescope, and the different forms in which they have been
proposed to be constructed.

Fig. 62 represents the reflecting telescope as originally proposed
by Gregory. ABEF represents a tube open at AF towards the object; at
the other end is placed a concave speculum BE, with a hole CD in its
centre, the focus of which is at _e_. A little beyond this focus,
towards the object end of the telescope AF, is placed another small
concave mirror G, having its polished face turned towards the great
speculum, and is supported by an arm GH fastened to a slider connected
with the tube. At the end of the great tube BE is screwed in a small
tube CDKI, containing a small plano-convex lens IK. Such are the
essential parts of this instrument and their relative positions. It
will be recollected in our description of the properties of concave
mirrors (see page 92), that, when rays proceed from a distant object,
and fall upon a concave-speculum, they paint an image or representation
of the object in its focus before the speculum. Now suppose two
parallel rays _ab_ falling on the speculum BE, in _cd_; they are
reflected to its focus _e_ where an inverted image of the object is
formed. This image is formed at a little more than the focal distance
of the small speculum from its surface, and serves as it were for an
object on which the small mirror may act. By the action of this mirror
this first image is reflected to a point about _f_, where a second
image is formed very large and erect. This image is magnified in the
proportion of _f_G to _e_G, the rays from which are transmitted to the
eye glass IK, through which the eye perceives the object clear and
distinct, after the proper adjustments have been made.

[Illustration: _fig. 62._]

[Illustration: _fig. 63._]

[Illustration: _fig. 64._]

[Illustration: _fig. 65._]

[Illustration: _figure 66._]

Suppose the focal distance of the great mirror was 9 inches, and the
focal distance of the small mirror 1-1/2 inch--were we to remove the
eye piece of this telescope, and look through the hole of the great
mirror, we should see the image of the object depicted upon the face of
the small speculum, and magnified, in the proportion of 9 to 1-1/2, or,
6 times, on the same principle as a common convex object glass 9 inches
focal length, with an eye glass whose focus is 1-1/2 inch magnifies 6
times. This may be regarded as the first part of the magnifying power.
If now, we suppose the small speculum placed a little more than 1-1/2
inch from the image formed by the great speculum, a second image is
formed about _f_, as much exceeding the first in its dimensions as it
exceeds it in distance from the small speculum, on the principle on
which the object glass of a compound microscope forms a large image
near the eye glass. Suppose this distance to be 9 times greater, then
the whole magnifying power will be compounded of 6 multiplied by 9, or
54 times. As a telescope it magnifies 6 times, and in the microscope
part 9 times.--Such is a _general_ idea of the Gregorian telescope, the
minute particulars and structure of which can only be clearly perceived
by a direct inspection of the instrument.

_The Newtonian Reflector._--This instrument is somewhat different
both in its form and in its mode of operation from that of Gregory.
It is represented in fig. 63, where BAEF is the tube, and BE, the
object concave mirror, which reflects the parallel rays _ab_ to a
_plane_ speculum G, placed 45°, or half a right angle to the axis of
the concave speculum. This small plane reflector must be of an oval
form, the length of the oval should be to the breadth as 7 to 5, on
account of the obliquity of its position. It is supported on an arm
fixed to the side of the tube; an eye-glass is placed in a small tube,
moveable in the larger tube, so as to be perpendicular to the axis of
the large reflector, the perpendicular line passing through the centre
of the small mirror. The small mirror is situated between the large
mirror and its focus, that its distance from this focal point may be
equal to the distance from the centre of the mirror to the focus of the
eye-glass. When the rays _ab_ from a distant object fall upon the large
speculum at _cd_, they are reflected towards a focus at _h_; but being
intercepted by the plane mirror G, they are reflected perpendicularly
to the eye-glass at I, in the side of the tube, and the image formed
near that position at _e_ is viewed through a small plano-convex lens.
The magnifying power of this telescope is in the proportion of the
focal distance of the speculum to that of the eye-glass. Thus, if the
focal distance of the speculum be 36 inches, and that of the eye-glass
1/3 of an inch, the magnifying power will be 108 times. It was this
form of the reflecting telescope, that Newton invented, which Sir. W.
Herschel adopted, and with which he made most of his observations and
discoveries.

_The Cassegrainian Reflector._--This mode of the reflecting telescope,
suggested by M. Cassegrain, a Frenchman, is represented in fig. 64. It
is constructed in the same way as the Gregorian, with the exception of
a small _convex_ speculum G being substituted in the room of the small
concave in Gregory’s construction. As the focus of a convex mirror is
negative, it is placed at a distance from the large speculum equal to
the difference of their foci, that is, if the focal length of the large
speculum be 18 inches, and that of the small convex 2 inches, they are
placed at 16 inches distant from each other, on a principle similar
to that of the Galilean telescope, in which the concave eye-glass is
placed within the focus of the object-glass by a space equal to the
focal length of the eye-glass. In this telescope, likewise, instead
of two there is only _one image_ formed, namely that in the focus
of the eye-glass; and, on this account some are of opinion that the
distinctness is considerably greater than in the Gregorian. Mr. Ramsden
was of opinion that this construction is preferable to either of the
former reflectors, because the aberrations of the two metals have a
tendency to correct each other, whereas in the Gregorian both the
metals being concave, any error in the specula will be doubled. It is
his opinion that the aberrations in the Cassegrainian construction
to that of the Gregorian is as 3 to 5. The length of this telescope
is shorter than that of a Gregorian of equal focal length, by twice
the focal length of the small mirror, and it shows every thing in
an _inverted_ position, and consequently is not adapted for viewing
terrestrial objects.

_Dr. Hook’s Reflector._--Before the reflecting telescope was much
known, Dr. Hook contrived one, the form of which is represented, fig.
65, which differs in little or nothing from the Gregorian, except that
the eye-glass I is placed in the hole of the great speculum BE.

_Martin’s Reflector._--Mr. Bengamin Martin, a distinguished writer on
optical and philosophical science, about a century ago, described a
new form of the reflecting telescope, approximating to the Newtonian
structure, which he contrived for his own use. It is represented in
fig. 66. ABEF is the tube, in which there is an opening or aperture OP,
in the upper part. Against this hole within the tube is placed a large
plane speculum GH, at half a right angle with the axis or sides of the
tubes, with a hole CD perforated through its middle. The parallel rays
_a b_ falling on the inclined plane GH are reflected perpendicularly
and parallel on the great speculum BE in the bottom of the tube. From
thence they are reflected converging to a focus _e_ through the hole of
the plane mirror CD, which being also the focus of the eye-glass IK,
the eye will perceive the object magnified and distinct.

In the figures referred to in the above descriptions, only one
eye-glass is represented to avoid complexity; but in most reflecting
telescopes, the eye-piece consists of a combination of two plano-convex
glasses, as in fig. 67, which produces a more correct and a larger
field of view than a single lens. This combination is generally known
by the name of the _Huygenian eye-piece_ which shall be described in
the section on the _eye-pieces_ of telescopes.

The following rule has been given for finding the magnifying power of
the Gregorian telescope:--Multiply the focal distance of the great
mirror by the distance of the small mirror from the image next the
eye; and multiply the focal distance of the small mirror by the focal
distance of the eye-glass; then divide the product of the former
multiplication by the product of the latter, and the quotient will
express the magnifying power. The following are the dimensions of one
of the reflecting telescopes constructed by Mr. Short--who was long
distinguished as the most eminent maker of such instruments, on a large
scale, and whose large reflectors are still to be found in various
observatories throughout Europe.

The focal distance of the great mirror 9.6 inches; or P _m_, fig. 67,
its breadth FD 2.3; the focal distance of the small mirror L _n_
1.5--or 1-1/2 inch--its breadth _g h_ 0.6--or 6/10 of an inch; the
breadth of the hole in the great mirror UV, 0.5--or half an inch--the
distance between the small mirror and the next eye-glass LR, 14.2; the
distance between the two eye-glasses SR, 2.4; the focal distance of the
eye-glass next the metal, 3.8.; and the focal distance of the eye-glass
next the eye, S _a_ 1.1, or one inch and one tenth. The magnifying
power of this telescope was about 60 times. Taking this telescope as a
standard, the following table of the dimensions and magnifying powers
of Gregorian reflecting telescopes, as constructed by Mr. Short, has
been computed.

[Illustration: _figure 67._]

  A: Focal distance of the great mirror.
  B: Breadth of the great mirror.
  C: Focus of the small speculum.
  D: Breadth of the hole in the great speculum.
  E: Distance between the small speculum and the first eye-glass.
  F: Focal distance of the glass next the metals.
  G: Focal distance of the glass next the eye.
  H: Distance between the plain sides of the two glasses.
  I: Magnifying power.
  J: Distance between the second glass and the small eye-hole.
  --------+--------+--------+--------+--------
          |        |        |        |
      A.  |   B.   |  C.    |   D.   |   E.
  --------+--------+--------+--------+--------
          |        |        |        |
   P  _m_ | D   F  | L  _n_ | U   V  | L   R
          |        |        |        |
  In. Dec.|In. Dec.|In. Dec.|In. Dec.|In. Dec.
          |        |        |        |
   5.  65 | 1.  54 | 1.  10 | 0.  31 | 8.  54
   9.  60 | 2.  30 | 1.  50 | 0.  39 |14.  61
  15.  50 | 3.  30 | 2.  14 | 0.  50 |23.  81
  36.  00 | 6.  26 | 3.  43 | 0.  65 |41.  16
  60.  00 | 9.  21 | 5.  00 | 0.  85 |68.  17

  --------+--------+---------+-----+------
          |        |         |     |
     F.   |   G.   |   H.    | I.  |  J.
  --------+--------+---------+-----+------
          |        |         |     |
     R    |   S    |  R   S  |     |
          |        |         |     |
  In. Dec.|In. Dec.|In. Dec. | In. |
          |        |         |     |
   2.  44 | 0.  81 | 1.  68  |  39 | 0. 41
   3.  13 | 1.  04 | 2.  09  |  60 | 0. 52
   3.  94 | 1.  31 | 2.  63  |  86 | 0. 66
   5.  12 | 1.  71 | 3.  41  | 165 | 0. 85
   6.  43 | 2.  14 | 4.  28  | 243 | 1. 07


Mr. Short--who was born in Edinburgh in 1710, and died near London,
1768--was considered as the most accurate constructor of reflecting
telescopes, during the period which intervened from 1732, to 1768. In
1743, he constructed a reflector for Lord Thomas Spencer, of 12 feet
focal length, for which he received 600 guineas. He made several other
telescopes of the same focal distance, with greater improvements and
higher magnifiers; and in 1752, finished one for the king of Spain, for
which, with its whole apparatus, he received £1200. This was considered
the noblest instrument of its kind that had then been constructed, and
perhaps it was never surpassed, till Herschel constructed his twenty
and forty feet reflectors. High as the prices of large telescopes now
are, Mr. Short charged for his instruments at a much higher rate than
opticians now do, although the price of labour, and every other article
required in the construction of a telescope, is now much dearer. But
he had then scarcely any competitor, and he spared neither trouble nor
expense to make his telescopes perfect, and put such a price upon them
as properly repaid him. The following table contains a statement of the
apertures, powers, and prices of Gregorian telescopes, as constructed
by Mr. James Short.[26]

  A: Number.
  B: Focal length in inches.
  C: Diameter of aperture in inches.
  D: Prices in guineas.
  -----+---------+------+-----------------------------------+----
       |         |      |                                   |
    A. |    B.   |  C.  |        Magnifying powers.         | D.
  -----+---------+------+-----------------------------------+----
    1  |   3     |  1.1 | 1 Power of               18 times |   3
    2  |   4-1/2 |  1.3 | 1 "                      25   "   |   4
    3  |   7     |  1.9 | 1 "                      40   "   |   6
    4  |   9-1/2 |  2.5 | 2 Powers       40  and   60   "   |   8
    5} |  12     |  3.0 | 2 "            55  and   85   "   |  10
    6} |  12     |  3.0 | 4 "  35,  55,  85, and  110   "   |  14
    7  |  18     |  3.8 | 4 "  55,  95, 130, and  200   "   |  20
    8  |  24     |  4.5 | 4 "  90, 150, 230, and  300   "   |  35
    9  |  36     |  6.3 | 4 " 100, 200, 300, and  400   "   |  75
   10  |  48     |  7.6 | 4 " 120, 260, 380, and  500   "   | 100
   11  |  72     | 12.2 | 4 " 200, 400, 600, and  800   "   | 300
   12  | 144     | 18.0 | 4 " 300, 600, 900, and 1200   "   | 800

From this table, it appears that Mr. Short charged 75 guineas for a 3
feet reflector, whereas such an instrument is now marked in the London
opticians’ catalogues at £23, when mounted on a common brass stand, and
£39. 18s., when accompanied with rack-work motions and other apparatus.
It is now generally understood that in the above table, Short always
greatly _overrated_ the higher powers of his telescopes. By experiment
they were generally found to magnify _much less_ than here expressed.

_General remarks on Gregorian Reflectors._--1. In regard to the hole
UV, of the great speculum--its diameter should be equal, or nearly
so, to that of the small speculum L, fig. 67. For if it be less, no
more parallel rays will be reflected than if it were equal to _g h_,
and it may do harm in contracting the visible area within too narrow
limits. Nor must it be larger than the mirror L, because some parallel
rays will then be lost, and those of most consequence as being nearest
the centre. 2. The small hole at _e_ to which the eye is applied,
must be nicely adjusted to the size of the cone of rays proceeding
from the nearest lens S. If it be larger, it will permit the foreign
light of the sky or other objects to enter the eye, so as to prevent
distinct vision; for the eye should receive no light, but what comes
from the surface of the small mirror L. If the hole be smaller than
the cylinder of rays at _e_ then some of the necessary light will be
excluded, and the object rendered more obscure. The diameter of this
hole may be found by dividing the aperture of the telescope in inches
by its magnifying power. Thus, if we divide the diameter of one of
Short’s telescopes, the diameter of whose large speculum is 2.30, by
60, the magnifying power, the quotient will be .0383, which is nearly
the 1/25 of an inch. Sometimes this hole is made so small as the 1/50
of an inch. When this hole is, by any derangement, shifted from its
proper position, it sometimes requires great nicety to adjust it, and,
before it is accurately adjusted, the telescope is unfit for accurate
observation. 3. It is usual to fix a plate with a hole in it, at _a b_,
the focus of the eye glass S, of such a diameter as will circumscribe
the image, so as to exhibit only that part of it which appears
distinct, and to exclude the superfluous rays. 4. There is an adjusting
screw on the outside of the great tube, connected with the small
speculum, by which that speculum may be pushed backwards or forwards to
adjust the instrument to distinct vision. The hand is applied for this
purpose at T.

_Newtonian Telescopes._--These telescopes are now more frequently
used for celestial observations than during the last century, when
Gregorian reflectors were generally preferred. Sir W. Herschel was
chiefly instrumental in introducing this form of the reflecting
telescope to the more particular attention of astronomers, by the
splendour and extent of the discoveries which it enabled him to make.
In this telescope there is no hole required in the middle of the great
speculum, as in the Gregorian construction, which circumstance secures
the use of all the rays which flow from the central parts of the mirror.

The following table contains a statement of the apertures and
magnifying powers of Newtonian Telescopes, and the focal distances of
their eye-glasses. The first column contains the focal length of the
great speculum in feet; the second, its linear aperture in inches;
the third, the focal distance of the single glass in decimals, or in
1000ths of an inch, and the fourth column, contains the magnifying
power. This portion of the table was constructed by using the
dimensions of Mr. Hadley’s Newtonian Telescope, formerly referred to,
as a standard--the focal distance of the great mirror being 62-1/2
inches, its medium aperture 5 inches, and power 208. The fifth, sixth,
and seventh columns contains the apertures of the concave speculum,
the focal lengths of the eye-glasses and the magnifying powers, as
calculated by Sir D. Brewster, from a telescope of Mr. Hauksbee, taken
as a standard; whose focal length was 3 feet 3 inches, its aperture
about 4 inches, and magnifying power 226 times.

  A: Focal distance  of concave metal.
  B: Aperture of concave metal.
  C: Focal distance of single eye-glass.
  D: Magnifying power.
  E: Aperture of the concave speculum.
  F: Focal length of the eye-glass.
  G: Magnifying power.
  --------+----------+----------++----------------------------------
          |          |          ||
          |          |          ||    Sir D. Brewster’s Numbers.
     A.   |    B.    |    C.    ++----------------------------------
          |          |          ||     |          |            |
          |          |          ||  D. |    E.    |     F.     | G.
  --------+----------+----------++-----+----------+------------+----
          |          |          ||     |          |            |
    Feet. | In. Dec. | In. Dec. ||     | In. Dec. | In. Dec.   |
    0-1/2 |  0.  86  | 0.  167  ||  36 |  1.  34  | 0.  107    |  56
    1     |  1.  44  | 0.  199  ||  60 |  2.  23  | 0.  129    |  93
    2     |  2.  45  | 0.  236  || 102 |  3.  79  | 0.  152    | 158
    3     |  3.  31  | 0.  261  || 138 |  5.  14  | 0.  168    | 214
    4     |  4.  10  | 0.  281  || 171 |  6.  36  | 0.  181    | 265
    5     |  4.  85  | 0.  297  || 202 |  7.  51  | 0.  192    | 313
    6     |  5.  57  | 0.  311  || 232 |  8.  64  | 0.  200=1/5| 360
    7     |  6.  24  | 0.  323  || 260 |  9.  67  | 0.  209    | 403
    8     |  6.  89  | 0.  334  || 287 | 10.  44  | 0.  218    | 445
    9     |  7.  54  | 0.  344  || 314 | 11.  69  | 0.  222    | 487
   10     |  8.  16  | 0.  353  || 340 | 12.  65  | 0.  228    | 527
   11     |  8.  76  | 0.  362  || 365 | 13.  58  | 0.  233    | 566
   12     |  9.  36  | 0.  367  || 390 | 14.  50  | 0.  238    | 604
   13     |  9.  94  | 0.  377  || 414 | 15.  41  | 0.  243    | 642
   14     | 10.  49  | 0.  384  || 437 | 16.  25  | 0.  248    | 677
   15     | 11.  04  | 0.  391  || 460 | 17.  11  | 0.  252    | 713
   16     | 11.  59  | 0.  397  || 483 | 17.  98  | 0.  256    | 749
   17     | 12.  14  | 0.  403  || 506 | 18.  82  | 0.  260    | 784
   18     | 12.  67  | 0.  409  || 528 | 19.  63  | 0.  264    | 818
   19     | 13.  20  | 0.  414  || 550 | 20.  45  | 0.  268    | 852
   20     | 13.  71  | 0.  420  || 571 | 21.  24  | 0.  271    | 885

One great advantage of reflecting telescopes above common refractors,
is, that they will admit of eye glasses of a much shorter focal
distance, and consequently, will magnify so much the more, for the
rays are not  by reflection from a concave mirror, if it be
ground to a true figure, as they are by passing through a convex
glass though figured and polished with the utmost exactness. It will
be perceived from the above table, that the focal length of the eye
glasses is very small, the lowest there stated being only about 1/10
of an inch, and the highest little more than 1/4 of an inch focal
distance. Sir W. Herschel obtained the high powers which he sometimes
put upon his telescopes, by using small double convex lenses for eye
glasses, some of which did not exceed the _one fiftieth of an inch_ in
focal length. When the focal length of the concave speculum, and that
of the eye glass are given, the magnifying power is found by dividing
the former by the latter, after having reduced the focal length of the
concave speculum to inches. Thus the 6 feet speculum, multiplied by 12,
produces 72 inches, which, divided by Brewster’s number for the focus
of the eye glass = 200, or 1/5 of an inch, produces a quotient of 360
as the magnifying power. It has been calculated that, if the metals
of a Newtonian telescope be worked as exquisitely as those in Sir W.
Herschel’s 7 feet reflector, the highest power that such a telescope
should bear with perfect distinctness, will be found by multiplying the
diameter of the great speculum in inches, by 74, and the focal distance
of the single eye glass may be found by dividing the focal distance of
the great mirror by the magnifying power. Thus 6.25--the aperture in
inches of Herschel’s 7 feet Newtonian--multiplied by 74 is 462-1/2, the
magnifying power; and 7 multiplied by 12, and divided by 462.5 is 0.182
of an inch, the focal distance of the single eye glass required. But
it is seldom that more than one half of this power can be applied with
effect to any of the planetary bodies. For general purposes the power
produced by multiplying the diameter of the speculum by 30, or 40, will
be found most satisfactory.

The following are the general prices of reflecting telescopes as made
by the London opticians.

                                                           £ _s._
  A four feet, seven inch aperture, Gregorian Reflector; with
  the vertical motions upon a new invented principle, as well as
  apparatus to render the tube more steady in observation;
  according to the additional apparatus of small speculums,
  eye-pieces, micrometers, &c. from                  80 to 120 0

  Three feet long, mounted on a plain brass stand         23   2

  Ditto, with rack-work motions, improved mounting, and
  metals                                                  39  18

  Two feet long without rack-work, and with 4 magnifying
  powers, improved                                        15  15

  Ditto with rack-work motion                             22   1

  Eighteen inch on a plain stand                           9   9

  Twelve inch Ditto                                        6   6

The above are the prices stated in Messrs. W. and S. Joneses catalogue.

The following list of prices of the various kinds of reflecting
telescopes is from Messrs. Tulley’s (of Islington) catalogue.

                                                                £  _s._

  1  foot _Gregorian_ reflector, on pillar and claw stand, metal
  2-1/2 inches diameter, packed in a mahogany box                6    6

  1-1/2 foot ditto, on pillar and claw stand, metal 3 inches
  diameter, packed in mahogany box                              11   11

  2 feet ditto, metal 4 inches diameter                         16   16

  Ditto, ditto, with rack-work motions                          25    4

  3 feet ditto, metal 5 inches diameter, with rack-work
  motions                                                       42    0

  Ditto, metal 6 inches diameter, on a tripod stand, with centre
  of gravity motion                                             68    5

  4 feet ditto, metal 7 inches diameter, as above              105    0

  6 feet ditto, metal 9 inches diameter, on an improved iron
  stand                                                        210    0

  7 feet _Newtonian_ reflectors, 6 inches aperture, mounted on
  a new and improved stand                                     105    0

  Ditto, ditto, metal 7 inches diameter                        126    0

  9 feet ditto, metal 9 inches diameter                        210    0

  10 feet ditto, metal 10 inches diameter                      315    0

  12 feet ditto, metal 12 inches diameter                      525    0

_Comparative brightness of achromatic and reflecting telescopes._ The
late astronomer royal, Dr. Maskelyne, from a comparison of a variety of
telescopes, was led to the following conclusion,--‘that the aperture
of a common reflecting telescope, in order to show objects as bright
as the achromatic must be to that of an achromatic telescope as 8 to
5,’--in other words, an achromatic whose object glass is 5 inches
diameter, will show objects with as great a degree of brightness as a
reflector whose large speculum is 8 inches in diameter. This result,
if correct, must be owing to the small number of rays reflected from
a speculum compared with the number transmitted through an achromatic
object glass.


SECT. 2.--THE HERSCHELIAN TELESCOPE.

Soon after Sir William Herschel commenced his astronomical career, he
introduced a new era in the history of reflecting telescopes. After he
had cast and polished an immense variety of specula for telescopes of
different sizes-he, at length, in the year 1782, finished a 20 feet
reflector with a large aperture. Being sensible of the vast quantity
of light which is lost by a second reflection from the small speculum,
he determined to throw it aside altogether, and mounted this 20 feet
reflector on a stand that admitted of being used without a small
speculum in making _front observations_--that is, in sitting with his
back to the object, and looking directly towards the surface of the
speculum. Many of his discoveries and measurements of double stars were
made with this instrument, till, at length, in the year 1785 he put
the finishing hand to that gigantic speculum, which soon became the
object of universal astonishment, and which was intended for his _forty
feet_ reflecting telescope; he had succeeded so well in constructing
reflecting telescopes of comparatively small aperture, that they would
bear higher magnifying powers than had ever previously been applied;
but he found that a deficiency of light could only be remedied by an
increased diameter of the large speculum, which therefore was his main
object, when he undertook to accomplish a work which to a man less
enterprising, would have appeared impracticable. The difficulties he
had to overcome were numerous; particularly in the operative department
of preparing, melting, annealing, grinding, and polishing a mass of
metal that was too unwieldly to be moved without the aid of mechanical
powers. At length, however, all difficulties having been overcome,
this magnificent instrument was completed with all its complicated
apparatus, and erected for observation, on the 28th of August, 1789,
and on the same day the sixth satellite of Saturn was detected, as a
prelude of still farther discoveries which were afterwards made by this
instrument, in the celestial regions.

It would be too tedious to attempt a description of all the machinery
and apparatus connected with this noble instrument. The reader
who wishes to peruse a minute description of the stairs, ladders,
platform, rollers, and of every circumstance relating to joiner’s work,
carpenter’s work, smith’s work, and other particulars connected with
the formation and erection of this telescope, will find the details
recorded in the 85th volume of the Philosophical Transactions of the
Royal Society of London, for 1795, in which there are sixty-three pages
of letter press, and eighteen plates illustrative of the subject. I
shall content myself with giving a short outline of the essential parts
belonging to this instrument.

The _tube_ of this telescope is made of rolled or sheet iron, joined
together without rivets; the thickness of the sheets is somewhat less
than 1/36 part of an inch, or 14 pounds weight for a square foot;
great care was taken that the cylindrical form should be secured, and
the whole was coated over three or four times with paint, inside and
outside, to secure it against the damp. This tube was removed from
the place in which it was formed by twenty-four men, divided into six
sets; so that two men on each side, with a pole of 5 feet long in
their hands, to which was affixed a piece of course cloth, 7 feet long
going under the tube, and joined to a pole 5 feet long, in the hands
of two other men, assisted in carrying the tube. The _length_ of this
tube is 39 feet 4 inches, the diameter 4 feet 10 inches; and, on a
moderate computation, it was ascertained, that a wooden tube of proper
dimensions would have exceeded an iron one in weight by at least 3000
pounds. Reckoning the circumference of the tube 15 feet, its length
39-1/3 feet, and 14 lib. for the weight of a square foot, it must have
contained 590 square feet, and weighed 8,260 pounds. Various hoops were
fixed within the tube, and longitudinal bars of iron connecting some of
them are attached to the two ends of the tube, by way of bracing the
sheets, and preserving the shape perfect, when the pulleys are applied
to give the necessary elevation at the upper end, and that the speculum
may be kept secure at the lower end. The lower end of the tube is
firmly supported on rollers that are capable of being moved forwards or
backwards by a double rack, connected with a set of wheels and pinions.
By an adjustment at the lower extremity of the tube, the speculum is
turned to a small inclination, so that the line of collimation may not
be coincident with the longitudinal axis of the tube, but may cross the
tube diagonally, and meet the eye in the air at about two inches from
the edge of the tube, which is the peculiarity of the construction,
that supersedes the necessity of applying a second reflector. Hence no
part of the head of the observer intercepts the incident rays, and the
observation is taken with the face looking at the speculum, the back
being turned to the object to be observed.

The large speculum is enclosed in a strong iron ring, braced across
with bars of iron, and an enclosure of iron and ten sheets makes a case
for it. It is lifted by three handles of iron attached to the sides of
the ring, and is put into and taken out of its proper place in the tube
by the help of a moveable crane, running on a carriage, which operation
requires great care. The speculum is made of a metallic composition,
and is 49-1/2 inches in diameter; but the concave polished surface is
only 48 inches, or 4 feet in diameter. Its thickness is 31 inches; and
when it came from the cast its weight was 2118 pounds. The metals for
its formation were procured at a warehouse in Thames Street, London,
where they kept ingots of two kinds ready made, one of white, and the
other of bell-metal; and it was composed of two ingots of bell-metal
for one of white. It was not to be expected that a speculum of such
large dimensions, could have a perfect figure imparted to its surface,
nor that the curve, whatever it might be, would remain identically the
same in changes of temperature; therefore we are not surprised when we
are told, that the magnifying powers used with this telescope seldom
exceeded 200; the quantity of light collected by so large a surface
being the principal aim of the maker. The raising of the balcony, on
which the observer stands, and the sliding of the lower end of the
tube, in which the speculum rests, are effected by separate tackles,
and require only occasional motions; but the elevation of the telescope
requires the main tackle to be employed, and the motion usually given
in altitude at once was two degrees; the breadth of the zone in which
the observations were made, as the motion of the sphere in right
ascension brought the objects into view. A star, however, could be
followed for about a quarter of an hour. Three persons were employed
in using this telescope, one to work the tackle, another to observe,
and a third to mark down the observations. The elevation was pointed
out by a small quadrant fixed to the main tube, near the lower end, but
the polar distance was indicated by a piece of machinery, worked by a
string, which continually indicated the degree and minute on a dial in
the small house adjoining, while the time was shown by a clock in the
same place, Miss Herschel performing the office of Registrar.

At the upper end the tube is open, and directed to the part of the
heavens intended for observation, and the observer, standing on the
foot board, looks down the tube, and perceives the object by rays
reflected from the speculum, through the eye glass at the opening
of the tube. When the telescope is directed to any objects near the
zenith, the observer is necessarily at an elevation at least 40 feet
from the ground. Near the place of the eye glass is the end of a tin
pipe, into which a mouth-piece may be placed, so that, during an
observation, a person may direct his voice into this pipe, while his
eye is at the glass. This pipe, which is 1-1/2 inch in diameter runs
down to the bottom of the tube, where it goes into a turning joint,
thence into a drawing tube, and out of this into another turning joint,
from whence it proceeds, by a set of sliding tubes towards the front of
the foundation timber. Its use is to convey the voice of the observer
to his assistants, for at the last place, it divides itself into two
branches, one going into the observatory, the other into the workman’s
room, ascending in both places through the floor, and terminates in
the usual shape of speaking trumpets. Though the voice passes in this
manner through a tube, with many inflections, and through not less than
115 feet, it requires very little exertion to be well understood.

To direct so unwieldy a body to any part of the heavens at pleasure,
many mechanical contrivances were evidently necessary. The whole
apparatus rests upon rollers, and care was previously taken of the
foundation in the ground. This consists of concentrical brick walls,
the outermost 42 feet, the innermost 21 feet in diameter, 2 feet 6
inches deep under ground, 2 feet 3 inches broad at the bottom, and 1
foot 2 inches at the top, capped with paving stones 3 inches thick, and
12-3/4 inches broad.

In the centre is a large post of oak, framed together with braces under
ground, and walled fast to brick-work to make it steady. Round this
centre the whole frame is moved horizontally by means of 20 rollers,
12 upon the outer, and 8 upon the inner wall. The vertical motion is
given to the instrument by means of ropes and pullies, passing over the
main beam supported by the ladders. These ladders are 49 feet long,
and there is a moveable gallery with 24 rollers to ease its motion.
There is a stair-case intended for persons who wish to ascend into the
gallery, without being obliged to go up the ladder. The ease with which
the horizontal and vertical motions may be communicated to the tube
may be conceived, from a remark of Sir W. Herschel, that, in the year
1789, he several times observed Saturn, two or three hours before and
after its meridian passage with one single person to continue, at his
directions, the necessary horizontal and vertical motions.

By this telescope the sixth and seventh satellites of Saturn were
discovered, only one of which is within the reach of the 20 feet
reflector, or even of a 25 feet instrument. The discovery of the
satellites of the planet Uranus, however, was made by the 20 feet
reflector, but only after it had been converted from the Newtonian to
the Herschelian construction--which affords a proof of the superiority
of the latter construction over the former when the same speculum is
used. Never had the heavens before been observed with so extraordinary
an instrument as the forty feet reflector. The nebulosities which
are found among the fixed stars, in various regions of the heavens,
appeared almost all to resolve themselves into an innumerable
multitude of stars; others, hitherto imperceptible, seemed to have
acquired a distinct light. On the entrance of Sirius into the field
of the telescope, the eye was so violently affected, that stars of
less magnitude could not immediately after be perceived; and it was
necessary to wait for 20 minutes before these stars could be observed.
The ring of Saturn had always before ceased to be visible when its
plane was directed towards the earth; but the feeble light which it
reflects in that position was enough for Herschel’s instrument, and the
ring, even then, still remained visible to him.

It has been generally considered that this telescope was capable of
carrying a power of 6000 times; and perhaps for the purpose of an
experiment, and for trying its effect on certain objects, such a power
may have been applied,--in which case the eye-glass must have been
only 2/25 of an inch focal distance, or somewhat less than one twelfth
of an inch. But such a power could not be generally applied, with any
good effect, to the planetary bodies; and I question much whether any
power above 1000 times was ever generally used. For, it is the quantity
of light which the telescope collects, more than the magnifying power,
that enables us to penetrate, with effect, into the distant spaces of
the firmament: and hence, as above stated, the power seldom exceeded
200, which on account of the large diameter of the speculum, would
enable the instrument to penetrate into the distant celestial spaces
perhaps further than if a power of as many thousands of times had been
applied.

Sir John Herschel, who inherits all the science, skill, and industry of
his father, some time ago ground and polished a new speculum for the 20
feet tube, formerly noticed, which is connected with a stand, pulleys
and other appendages, similar to those above described, though of
smaller dimensions. This telescope shows the double stars exceedingly
well defined, and was one of the principal instruments used in forming
his catalogue of these objects which was presented to the Royal
Society, in conjunction with that of Sir James South, about the year
1828. I suppose, it is likewise the same telescope with which Sir John
lately made his Sidereal observations at the Cape of Good Hope.


SECT. 3.--RAMAGE’S LARGE REFLECTING TELESCOPE.

The largest _front view_ reflecting telescope in this country--next
to Herschel’s 40 feet instrument--is that which was erected at the
Royal Observatory at Greenwich, in the year 1820, by Mr. Ramage of
Aberdeen. The diameter of the concave reflector is 15 inches, and its
focal length 25 feet. It is erected on machinery which bears a certain
resemblance to that of Herschel’s, which we have now described; but the
mechanical arrangements are greatly simplified, so that the instrument
is manageable by an observer without an assistant. The tube is composed
of a twelve-sided prism of deal 5/8 inch thick. At the mouth is a
double cylinder of different diameters on the same axis; around this a
cord is wound by a winch, and passes up from the small cylinder, over
a pulley, and down through another pulley on to the large cylinder.
When the winch, therefore, is turned to raise the telescope, the
endless cord is unwound from the smaller cylinder, and wound on to the
larger, the difference of the size of the two cylinders will be double
the quantity raised, and a mechanical force to any extent may thus be
obtained, by duly proportioning the diameters of the two cylinders: by
this contrivance the necessity of an assistant is superseded. The view
through this instrument first astonished those observers who had not
been accustomed to examine a heavenly body with a telescope possessing
so much light; and its performance was deemed quite extraordinary. But
when the first impression had subsided, and different trials had been
made in different states of the atmosphere, it was discovered that the
central portion of the speculum was more perfectly figured than the
ring bordering on the extreme edges. When the aperture was limited to
ten or twelve inches, the performance as to the distinctness in its
defining power, was greatly improved, and the light was so brilliant,
that the Astronomer Royal was disposed to entertain an opinion, that it
might equal that of a good achromatic refractor of the same dimensions.
When, however, very small and obscure objects are to be observed,
the whole light of the entire aperture may be used with advantage on
favourable evenings.

The eye-pieces adapted to this telescope have powers which magnify
the object linearly from 100 to 1500 times, which are competent to
fulfil all the purposes of vision when cleared of aberration. When the
telescope is placed in the plane of the meridian and elevated together
with the gallery, into any required altitude, the _meridional sweeps_,
formerly practised by Sir W. Herschel, and continued by Sir John with
great success, in the examination of double stars and nebula, may be
managed with great ease.

Mr. Ramage had a telescope of about the same size, erected in an open
space in Aberdeen, which I had an opportunity of inspecting when I
paid a visit to that gentleman in 1833; but cloudy weather prevented
my obtaining a view of any celestial bodies through it. He showed me
at that time two or three large speculums, from 12 to 18 inches in
diameter, which he had finished some time before, and which appeared
most beautifully polished. He told me, too, that he had ground and
polished them simply with his hand, without the aid of any machinery
or mechanical power--a circumstance which, he said, astonished the
opticians of London, when it was stated, and which they considered as
almost incredible. His experience in casting and polishing metals of
various sizes, during a period of 15 or 16 years, qualified him to
prepare specula of great lustre, and with an unusually high polish. It
has been asserted that a fifty feet telescope by Ramage of 21 inches
aperture was intended to be substituted for the 25 feet instrument
erected at Greenwich, and the speculum it is understood, was prepared,
and ready for use, provided the Navy Board was disposed to defray the
expense of carrying the plan into execution. But, unfortunately, this
ingenious artist was unexpectedly cut off in the midst of his career,
about the year 1835.


SECT. 4.--THE AERIAL REFLECTOR--CONSTRUCTED BY THE AUTHOR.

A particular description of this telescope was given in the ‘Edinburgh
New Philosophical Journal’ for April--July, 1826, conducted by
Professor Jameson, the greater part of which was copied in the ‘London
Encyclopedia,’ ] under the article _Telescope_. From this description I
shall endeavour to condense a brief account of this instrument with a
few additional remarks.

About the year 1822, an old speculum 27 inches in focal length, very
imperfectly polished happened accidentally to come into my possession;
and feeling no inclination to fit it up in the Gregorian form, I formed
the resolution of throwing aside the small speculum, and attempting
the _front view_ notwithstanding the uniform assertion of opticians,
that such an attempt in instruments of a small size is impracticable.
I had some ground for expecting success in this attempt, from several
experiments I had previously made, particularly from some modifications
I had made in the construction of astronomical eye-pieces, which have
a tendency to correct the aberration of the rays of light, when they
proceed somewhat obliquely from a lens or speculum. In the first
instance, I placed the speculum at the one end of a tube of the form
of a segment of a cone--the end next the eye being somewhat wider
than that at which the speculum was fixed, and its length about an
inch shorter than the focal distance of the mirror. A small tube for
receiving the different eye-pieces was fixed in the inside of the large
tube at the end next the eye, and connected with an apparatus by which
it could occasionally be moved either in a vertical or horizontal
direction. With the instrument fitted up in this manner, I obtained
some interesting views of the moon, and of terrestrial objects. But
finding that one side of the tube intercepted a considerable portion of
light from the object, I determined to throw aside the tube altogether,
and to fit up the instrument on a different plan.

A short mahogany tube, about 3 inches long, was prepared, to serve as
a socket for holding the speculum. To the side of this tube an arm was
attached, about the length of the focal distance of the mirror, at
the extremity of which a brass tube for receiving the eye-pieces, was
fixed, connected with screws and sockets, by which it might be raised
or depressed, and turned to the right hand or to the left, and with
adjusting apparatus by which it might be brought nearer to or farther
from the speculum. Fig. 69 exhibits a general representation of the
instrument in profile. AB is the short tube which holds the speculum;
CD the arm which carries the eye-tubes, which consists of two distinct
pieces of mahogany; the part D being capable of sliding along the under
side of C, through the brass sockets EF. To the under part of the
socket F is attached a brass nut with a female screw, in which the male
screw _ab_ acts by applying the hand to the knob _c_, which serves for
adjusting the instrument to distinct vision. G is the brass tube which
receives the eye-pieces. It is supported by a strong brass wire _de_,
which passes through a nut connected with another strong wire, which
passes through the arm D. By means of the nut _f_ this tube may be
elevated or depressed, and firmly fixed in its proper position; and by
the nut _d_ it may be brought nearer to or further from, the arm D.

[Illustration: _figure 69._]

By the same apparatus, it is also rendered capable of being moved
either in a vertical or horizontal direction: but when it is once
adjusted to its proper position, it must be firmly fixed, and requires
no further attention. The eye-piece represented in this figure is the
one used for terrestrial objects, which consists of the tubes belonging
to a pocket achromatic telescope. When an astronomical eye-piece is
used, the length of the instrument extends only to the point I. In
looking through this telescope, the right eye is applied at the point
H, and the observer’s head is understood to be uncovered, or, at
least, tightly covered with a thin cap. For those who use only the
left eye, the arm would require to be placed on the opposite side of
the tube, or the arm, along with the tube, be made to turn round 180
degrees.

[Illustration: _figure 70._]

Fig. 70 represents a front, or rather an oblique view of the
instrument, in which the position of the speculum may be seen. All the
specula which I fitted up in this form, having been originally intended
for Gregorian reflectors, have holes in their centres. The eye-piece
is therefore directed to a point nearly equi-distant from the hole to
the left hand edge of the speculum, that is, to the point _a_. In one
of these instruments fitted up with a four feet speculum, the line
of vision is directed to the point _b_ on the opposite side of the
speculum, but, in this case, the eye-tube is removed farther from the
arm, than in the former case. The hole in the centre of the speculum is
obviously a defect in this construction of a reflecting telescope, as
it prevents us from obtaining the full advantage of the rays which fall
near the centre of the mirror; yet the performance of the instruments,
even with this disadvantage, is superior to what we should previously
have been led to expect.

The principal nicety in the construction of this instrument, consists
in the adjustment and proper direction of the eye-tube. There is only
one position in which vision will be perfectly distinct. It must be
neither too high nor too low,--it must be fixed at a certain distance
from the arm,--and must be directed to a certain point of the speculum.
This position must be ultimately determined by experiment, when viewing
terrestrial objects. A person unacquainted with this construction
of the telescope, would, perhaps, find it difficult, in the first
instance, to make this adjustment; but were it at any time deranged,
through accident or otherwise, I can easily make the adjustment anew,
in the course of a minute or two.

In pointing this telescope to the object intended to be viewed, the
eye is applied at K, fig, 69, and looking along the arm, towards the
eye-piece, till it nearly coincide with the object, it will, in most
cases, be readily found. In this way I can easily point this instrument
to Jupiter or Saturn, or to any of the other planets, visible to the
naked eye, even when a power of 160 or 170 times is applied. When high
magnifying powers, however, are used, it may be expedient to fix,
on the upper part of the short tube in which the speculum rests, a
Finder, such as that which is used in Newtonian telescopes. When the
moon is the object intended to be viewed, she may be instantly found by
moving the instrument till her reflected image be seen from the eye-end
of the telescope on the face of the mirror.

I have fitted up several instruments of the above description with
specula of 16, 27, 35, and 49 inches focal distance. One of these
having a speculum of 27 inches focal length, and an astronomical
eye-piece, producing a magnifying power of about 90 times, serves
as a good astronomical telescope. By this instrument the belts
and satellites of Jupiter, the ring of Saturn, and the mountains
and cavities of the moon, may be contemplated with great ease and
distinctness. With a magnifying power of 35 or 40 times, terrestrial
objects appear remarkably bright and well-defined. When compared with
a Gregorian, the quantity of light upon the object appears nearly
doubled, and the image is equally distinct--although the speculum has
several blemishes, and its surface is but imperfectly polished. It
represents objects in their natural colours, without that dingy and
yellowish tinge which appears when looking through a Gregorian. Another
of these instruments is about four feet long. The speculum which
belongs to it is a very old one: when it came into my possession, it
was so completely tarnished, as scarcely to reflect a ray of light.
After it was cleaned, it appeared to be scarcely half polished, and
its surface is covered with yellowish stains which cannot be erased.
Were it fitted up upon the Gregorian plan, it would, I presume, be
of very little use, unless when a very small magnifying power was
applied. Yet, in its present form, it bears, with distinctness, a
magnifying power of 130 times, and is equal in its performance to a
3-1/2 feet achromatic. It exhibits distinct and interesting views of
the diversities of shade, and of the mountains, vales, cavities, and
other inequalities of the moon’s surface. With a power of about 50
times, and a terrestrial eye-piece, it forms an excellent telescope
for land objects, and exhibits them in a brilliant and novel aspect.
The smallest instrument I have attempted to construct on this plan,
is only 5-1/2 inches focal distance, and 1-3/4 inch diameter. With a
magnifying power of about 15 times, it shows terrestrial objects with
distinctness and brilliancy. But I should deem it inexpedient to fit
up any instrument of this description with specula of a shorter focal
distance than 20 or 24 inches. The longer the focal distance the more
distinctness may be expected, although the aperture of the speculum
should be comparatively small.

The following are some of the properties and advantages peculiar to
this construction of the reflecting telescope.

1. It is _extremely simple_, and may be fitted up at a comparatively
_small expense_. Instead of large and expensive brass tubes, such as
are used in the Gregorian and Newtonian construction, little more is
required than a short mahogany tube, two or three inches long, to
serve as a socket for the speculum, with an arm connected with it
about the focal length of the speculum. The expense of small specula,
either plain or concave, is saved, together with the numerous screws,
springs, &c., for centering the two specula, and placing the small
mirror parallel to the large one. The only adjustment requisite in this
construction, is that of the eye-tube to the speculum; and, by means of
the simple apparatus above described, it can be effected in the course
of a few minutes. Almost the whole expense of the instrument consists
in the price of the speculum and the eye-pieces. The expense of fitting
up the four feet speculum, alluded to above--_exclusive of speculum and
eye-piece_--but including mahogany tube and arm, brass sockets, screws,
eye-tube, brass joint, and a cast-iron stand painted and varnished,
did not amount to £1 : 8_s_. A Gregorian of the same size would have
required a brass tube at least 4-1/2 feet in length, which would cost 5
or 6 guineas, besides the apparatus connected with the small speculum,
and the additional expense connected with the fitting up of the joint
and stand requisite for supporting and steadying so unwieldy an
instrument. While the one instrument would require two persons to carry
it from one room to another, and would occupy a considerable space in
an ordinary apartment, the other can be moved, with the utmost ease,
with one hand, to any moderate distance, and the space it occupies is
extremely small.

2. _It is more convenient for viewing celestial objects at a high
altitude, than other telescopes._ When we look through a Gregorian
reflector or an achromatic telescope of 4 or 5 feet in length, to an
object elevated 50 or 60 degrees above the horizon, the body requires
to be placed in an uneasy and distorted position, and the eye is
somewhat strained, while the observation is continued. But when
viewing similar objects by the _Aerial Reflector_, we can either stand
perfectly erect, or sit on a chair, with the same ease as we sit at a
desk when reading a book or writing a letter. In this way, the surface
of the moon or any of the planets, may be contemplated for an hour or
two, without the least weariness or fatigue. A delineation of the
lunar surface may be taken with this instrument with more ease and
accuracy than with any other instrument, as the observer can sketch
the outline of the object by one eye on a tablet placed a little below
the eye-piece, while the other eye is looking at the object. For the
purpose of accommodating the instrument to a sitting or standing
posture a small table was constructed, capable of being elevated or
depressed at pleasure, on which the stand of the telescope is placed.
When the telescope is 4 or 5 feet long, and the object at a very high
elevation, the instrument may be placed on the floor of the apartment,
and the observer will stand in an erect position.

3. This instrument is considerably _shorter_ than a Gregorian telescope
whose mirror is of the same focal length. When an astronomical
eye-piece is used, the whole length of the instrument is nothing more
than the focal length of the speculum. But a Gregorian whose large
speculum is 4 feet focus, will be nearly 5 feet in length, including
the eye-piece.

4. The Aerial Reflector far excels the Gregorian in brightness. The
deficiency of light in the Gregorians is owing to the second reflection
from the small mirror; for it has been proved by experiment that nearly
the one half of the rays of light which fall upon a reflecting surface
is lost by a second reflection. The image of the object may also be
presumed to be more correct, as it is not liable to any distortion by
being reflected from another speculum.

5. There is _less tremor_ in these telescopes than in Gregorian
Reflectors. One cause, among others, of the tremors complained of in
Gregorians is, I presume, the formation of a second image at a great
distance from the first, besides that which arises from the elastic
tremor of the small speculum, when carried by an arm supported only
at one end. But as the image formed by the speculum in the aerial
telescope is viewed _directly_, without being exposed to any subsequent
reflection, it is not so liable to the tremors which are so frequently
experienced in other reflectors. Notwithstanding the length of the arm
of the 4 feet telescope above mentioned, a celestial object appears
remarkably steady, when passing across the field of view, especially
when it is at a moderate degree of altitude; and it is easily kept in
the field by a gentle motion applied to the arm of the instrument.

In prosecuting my experiments in relation to these instruments, I
wished to ascertain what effect might be produced by using a _part of
a speculum_ instead of the whole. For this purpose, I cut a speculum,
three feet in focal length, through the centre, so as to divide it
into two equal parts, and fitted up each part as a distinct telescope;
so that I obtained two telescopes from one speculum. In this case I
found that each half of the speculum performed nearly as well as the
whole speculum had done before, at least there appeared to be _no
very sensible_ diminution in the _brightness_ of the object, when
viewed with a moderate power, and the image was equally accurate and
distinct; so that if _economy_ were a particular object aimed at in
the construction of these instruments, two good telescopes might be
obtained from one speculum; or if a speculum happened to be broken
accidentally into large fragments, one or more of the fragments might
be fitted up on this principle to serve as a tolerably good telescope.

From the experiments I have made in reference to these instruments, it
is demonstrable, that _a tube is not necessary_ in the construction of
a reflecting telescope--at least on the principle now stated--whether
it be used by day or by night for terrestrial or celestial objects;
for I have frequently used these telescopes in the open air in the day
time, without any inconvenience from extraneous light. Therefore, were
a reflecting telescope of 50 or 60 feet in length to be constructed, it
might be fitted up at a comparatively small expence, after the expense
of the metallic substances, and of casting, grinding, and polishing
the speculum is defrayed. The largest instrument of this description
which has hitherto been constructed is the 40 feet reflector of Sir
W. Herschel. This complicated and most unwieldy instrument had a tube
of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in
circumference, and weighed about 8000 pounds. Now, I conceive that
such enormous tubes, in instruments of such dimensions, are altogether
unnecessary. Nothing more is requisite than a short tube for holding
the speculum. Connected with one side of this tube (or with both sides
were it found necessary), two strong bars of wood, projecting a few
feet beyond the speculum end, and extending in front as far as the
focal length of the mirror, and connected by cross bars of wood, iron
or brass--would be quite sufficient for a support to the eye-piece, and
for directing the motion of the instrument. A telescope of 40 or 50
feet in length, constructed on this plan, would not require one fifth
of the expense, nor one fourth of the apparatus and mechanical power
for moving it to any required position, which were found necessary
in the construction of Sir W. Herschel’s large reflecting telescope.
The idea here suggested will perhaps be more readily appreciated by
an inspection of fig. 71, where A is the short tube, BC and DE the
two large bars or arms, connected with cross bars, for the purpose of
securing strength and steadiness. At I and K, behind the speculum,
weights might be applied, if necessary, for counterbalancing the lever
power of the long arm. F represents the position of the eye-piece,
and GH the joint and part of the pedestal on which the instrument is
placed. With regard to telescopes of smaller dimensions, as from 5
to 15 feet in focal length--with the exception of the expense of the
specula and eye-pieces--they might be fitted up for a sum not greater
than from 3 to 10 or 15 guineas.

[Illustration: _figure 71._]

Were any person to attempt the construction of those telescopes, it
is possible he might not succeed in his first attempts without more
minute directions than I have yet given. The following directions
may perhaps tend to guide the experimenter in adjusting the eye-tube
to the speculum, which is a point that requires to be particularly
attended to, and on which depends the accurate performance of the
instrument. After having fixed the eye-piece nearly in the position
it should occupy, and directed the instrument to a particular object,
look along the arm of the telescope, from K (fig. 69.) to the extremity
of the eye-piece at H, and observe, whether it nearly coincides with
the object. If the object appear lower than this line of vision, the
eye-piece must be lowered, and if higher, it must be raised, by means
of the nuts and screws at _gd_ and _fe_, till the object and the line
of vision now stated nearly coincide. The eye-piece should be directed
as nearly perpendicular to the front of the speculum as possible, but
so that the reflected image of one’s head from the mirror shall not
interfere to obstruct the rays from the object. An object may be seen
with an approximate degree of distinctness, but not accurately, unless
this adjustment be pretty accurately made. The astronomical eye-pieces
used for these telescopes are fitted with a brass cap which slides on
the end next the eye, and is capable of being brought nearer to or
farther from the first eye-glass. In the centre of this cap, next the
eye, is a small hole, about the 1/40th or 1/50th of an inch diameter,
or about as wide as to admit the point of a pin or a moderate-sized
needle. The distance of this hole from the lens next the eye must be
adjusted by trial, till the whole field of view appear distinct. A
common astronomical eye-piece, without this addition, does not answer
well. I find by experience, that terrestrial eye-pieces, such as those
used in good achromatic telescopes, are, on the whole, best adapted to
this construction of a reflecting telescope.

I have sometimes used these instruments for the purpose of viewing
perspective prints, which they exhibit in a beautiful and interesting
manner. If a  perspective be placed at one end of a large
room or gallery, and strongly illuminated either by the sun or by two
candles, and one of the reflectors furnished with a _small magnifying
power_, placed at the opposite end of the room--the representation of
a street or a landscape will be seen in its true perspective, and will
appear even more pleasant and interesting than when viewed through
the common _optical diagonal machine_. If an inverting eye-piece be
used--which is most eligible in this experiment--the print, of course,
must be placed in an inverted position.

That reflecting telescopes of the descriptions now stated are original
in their construction, appears from the uniform language of optical
writers, some of whom have pronounced such attempts to be altogether
impracticable. Sir David Brewster, one of the latest and most
respectable writers on this subject, in the ‘Edinburgh Encyclopedia’
art _optics_, and in the last edition of his _appendix_ to ‘Ferguson’s
Lectures,’ has the following remarks:--‘If we could dispense with
the use of the small specula in telescopes of moderate length, by
inclining the great speculum, and using an oblique, and consequently
a _distorted_ reflection, as proposed first by La Maire, we should
consider the Newtonian telescope as perfect; and on a large scale, or
when the instrument exceeds 20 feet, it has undoubtedly this character,
as nothing can be more simple than to magnify, by a single eye-glass,
the image formed by a single speculum. As the _front view is quite
impracticable_, and indeed _has never been attempted_ in instruments
of a small size, it becomes of great practicable consequence to remove
as much as possible, the evils which arise from the use of a small
speculum,’ &c.

The instruments now described have effectuated, in some degree, the
desirable object alluded to by this distinguished philosopher, and
the mode of construction is neither that of Sir W. Herschel’s front
view, nor does it coincide with that proposed by La Maire, which
appears to have been a mere hint that was never realized in the
construction of reflecting telescopes of a small size. The simplicity
of the construction of these instruments, and the excellence of their
performance, have been much admired by several scientific gentlemen and
others to whom they have been exhibited. Prior to the description of
them in the Edin. Philos. Journal, they were exhibited in the Calton
Hill Observatory, Edinburgh, in the presence of Professor Wallace,
and another gentleman, who compared their performance with that of an
excellent Gregorian. As this instrument is distinguished from every
other telescope, in being used without a tube, it has been denominated
‘_The aerial reflector_.’


SECT. 4.--EARL OF ROSSE’S REFLECTING TELESCOPES.

This nobleman, unlike many of his compeers, has, for a considerable
number of years past, devoted his attention to the pursuits of science,
and particularly to the improvement of reflecting telescopes. He is
evidently possessed of high mathematical attainments, combined with
an uncommon degree of mechanical ingenuity. About 14 or 15 years ago,
he engaged in various experiments with the view of counteracting the
effects of the spherical aberration of the specula of reflecting
telescopes--which imperfection, if it could be completely remedied,
would render the reflecting telescope almost a perfect instrument,
as it is not affected by the different refrangibility of the rays of
light. His method, we believe, consisted in forming a large speculum of
two or three separate pieces of metal, which were afterwards accurately
combined into one--a central part which was surrounded by one or two
rings ground on the same tool. When the images formed by the separate
pieces, were made exactly to coincide, the image of the object towards
which the whole speculum was directed, was then found to be as distinct
as either image had been when separate. But at the period referred to,
a sufficient number of experiments had not been made to determine that
his lordship had completely accomplished the object he intended.

Great interest, however, has of late been excited by the improvements
which his lordship has made in the formation of specula. Sir W.
Herschel never made public the means by which he succeeded in giving
such gigantic developement to the reflecting telescope: and therefore
the construction of a _large_ reflector has been considered as a
perilous adventure. But, according to a report of Dr. Robinson of
Armagh, to the Irish academy, the Earl of Rosse has overcome the
difficulties which have hitherto been met with, and carried to an
extent which even Herschel himself did not venture to contemplate,
the illuminating power of this telescope, along with a sharpness
of definition little inferior to that of the achromatic; and it is
scarcely possible, he observes, to preserve the necessary sobriety of
language in speaking of the moon’s appearance with this instrument,
which Dr. Robinson believes to be the most powerful ever constructed.
The difficulty of constructing large specula, and of imparting to them
the requisite degree of polish, has hitherto been considered so great,
that from 8 to 12 inches diameter has been in general their utmost
size. Indeed, except with the greatest reluctance, London opticians
would not accept of orders for specula of more than 9 inches in
diameter. It appears, however, that the Earl of Rosse has succeeded,
by a peculiar method of moulding, in casting object-mirrors of true
_speculum metal of three feet in diameter, and of a weight exceeding
17 cwt_. He is about to construct a telescope, the speculum of which
is _six feet_ in diameter, _fifty feet focal distance_, and of the
weight of _four tons_; and from what he has already accomplished, it
is not doubted that he possesses the power to carry his design into
effect. These great masses of metal, which, in the hands of all other
makers of specula would have been as untractable as so much unannealed
flint-glass, the Earl of Rosse has further succeeded in bringing to
the highest degree of polish, and the utmost perfection of curvature
by means of machinery. The process is conducted under water, by which
means those variations of temperature, so fatal to the finest specula
hitherto attempted, are effectually guarded against. To convince Dr.
Robinson of the efficacy of this machinery, the earl took the three
feet speculum out of its telescope, destroyed its polished surface,
and placed it under the mechanical polisher. In six hours it was
taken out with a perfect new surface as bright as the original. Under
the old system of hand-polishing, it might have required months,
and even years, to effect this restoration. Even before achieving
these extraordinary triumphs on the solid substance, his lordship
had constructed a six feet reflector by covering a curved surface of
brass with squares of the true _speculum metal_, which gave an immense
quantity of light, though subject to some irregularities, arising
from the number of joinings necessary in such a mosaic work. Of the
performance of his lordship’s great telescope, mounted with this
reflector, those who have seen it speak in terms of high admiration;
but in reference to the smaller and more perfect instrument, furnished
with the solid three feet speculum, the language of the Armagh
astronomer assumes a tone of enthusiasm and even of sublimity. By means
of this exquisite instrument, Dr. Robinson and Sir J. South, in the
intervals of a rather unfavourable night, saw several new stars, and
corrected numerous errors of other observers. For example, the planet
Uranus, supposed to possess a ring similar to that of Saturn, was found
not to have any such appendage; and those nebulæ, hitherto regarded,
from their apparently circular outline, as ‘coalescing systems,’
appeared, when tested by the three feet speculum, to be very far
indeed from presenting a globular appearance; numerous off-shoots and
appendages, invisible by other telescopes, appearing in all directions
radiating from their edges. Such discoveries, which reflect great
honour on the Earl of Rosse, will doubtless have great effect on the
interests of astronomical science.[27]


SECT. 5.--REFLECTING TELESCOPES WITH GLASS SPECULA.

After making a variety of experiments with aerial telescopes
constructed of metallic specula of different focal lengths, I
constructed a telescope on the same plan, with a concave glass mirror.
Having obtained a fragment of a very large convex mirror which happened
accidentally to have been broken, I caused the convex side to be
foliated, or silverised, and found its focal length to be about 27
inches. This mirror, which was about 5 inches diameter, I placed in one
of the aerial reflectors, instead of the metallic speculum, and tried
its effects with different terrestrial eye-pieces. With a power of
about 35 or 40 times, it gave a beautiful and splendid view of distant
terrestrial objects--the quantity of light reflected from them, being
considerably greater than when a metallic speculum was used, and they
appeared on the whole well-defined. The only imperfection--as I had
foreseen--consisted in a double image being formed of objects which
were remarkably bright and white, such as a light-house whitened on
the outside, and strongly illuminated by the sun. One of the images
was bright and the other faint. This was obviously owing to the two
reflections from the two surfaces of the mirror--one from the convex
silverised side, and the other from the concave side next the eye,
which produced the faint image--which circumstance has been generally
considered as a sufficient reason for rejecting the use of glass
specula in telescopes. But although very bright objects exhibited
a double image, almost all the other objects in the terrestrial
landscape appeared quite distinct and without any secondary image, so
that a common observer could scarcely have noticed any imperfection.
When the instrument, however, was directed to celestial objects, the
secondary image was somewhat vivid, so that every object appeared
double. Jupiter appeared with two bodies, at a little distance from
each other, and his four satellites appeared increased to eight.
The moon likewise appeared as a double orb, but the principal image
was distinct and well-defined. Such a telescope, therefore, was not
well-adapted for celestial observations, but might answer well enough
for viewing terrestrial objects.

Considering that the injurious effects of the secondary image arose
from the images reflected from the two surfaces being formed near the
same point, and at nearly the same focal distance, I formed a plan for
destroying the secondary image, or at least counteracting its effects,
by forming the concavity of the mirror next the eye of a portion of a
sphere _different_ from that of the convex side which was silverised,
and from which the principal image is formed. But, for a long time, I
could find no opticians possessed of tools of a sufficient length of
radii for accomplishing my design. At length a London working optician
undertook to finish a glass speculum, according to my directions,
which were, that the convex surface of the mirror should be ground on
a tool which would produce a focal distance by reflection of about 4
feet; and that the concave surface should have its focal distance at
about 3 feet 3 inches, so that the secondary image might be formed
at about 9 inches, within the focal distance of the silverised side,
and not interfere to disturb the principal image. But, either from
ignorance or inattention, the artist mistook the radius for the half
radius of concavity, and the speculum turned out to be only 23 inches
focal distance by reflection. This mirror was fitted up as a telescope,
on the aerial plan, and I found, as I expected, the secondary image
completely destroyed. It produced a very beautiful and brilliant view
of land objects, and even the brightest objects exhibited no double
image. The mirror was nearly 5 inches in diameter, but the image was
most accurately defined when the aperture was contracted to about 3
inches. It was fitted with a terrestrial eye-piece which produced a
magnifying power of about 25 times. When directed to the moon, it
gave a very distinct and luminous view of that orb, without the least
appearance of a secondary image. But as the focal distance of the
speculum was scarcely half the length I had prescribed, I did not
apply to it any high astronomical powers; as I find, that these can
only be applied with effect, in this construction, to a speculum of a
considerable focal length. Happening to have at hand a convex lens 10
feet focal length, and 4 inches in diameter--the one side of which had
been ground to a certain degree of concavity--I caused the convex side
to be foliated, which produced a focus by reflection, at 13-1/2 inches
distant. To this mirror I applied terrestrial powers of 15 and 24, with
considerable distinctness. The power of 15 produced a very brilliant
and distinct view of land objects. Had the mirror been at least 3 times
the focal length, it would have formed an excellent telescope, with the
same aperture.


SECT. 6.--A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYE-PIECE.

[Illustration: _figure 72._]

On the same principle as that by which a refracting telescope may be
constructed by means of a single lens--as represented fig. 51, (page
234) we may form a telescope by reflection with a single mirror, and
without an eye-piece. Let AB, fig. 72, represent a large concave
speculum, and C its focus--if an eye be placed at D, about 8 or 10
inches within the focal point C, all the objects in the direction of C,
or behind the spectator, will be seen magnified by reflection on the
face of the mirror, and strongly illuminated. The magnifying power, in
this case, will be nearly in the proportion of the focal length of the
mirror to the focal length of the eye for near objects. If for example,
the focal distance of the mirror be 8 feet, and the distance from the
eye at which we see near objects most distinctly, be 8 inches--the
magnifying power will be in the ratio of 8 to 96, or 12 times. I have
a glass mirror of this description, whose focal length is 4 feet 8
inches, and diameter 6 inches, which magnifies distant objects about 7
times, takes in a large field of view, and exhibits objects with great
brilliancy. It presents a very distinct picture of the moon, showing
the different streaks of light and shade upon her surface; and, in some
cases, shows the larger spots which traverse the solar disc. This mode
of viewing objects is extremely easy and pleasant, especially when the
mirror is of a large diameter; and the observer is at first struck and
gratified with the novel aspect in which the objects appear.

Were a concave mirror of this description--whether of glass or of
speculum metal--to be formed to a very long focus, the magnifying
power would be considerable. One of 50 feet focal length, and of a
corresponding diameter, might produce a magnifying power, to certain
eyes, of about 75 times; and, from the quantity of light with which the
object would be seen, its effect would be much greater than the same
power applied to a common telescope. Sir W. Herschel states, that, on
one occasion, by looking with his naked eye on the speculum of his 40
feet Reflector, without the interposition of any lens or mirror, he
perceived distinctly one of the satellites of Saturn, which requires
the application of a considerable power to be seen by an ordinary
telescope. Such an instrument is one of the most simple forms of a
telescope, and would exhibit a brilliant and interesting view of the
moon, or of terrestrial objects.


PRICES OF REFLECTING TELESCOPES.

1. Prices as stated by Messrs. W. and S. Jones, Holborn, London.

                                                              £  s.
  A 4 feet, 7 inch aperture Gregorian reflector, with the
  vertical motions upon a new invented principle, as well as
  apparatus to render the tube more steady for observation,
  according to the additional apparatus of small speculums,
  eye-pieces, micrometers, &c.              from 80_l._ to  120   0

  Three feet long, mounted on a plain brass stand            23   2

  Ditto with rack-work motions, improved mountings and
  metals                                                     39  18

  Two feet long without rack-work, and with 4 magnifying
  powers, improved                                           15  15

  Ditto improved, with rack-work motions                     22   1

  Eighteen inch, on a plain stand                             9   9

  Twelve inch ditto                                           6   6

2. Prices as stated by Messrs. Tulley, Islington.

                                                              £  s.
  1 foot Gregorian Reflector, on pillar-and-claw stand, metal
  2-1/2 inches diameter, packed in a mahogany box             6   6

  1-1/2  foot ditto on pillar and claw stand, metal 3 inches
  diameter, packed in a mahogany box                         11  11

  2 feet ditto, metal 4 inches diameter                      16  16

  Ditto with rack-work motions                               25   4

  3 feet ditto, metal 5 inches diameter, rack-work motions   42   0

  4 feet ditto, metal 7 inches diameter, on a tripod stand
  with centre of gravity motion                             105   0

  6 feet ditto, metal 9 inches diameter                     210   0

  7 feet _Newtonian_, 6 inches aperture                     105   0

  12 feet ditto, metal 12 inches diameter                   525   0

3. Prices stated by Mr. G. Dollond, St. Paul’s Church Yard.

                                                              £  s.
  Reflecting telescopes 14 inches long, in a mahogany box     9   9

  Ditto, 18 inches                                           12  12

  Ditto 2 feet                                               18  18

  Ditto with 4 different powers, and rack-work stand
  supporting the telescope in the centre of gravity          36  15

  Ditto 3 feet, with ditto                                   50   0

4. Prices of single speculums and reflecting telescopes, as made by Mr.
Grub, Charlemont Bridge works, Dublin.

  -----------------------------------------+-+----------------------------------------
          NEWTONIAN TELESCOPES.            | |       GREGORIAN REFLECTORS.
  --------+---------+----------+-----------+ +--------+---------+----------+----------
  Diameter| Focal   | Price of | Price of  | |Diameter| Focal   | Price of | Price of
  in      | length  | Mirrors  | telescope | |in      | length  | Mirrors  | telescope
  inches. | in feet.| alone.   | complete  | |inches. | in feet.| alone.   | complete
          |         |          | without   | |        |         |          | without
          |         |          | stand.    | |        |         |          | stand.
  --------+---------+----------+-----------+ +--------+---------+----------+----------
          |         |  £    s. |  £    s.  | |        |         |  £    s. |  £    s.
          |         |          |           | |    6   |    3    |  17   10 |  25    0
          |         |          |           | |        |         |          |
      7   |    7    |  17   10 |  27   10  | |    7   |    3    |  25    0 |  34    0
          |         |          |           | |        |         |          |
      9   |   10    |  25    0 |  40    0  | |    9   |   4-1/2 |  35    0 |  50    0
          |         |          |           | |        |         |          |
     12   |   12    |  60    0 |  90    0  | |   12   |    7    |  70    0 | 100    0
          |         |          |           | |        |         |          |
     15   |   15    | 120    0 | 170    0  | |   15   |    9    | 150    0 | 200    0
          |         |          |           | |        |         |          |
     18   |   18    | 200    0 | 260    0  | |   18   |   12    | 240    0 | 300    0


ON THE EYE-PIECES OF TELESCOPES.

Although the performance of telescopes chiefly depends on the goodness
of the object-glass, or the object-speculum of the instrument, yet it
is of considerable importance, in order to distinct vision, and to
obtain a large and uniformly distinct field of view, that the eye-piece
be properly constructed. The different kinds of eye-pieces may be
arranged into two general divisions--_Astronomical_ and _terrestrial_.

1. _Astronomical eye-pieces._--The most simple astronomical eye-piece
is that which consists of a single convex lens; and when the focal
distance of this lens, and that of the object-glass of the instrument
is accurately ascertained, the magnifying power may be nicely
determined, by dividing the focal length of the object-lens by that
of the eye-glass. But, as the pencil of white light transmitted by
the object-glass, will be divided by the eye-glass into its component
colours, the object will appear bordered with  fringes, and the
distinctness of vision consequently injured. Besides, the spherical
aberration, when a single lens is used, is much greater than when two
or more glasses are employed. Hence astronomical eye-pieces are now
formed by a combination of at least two lenses.

[Illustration: _figure 73._]

The combination of lenses now generally used for astronomical purposes,
is that which is usually denominated the _Huygenian eye-piece_, having
been first proposed by the celebrated Huygens, as a great improvement
on the single lens eye-piece. The following figure (73) represents a
section of this eye-piece. Let AB be a compounded pencil of white light
proceeding from the object-glass; BF a plano-convex field-glass, with
its plane side next the eye-glass E. The red rays of the pencil AB,
after refraction would cross the axis in R, and the violet rays in V,
but meeting the eye-glass E, the red rays will be refracted to O, and
the violet nearly in the same direction, when they will cross each
other about the point O, in the axis, and unite. The distance of the
two glasses FE, to produce this correction, when made of crown glass,
must be equal to half the sum of their focal distances nearly. For
example, suppose the focal distance of the largest, or field lens, to
be 3 inches, and the focal distance of the lens next the eye, 1 inch,
the two lenses should be placed exactly at the distance of 2 inches;
the sum of their focal length being 4, the half of which is 2. In other
words, the glass next the eye should be placed as much _within_ the
focus of the field-glass as is equal to its own focal distance. The
focal length of a single lens, that has the same magnifying power as
this compound eye-piece--is equal to twice the product of the focal
lengths of the two lenses, divided by the sum of the same numbers.
Or, it is equal to half the focal length of the field-glass. Thus, in
reference to the preceding example, twice the product of the focal
length of the two lenses--is equal to 6, and their sum is 4. The former
number divided by the latter, produces a quotient of 1-1/2, which
is the focal length of a single lens, which would produce the same
magnifying power as the eye-piece; and 1½ is just half the focal length
of the field-glass. The proportion of the focal lengths of the two
lenses to each other, according to Huygens, should be as 3 to 1; that
is, if the field-glass be 4-1/2 inches, the eye-glass should be 1-1/2;
and this is the proportion most generally adopted. But some opticians
have recommended that the proportions should be as 3 to 2. Boscovich
recommended two similar lenses; and in this case the distance between
them was equal to half the sum of their focal distances, as in the
Huygenian eye-piece.

The image is formed at IM, at the focal distance of the lens next the
eye, and at the same distance from the field-glass. When distinct
vision is the principal object of an achromatic telescope, the two
lenses are usually both plano-convex, and fixed with their curved
faces towards the object glass, as in the figure. Sometimes, however,
they consist of what is called _crossed_ lenses, that is lenses ground
on one side to a short focus, and on the other side to a pretty
long focus, the sides with the deepest curves being turned towards
the object glass. A diaphragm, or aperture of a proper diameter, is
placed at the focus of the eye lens, where the image formed by the
object-glass falls, for the purpose of cutting off the extreme rays of
the field lens, and rendering every part of the field of view equally
distinct. This is likewise the form of the eye-piece generally applied
to Gregorian reflectors. In short, when accurately constructed, it is
applicable to telescopes of every description. This eye-piece, having
the image viewed, by the eye behind the inner lens, is generally called
the _negative_ eye-piece, and is that which the optical-instrument
makers usually supply, of three or four different sizes, for so many
magnifying powers, to be applied to different celestial objects,
according to their nature or the state of the atmosphere in which they
are used.

_Ramsden’s eye-piece._--There is another modification of lenses,
known by the name of the _Positive_, or Ramsden’s eye-piece, which is
much used in Transit instruments, and telescopes which are furnished
with micrometers, and which affords equally good vision as the other
eye-piece. In this construction the lenses are plano-convex, and
nearly of the same focus, but are placed at a distance from each other
less than the focal distance of the glass next the eye, so that the
image of the object viewed is beyond both the lenses, when measuring
from the eye. The flat faces of the two lenses are turned into
contrary directions in this eye-piece--one facing the object-glass,
and the other the eye of the observer; and as the image formed at
the focus of the object-glass, lies parallel to the flat face of the
contiguous lens, every part of the field of view is distinct at the
same adjustment, or, as opticians say, there is a _flat field_, which,
without a diaphragm, prevents distortion of the object. This eye-piece
is represented in fig. 74, where AB and CD are two plano-convex
lenses, with their convex sides inwards. They have nearly the same
focal length, and are placed at a distance from each other, equal to
about two thirds of the focal length of either. The focal length of
an equivalent single lens is equal to three fourths the focal length
of either lens, supposing them to have equal focal distances. This
eye-piece is generally applied, when wires of spider’s lines are used
in the common focus; as the piece containing the lenses can be taken
out without disturbing the lines, and is adjustable for distinct
vision; and whatever may be the measure of any object given by the
wire micrometer, at the solar focus, it is not altered by a change of
the magnifying power, when a second eye-piece of this construction is
substituted.

[Illustration: _figure 74._]

[Illustration: _figure 75._]

[Illustration: _figure 76._]

_Aberration of lenses._--In connection with the above descriptions,
the following statements respecting the spherical aberration of lenses
may not be inappropriate. Mr. John Dollond, in a letter to Mr. Short,
remarks, that ‘the aberration in a single lens is as the cube of the
refracted angle; but if the refraction be caused by two lenses, the sum
of the cubes of each half will be 1/4 of the refracted angle, twice
the cube of 1 being 1/4 the cube of 2. So three times the cube of 1 is
only _one ninth_ of the cube of 3.’ &c. Hence the indistinctness of the
borders of the field of view of a telescope is diminished by increasing
the number of lenses in an eye piece. Sir J. Herschel has shown that if
two plano-convex lenses are put together as in fig. 75, the aberration
will be only 0.2481, or _one fourth_ of that of a single lens in its
best form. The focal length of the first of these lenses, must be to
that of the second as 1 to 2.3. If their focal lengths are equal, the
aberration will be 0.603, or nearly one half. The spherical aberration,
however, may be _entirely_ destroyed by combining a meniscus and double
convex lens, as shown in fig. 76, the convex sides being turned to the
eye when they are used as lenses, and to parallel rays, when they are
used as burning glasses. Sir J. Herschel has computed the following
curvatures for such lenses.

  _Focal length of the convex lens_    +10.000
  Radius of its first surface           +5.833
  Radius of its second surface         -35.000
  _Focal length of the meniscus_       +17.829
  Radius of its first surface           +3.688
  Radius of its second surface          +6.294
  Focal length of the compound lens     +6.407

On the general principles above stated, a good astronomical eye-piece
may be easily constructed with two proper lenses, either according to
the plan of Huygens or that of Ramsden; and, from what has been now
stated it is demonstrably certain, that, in all cases where two glasses
are properly combined, such an eye-piece is superior to a single lens,
both in point of distinctness, and of the enlargement of the field of
view. I lately fitted up an eye-piece, on Ramsden’s principle, with
two lenses, each about 3 inches focal length, and 1-3/8 inch diameter,
placed at half an inch distant, with their convex surfaces facing
each other as in fig. 74, which forms an excellent eye-piece for an
achromatic telescope, 6 feet 8 inches focal distance, and 4 inches
aperture, particularly for viewing clusters of stars, the Milky Way,
and the large nebulæ. The field of view is large, the magnifying power
is only between 50 and 60 times, and the quantity of light being so
great, every celestial object appears with great brilliancy, and it is
in general much preferable, when applied to the stars than any of the
higher powers. When applied to _Presepe_ in Cancer, it exhibits that
group at one view, as consisting of nearly a 100 stars which exhibit a
beautiful and most striking appearance.

It may appear a curious circumstance that any eye-piece which is good
with a short telescope, is also good with a long one, but that the
reverse is not true; for it is found to be more difficult to make
a good eye-piece for a short than for a long focal distance of the
object-glass.

Celestial eye-pieces are sometimes constructed so as to produce
_variable powers_. This is effected by giving a motion to the lens next
the eye, so as to remove it nearer to or farther from the field lens;
for at every different distance at which it is placed from the other
lens, the magnifying power will either be increased or diminished. The
greatest power is when the two lenses are nearly in contact, and the
power diminishes in proportion to the distance at which the glass next
the eye is removed from the other. The scale of distance, however,
between the two lenses, cannot be greater than the focal distance of
the field, or inner glass; for if it were, the lenses would no longer
form an eye-piece, but would be changed into an inverting opera-glass.
For effecting the purpose now stated, the eye-glass is fixed in a
tube which slides upon an interior tube on which is marked a scale of
distances, corresponding to certain magnifying powers; and, in this way
an eye-piece may be made to magnify about double the number of times,
when the lenses are in one position than when they are in another--as,
for example, all the powers from 36 to 72 times may be thus applied,
merely by regulating the distance between the two lenses. When the
glasses are varied in this manner the eye-piece becomes sometimes a
_positive_ eye-piece, like Ramsden’s, and sometimes a negative one like
that of Huygens.

_Diagonal eye-pieces._ The eye-pieces to which we have now adverted,
when adapted to refracting telescopes, both reverse and invert the
object, and therefore are not calculated for showing terrestrial
objects in their natural position. But as the heavenly bodies are of a
spherical form, this circumstance detracts nothing from their utility.
When the celestial object, however, is at a high altitude, the observer
is obliged to place his head in a very inconvenient position, and to
direct his eye nearly upwards; in which position he cannot remain long
at ease, or observe with a steady eye. To remedy this inconvenience,
the diagonal eye-piece has been invented, which admits of the eye
being applied at the side--or at the upper part of the eye-piece,
instead of the end; and when such an eye-piece is used, it is of no
importance in what direction the telescope is elevated, as the observer
can then either sit or stand erect, and look down upon the object with
the utmost ease. This object is effected by placing a flat piece of
polished speculum-metal at an angle of 45 degrees in respect to the two
lenses of the eye-piece, which alters the direction of the converging
rays, and forms an image which becomes erect with respect to altitude,
but is reversed with respect to azimuth;--that is, in other words,
when we look down upon the objects in the field of view, they appear
erect; but that part of an object which is in reality on our right hand
appears on our left; and if it be in motion, its _apparent_ is opposite
to its _real_ motion; if it be moving towards the west, it will seem to
move towards the east.

There are three situations in which the diagonal reflector in this
eye-piece may be placed. It may be placed either 1. before the
eye-piece,--or 2. behind it,--or 3. between the two lenses of which
the eye-piece consists. The most common position of the reflector is
between the lenses; and this may be done both in the negative and the
positive eye-pieces; but as the distance between the two lenses is
necessarily considerable, to make room for the diagonal position of the
reflector, the magnifying power cannot be great; otherwise, a diagonal
eye-piece of this construction remains always in adjustment, and is
useful in all cases where a high power is not required. The following
is a description and representation of a diagonal eye-piece of this
kind in my possession.

[Illustration: _figure 77._]

In fig. 77, AB represents the plano-convex lens next the object, which
is about 2 inches in focal length, and 3/4 inch in diameter; CD, a
plain metallic speculum of an oval form, well polished, and placed at
half a right angle to the axis of the tube; and EF another plano-convex
lens, about 1-1/2 inch focal distance. The centre of the speculum is
about 1-1/4 inch from the lens AB, and about 1/2 or 1/3 inch from EF;
so that this eye-piece is a _positive_ one, on the principle proposed
by Ramsden. The rays proceeding from the lens AB, and falling upon the
speculum, are reflected in a perpendicular direction to the lens EF,
where they enter the eye at G, which looks down upon the object through
the side of the tube. The real size of this eye-piece is much about the
same as that represented in the figure. When applied to an achromatic
telescope of 44-1/2 inches focal distance it produces a magnifying
power of 36 times, and exhibits a very beautiful view of the whole
of the full moon. It likewise presents a very pleasing prospect of
terrestrial objects, which appear as if situated immediately below us.

[Illustration: _figure 78._]

Another plan of the diagonal eye-piece is represented in fig. 78, where
the speculum is fixed _within_ the sliding tube which receives the
eye-piece, or immediately below it. The part of the tube at AB slides
into the tube of the telescope, CD is the speculum placed at half a
right angle to the axis of the tube, and EF, the tube containing the
lenses, which stands at right angles to the position of the telescope,
and slides into an exterior tube, and the eye is applied at G. This
construction of the diagonal eye-piece may be used with any eye-piece
whatever, whether the Huygenian or that of Ramsden. It will admit of
any magnifying power, and if several different eye-pieces be fitted
to the sliding tube, they may be changed at pleasure. This form of
the diagonal eye-piece, I therefore consider as the best and the
most convenient construction, although it is not commonly adopted by
opticians.

When any of these eye-pieces are applied to a telescope, with the lens
E on the upper part of it, we look down upon the object, if it be a
terrestrial one, as if it were under our feet. If we turn the eye-piece
round in its socket a quarter of a circle towards the left, an object
directly before us in the south, will appear as if it were in the
_west_ and turned upside down. If, from this position, it is turned
round a semicircle towards the right, and the eye applied, the same
object will appear as if it were situated in the east, and inverted;
and if it be turned round another quadrant, till it be directly
opposite to its first position, and the eye applied from below, the
object or landscape will appear as if suspended in the atmosphere above
us. This eye-piece, therefore, is capable of exhibiting objects in a
great variety of aspects, and the use of it is both pleasant and easy
for the observer. But there is a considerable loss of light, occasioned
by the reflection from the speculum, which is sensibly felt when very
high powers are applied; and therefore when very small stars are to be
observed, such as some of those connected with double or triple stars,
the observer should not study his own ease so much as the quantity of
light he can retain with a high power, which object is best attained
with an ordinary eye-piece and a telescope of large aperture.

We have said that a diagonal eye-piece may be constructed with a
reflector _before_ the eye-piece. In this case, the speculum is
sometimes made to slide before the eye at the requisite angle of
reclination, in which application each eye-piece must necessarily have
a groove to receive it, and the eye must be applied without a hole
to direct it, but it may be put on and taken off without disturbing
the adjustment for distinct vision, and is very simple in its
application. But, on the whole, the form represented in fig. 78, is
the most convenient, and should generally be preferred, as any common
astronomical eye-piece can be applied to it. I have used a diagonal
eye-piece of this kind, with good effect, when a power of 180 has been
applied to the sun and other celestial objects.

Instead of a metallic speculum, a _rectangular prism of glass_
is sometimes substituted; for the rays of light are then bent by
reflection from the second polished surface, which ought to be _dry_,
and undergo two refractions which achromatise them; and the same effect
is thus produced as by polished metal. Ramsden sometimes gave one of
the polished faces of a right angled prism a curve, which prism served
instead of a lens in an eye-piece, and also performed the office of a
reflector. A semi-globe, or what has been called a Bull’s eye, has also
been used as a diagonal eye-piece, and when the curve is well-formed,
and the glass good, it is achromatic, and is said to perform pretty
well, but it is not superior to the forms already described.


SECT. 2.--TERRESTRIAL EYE-PIECES.

When describing the common refracting telescope, (p. 228.) I have
noticed that three eye-glasses, placed at double their focal distances
from each other, formerly constituted the terrestrial eye-piece,
as represented in fig. 47. But this construction, especially for
achromatic instruments, has now become obsolete, and is never used,
except in small pocket spy-glasses formed with a single object lens. In
its place a four glassed eye-piece has been substituted, which is now
universally used in all good telescopes, and which, besides improving
the vision and producing an erect position of the images of objects,
presents a considerably larger field of view. During the progressive
stages of improvement made in the construction of erect eye-pieces by
Dollond and Ramsden, three, four, and five lenses were successively
introduced; and hence, in some of the old telescopes constructed by
these artists, we frequently find five lenses of different descriptions
composing the eye-piece. But four lenses, arranged in the manner I
am now about to describe, have ultimately obtained the preference.
In a telescope having a celestial eye-piece of the Huygenian form,
the image that is formed in the focus of the object glass, is that
which is seen magnified, and in an inverted position; but when a four
glassed eye-piece is used, which produces an erect view of the object,
the image is repeated, and the _second_ image, which is formed by
the inner pair of lenses AB on an enlarged scale, is that which the
pair of lenses CD at the eye-end render visible on a scale still more
enlarged. The modern terrestrial eye-piece, represented in fig. 79,
is, in fact, nothing else than a compound microscope, consisting of an
object lens, an amplifying lens, and an eye-piece composed of a pair
of lenses on the principle of the Huygenian eye-piece. Its properties
will be best understood by considering the first image of an object,
which is formed in the focus of the object glass, as a small luminous
object to be rendered visible, in a magnified state, by a compound
microscope. The object to be magnified may be considered as placed near
the point A, and the magnified image at _i_, which is viewed by the
lens D. Hence, if we look through such an eye-piece at a small object
placed very near the lens A, we shall find that it acts as a compound
microscope of a moderate magnifying power increasing, in some cases,
the diameter of the object about 10 times, and 100 times in surface.

[Illustration: _figure 79._]

In order to distinguish the different lenses in this eye-piece, we may
call the lens A, which is next to the first image, the _object-lens_,
the next to it B, the _amplifying-lens_, the third, or C, the
_field-lens_, and the one next the eye, D, the _eye-lens_. The first
image formed a little before A, may be denominated the _radiant_, or
the object from which the rays proceed. Now, it is well known as a
principle in optics, that if the radiant be brought nearer to the lens
than its principal focus, the emerging rays will _diverge_, and, on
the contrary, if the radiant be put farther from the lens than its
principal focal distance, the emerging rays will _converge_ to a point
at a distance beyond the lens, which will depend on the distance of the
radiant from the first face of the lens. In this place an image of the
radiant will be formed by the concurrence of the converging rays, but
in a contrary position; and the length of the image will exceed the
length of the radiant in the same proportion, as the distance of the
image from the radiant exceeds that of the radiant from the lens. This
secondary image of the radiant at _i_, is not well-defined, when only
one lens, as A, is used, owing to the great spherical aberrations, and
therefore the amplifying lens is placed at the distance of the shorter
conjugate focus, with an intervening diaphragm of a small diameter at
the place of the principal focus; the uses of which lens and diaphragm
are, first to cut off the  rays that are occasioned by the
dispersive property of the object lens,--and secondly, to bring the
rays to a shorter conjugate focus for the place of the image, than
would have taken place with a single lens having only one refraction.
As the secondary image is in this way much better defined and free from
colouration, the addition of this second lens is a great improvement to
vision. For this reason I am clearly of opinion, that the object glass
of a compound microscope, instead of consisting of a small single lens,
should be formed of two lenses on the principle now stated, which would
unquestionably add to the distinctness of vision.

With respect to _the proportions of the focal lengths of the lenses_
in this four glass eye-piece, Mr. Coddington states, that if the focal
lengths, reckoning from A to D, fig. 79, be as the numbers 3, 4, 4 and
3, and the distances between them on the same scale, 4, 6, and 5, 2,
the radii, reckoning from the outer surface of A, should be thus:--

  A {First surface  27 }nearly plano-convex.
    {Second surface  1 }

  B {First surface   9 }a miniscus.
    {Second surface  4 }

  C {First surface    1 }nearly plano-convex.
    {Second surface  21 }

  D {First surface    1 }double convex.
    {Second surface  24 }

Sir D. Brewster states, that a good achromatic eye-piece may be made of
4 lenses, if their focal lengths, reckoning from that next the object,
be as the numbers 14, 21, 27, 32; their distances 23, 44, 40; their
apertures 5.6; 3.4; 13.5; 2.6; and the aperture of the diaphragm placed
in the interior focus of the fourth eye-glass, 7. Another proportion
may be stated:--Suppose the lens next the object A, to be 1-7/8 inch
focal length, then B may be 2-1/2 inches, C 2 inches, and D 1-1/2; and
their distances AB 2-1/2; BC 3-5/8; and CD 2-3/8. In one of Ramsden’s
small telescopes, whose object glass was 8-1/2 inches in focal length,
and its magnifying power 15.4, the focal lengths of the eye glasses
were A 0.775 of an inch, B 1.025, C 1.01, D 0.79;--the distances AB
1.18, BC 1.83, and CD 1.105. In the excellent achromatic telescope
of Dollond’s construction which belonged to the Duc de Chaulnes, the
focal lengths of the eye glasses, beginning with that next the object,
were 14-1/4 lines, 19, 22-3/4, 14; their distances 22.48 lines, 46.17,
21.45, and their thickness at the centre, 1.23 lines, 1.25, 1.47. The
fourth lens was plano-convex, with the plane side to the eye, and the
rest were double convex lenses. This telescope was in focal length 3
feet 5-1/2 inches.

The magnifying power of this eye-piece, as usually made, differs only
in a small degree from what would be produced by using the first or
the fourth glass alone, in which case the magnifying power would be
somewhat greater, but the vision less distinct, and were the lens next
the eye used alone without the field glass, the field of view would
be much contracted. Stops should be placed between the lenses A and B,
near to B, and a larger one between C and D, to prevent any false light
from passing through the lenses to the eye. The more stops that are
introduced into a telescope--which should all be blackened--provided
they do not hinder the pencils of light proceeding from the object, the
better will the instrument perform.

For the information of amateur constructors of telescopes, I shall
here state the dimensions of two or three four glassed eye-pieces in
my possession, which perform with great distinctness, and present a
pretty large field of view. In one of these, adapted to a 44-1/2 inch
achromatic, the lens A, next the object, is 1-7/8 inch, focal length,
and about 1 inch diameter, with the plane side, next the object. The
focal length of the lens B 2-1/10 inches, diameter 7/10 inch, with its
plane side next A; distance of these lenses from each other 2-4/10
inches. Distance of the field lens C from the lens B 5-1/2 inches. The
small hole or diaphragm between A and B is at the focus of A, and is
about 1/6 inch diameter, and about 3/8 of an inch from the lens B. The
field lens C is 2 inches focal length, and 1-1/4 inch diameter, with
its plane side next the eye. The lens next the eye D is 1 inch focal
distance, 1/2 inch diameter, and is distant from the field glass 1-3/4
inch, with its plane side next the eye. The magnifying power of this
eye-piece is equivalent to that of a single lens whose focal length is
half an inch, and with the 44-1/2 inch object glass produces a power of
about 90 times. The lens next the eye can be changed for another 1-3/8
inch focal length, which produces a power of 65; and the two glasses
CD can be changed for another set, of a longer focal distance which
produces a power of 45 times. The whole length of this eye-piece is
11-1/2 inches.

In another eye-piece, adapted to a pocket achromatic, whose object
glass is 9 inches focal length, the lens A is 1 inch focal length,
and 1/2 inch diameter; the lens B 1-1/4 inch, and 1/2 inch diameter,
their distance 1-1/2 inch, the lens C 1-1/10 inch focal length, and
5/8 inch diameter; the eye-lens D 5/8 inch focal length, and 3/8 inch
diameter; distance between C and D 1-1/8 inch. The distance between B
and C 1-3/4 inch. The whole length of this eye-piece is 4-1/2 inches,
and its power is nearly equal to that of a single lens of 1/2 or 6/10
of an inch focal length, the magnifying power of the telescope being
about 16 times. Another eye-piece of much larger dimensions, has the
lens A of 2-1/2 inches focal length, and 3/4 inch diameter: the lens
B 2-3/4 inches focus and 5/8 inch diameter; and their distance 2-3/4
inches; the lens C 2-5/8 inches focus and 1-1/8 inch diameter; the lens
D 1-3/4 inch focus and 3/4 inch diameter; distance from each other
2-3/4 inches. The distance between the lenses B and C is 4 inches. The
magnifying power is equal to that of a single lens 1-1/8 inch focal
distance. When applied to an achromatic object glass 6 feet 7 inches
focal length, it produces a power of about 70 times. This eye-piece has
a moveable tube 9 inches in length in which the two lenses next the
eye are contained, by pulling out which, and consequently increasing
the distance between the lenses B and C, the magnifying power may be
increased to 100, 120 or 140, according to the distance to which this
moveable tube is drawn out. It has also a second and third set of
lenses, corresponding to C and D of a shorter focal distance, which
produce higher magnifying powers on a principle to be afterwards
explained.


_Description of an eye-piece, &c. of an old Dutch Achromatic Telescope._

About twenty or thirty years ago, I purchased, in an optician’s shop
in Edinburgh, a small achromatic telescope, made in Amsterdam, which
was supposed, by the optician, to have been constructed prior to the
invention of achromatic telescopes by Mr. Dollond. It is mounted wholly
of brass, and in all its parts is a piece of beautiful and exquisite
workmanship, and the utmost care seems to have been taken to have all
the glasses and diaphragms accurately adjusted. The object glass is a
double achromatic, 6-1/2 inches focal distance and 1 inch diameter,
but the clear aperture is only 7/8 inch diameter. It is perfectly
achromatic, and would bear a power of 50 times, if it had a sufficient
quantity of light. The following inscription is engraved on the tube
adjacent to the object glass:--“_Jan van Deyl en Zoon Invenit et
Fecit, Amsterdam, Ao. 1769_.” Although Dollond exhibited the principle
of an achromatic telescope, eight or ten years before the date here
specified, yet it is not improbable that the artist whose name is
here stated, may not have heard of Dollond’s invention; and that he
was really, as he assumes, one of the inventors of the achromatic
telescope. For, the invention of this telescope by Dollond was not very
generally known, except among philosophers and the London opticians,
till a number of years after the date above stated. Euler, in his
“Letters to a German Princess”--in which telescopes are particularly
described, makes no mention of, nor the least allusion to the invention
of Dollond, though this was a subject which particularly engaged his
attention. Now, these letters were written in 1762, but were not
published till 1770. When alluding to the defects in telescopes arising
from the different refrangibility of the rays of light, in Letter
43, and that they might possibly be rectified by means of different
transparent substances, he says, ‘But neither theory nor practice have
hitherto been carried to the degree of perfection necessary to the
execution of a structure which should remedy these defects.’ Mr. B.
Martin, in his ‘Gentleman and Lady’s Philosophy,’ published in 1781,
alludes to the achromatic telescope, but speaks of it as it were but
very little, if at all superior to the common refracting telescope. And
therefore, I think it highly probable that Jan van Deyl, was really an
inventor of an achromatic telescope, before he had any notice of what
Dollond and others had done in this way some short time before.

But my principal object in adverting to this telescope, is to describe
the structure of the eye-piece, which is a very fine one, and which
is somewhat different from the achromatic eye-piece above described.
It consists of four glasses, two combined next the eye, and two next
the object. Each of these combinations forms an astronomical eye-piece
nearly similar to the Huygenian. The lens A, next the object, fig. 80,
is 5/8 inch focal distance, and 4/10 inch diameter; the lens B 3/8 inch
focus, and 1/5 inch diameter, and the distance between them somewhat
less than 5/8 inch; the diameter of the aperture _e_ about 1/15 of an
inch. This combination forms an excellent astronomical eye-piece, with
a large flat field, and its magnifying power is equivalent to that of
a single lens 5/8 or 6/8 focal length. The lens C is 1/2 inch focal
length, and 4/10 inch diameter; the lens D 1/4 inch focus, and about
1/5 inch diameter; their distance about 1/2 inch, or a small fraction
more. The hole at _d_ is about 1/20 or 1/25 of an inch diameter, and
the distance between the lenses B and C about 1-1/2 inch. The whole
length of the eye-piece is 3-1/4 inches--exactly the same size as
represented in the engraving. Its magnifying power is equal to that of
a single lens 1/4 inch focal length; and consequently the telescope,
though only 9-1/2 inches long, magnifies 26 times, with great
distinctness, though there is a little deficiency of light when viewing
land objects, which are not well illuminated.

[Illustration: _figure 80._]

The glasses of this telescope are all plano-convex, with their
convex-sides towards the object--except the lens D, which is double
convex, but flattest on the side next the eye, and they are all very
accurately finished. The two lenses C and D form an astronomical
eye-piece nearly similar to that formed by the lenses A and B. The
focus of the telescope is adjusted by a screw, the threads of which are
formed upon the outside of a tube into which the eye-piece slides. The
eye-piece and apparatus connected with it, is screwed into the inside
of the main tube, when not in use, when the instrument forms a compact
brass cylinder 6 inches long, which is enclosed in a fish-skin case,
lined with silk velvet, which opens with hinges.

The lenses in the eye-pieces formerly described, though stated to be
plano-convexes, are for the most part _crossed glasses_, that is
ground on tools of a long focus on the one side, and to a short focus
on the other. The construction of the eye-piece of the Dutch telescope
above described, is one which might be adopted with a good effect
in most of our achromatic telescopes; and I am persuaded, from the
application I have made of it to various telescopes, that it is even
superior, in distinctness and accuracy, and in the _flatness of field_
which it produces to the eye-piece in common use. The two astronomical
eye-pieces of which it consists, when applied to large achromatic
telescopes, perform with great accuracy, and are excellently adapted
for celestial observations.


SECT. 3.--DESCRIPTION OF THE PANCRATIC EYE-TUBE.

From what we have stated, when describing the common terrestrial
eye-piece now applied to achromatic instruments, (p. 349, fig. 79.), it
appears obvious, that any variety of magnifying powers, within certain
limits, may be obtained by removing the set of lenses CD, fig. 79,
nearer to or farther from the tube which contains the lenses A and B,
on the same principle as the magnifying power of a compound microscope
is increased by removing the eye-glasses to a greater distance from
the object-lens. If then, the pair of eye-lenses CD be attached to an
inner tube that will draw out and increase their distance from the
inner pair of lenses, as the tube _a b c d_, the magnifying power may
be indefinitely increased or diminished, by pushing in or drawing out
the sliding tube, and a scale might be placed on this tube, which, if
divided into equal intervals, will be a scale of magnifying powers, by
which the power of the telescope will be seen at every division, when
the lowest power is once determined.

Sir David Brewster, in his ‘Treatise on New Philosophical instruments,’
Book i. chap. vii. page 59, published in 1813, has adverted to this
circumstance, in his description of an ‘Eye-piece wire micrometer,’ and
complains of Mr. Ezekiel Walker, having in the ‘Philosophical Magazine’
for August, 1811, described such an instrument as an invention of his
own. Dr. Kitchener some years afterwards, described what he called a
Pancratic or omnipotent eye-piece, and got one made by Dollond, with a
few modifications different from that suggested by Brewster and Walker,
which were little else than cutting the single tube into several parts,
and giving it the _appearance_ of a new invention. In fact, none of
these gentlemen had a right to claim it as his peculiar invention, as
the principle was known and recognised long before. I had increased the
magnifying powers of telescopes, on the same principle, several years
before any of these gentlemen communicated their views on the subject,
although I never formally constructed a scale of powers. Mr. B. Martin,
who died in 1782, proposed many years before, such a moveable interior
tube as that alluded to, for varying the magnifying power.

In order to give the reader a more specific idea of this contrivance,
I shall present him with a figure and description of one of Dr.
Kitchener’s Pancratic eye-pieces, copied from one lately in my
possession. The following are the exact dimensions of this instrument,
with the focal distances, &c. of the glasses, &c. of which it is
composed.

[Illustration: _fig. 81._]

                                                      In.  Tenths.
  Length of the whole eye-piece, consisting of
  four tubes, when fully drawn out, or the distance
  from A to B. fig. 81.                                14       4

  Length of the three tubes on which the
  scale is engraved, from the commencement of
  the divisions at B to their termination at C.         9      15

  Each division into tens is equal to 3-10ths of an inch.

  When the three inner tubes are shut up to
  C, the length of the eye-piece is exactly             5       5

  When these tubes are thus shut up, the magnifying
  power for a 3-1/2 feet achromatic is 100
  times, which is the smallest power. When the
  inner tube is drawn out 1/3 of an inch, or to
  the first division, the power is 110, &c.

  Focal distance of the lens next the object            1       0

  Breadth of Ditto.                                     0      65

  The plane side of this glass is next the object.

  Focal distance of the second glass from the
  object                                                1       5

  This glass is double and equally convex,
  Breadth                                               0       5

  Distance between these two glasses                    1       7

  Focal distance of the third or field lens,
  which is plane on the side next the eye               1       1

  Breadth of Ditto.                                     0      55

  Focal distance of the lens next the eye               0       6

  Breadth                                               0      43

  This glass is plane on the side next the eye.

  Distance between the third and fourth glasses.        1       1

From the figure and description, the reader will be at no loss to
perceive how the magnifying power is ascertained by this eye-piece.
If the lowest power for a 44 inch telescope be found to be 100, when
the three sliding tubes are shut into the larger one, then by drawing
out the tube next the eye 4 divisions, a power of 140 is produced; by
drawing out the tube next the eye its whole length, and the second
tube to the division marked 220, a power of 220 times is produced, and
drawing out all the tubes to their utmost extent, as represented in
the figure, a power of 400 is obtained. These powers are by far too
high for such a telescope, as the powers between 300 and 400 can seldom
or never be used. Were the scale to begin at 50, and terminate at
200, it would be much better adapted to a 3-1/2 feet telescope. Each
alteration of the magnifying power requires a new adjustment of the
eye-piece for distinct vision. As the magnifying power is increased,
the distance between the eye-glass and the object-glass must be
diminished. Dr. Kitchener says, that ‘the pancratic eye tube gives a
better defined image of a fixed star, and shows double stars decidedly
more distinct and perfectly separated than any other eye tube, and
that such tubes will probably enable us to determine the distances
of these objects from each other, in a more perfect manner than has
been possible heretofore.’ These tubes are made by Dollond, London,
and are sold for two guineas each. But I do not think they excel, in
distinctness, those which are occasionally made by Mr. Tulley and other
opticians.




CHAPTER VI.


MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.

The following remarks, chiefly in regard to the manner of using
telescopes, may perhaps be useful to young observers, who are not much
accustomed to the mode of managing these instruments.

1. _Adjustments requisite to be attended to in the use of telescopes._
When near objects are viewed with a considerable magnifying power, the
eye-tube requires to be removed farther from the object-glass than
when very distant objects are contemplated. When the telescope is
adjusted for an object, 6, 8, or 10 miles distant, a very considerable
alteration in the adjustment is requisite in order to see distinctly
an object at the distance of two or three hundred yards, especially if
the instrument is furnished with a high magnifying power. In this last
case, the eye-tube requires to be drawn out to a considerable distance
beyond the focus for parallel rays. I have found that, in a telescope
which magnifies 70 times, when adjusted for an object at the distance
of two miles, the adjustment requires to be altered fully one inch in
order to perceive distinctly an object at the distance of two or three
hundred yards; that is, the tube must be drawn, in this case, an inch
farther from the object-glass, and pushed in the same extent, when we
wish to view an object at the distance of two or three miles. These
adjustments are made, in pocket perspectives, by gently sliding the
eye-tube in or out, by giving it a gentle circular or spiral motion
till the object appear distinct. In using telescopes which are held
in the hand, the best plan is to draw all the tubes out to their full
length, and then, looking at the object, with the left hand supporting
the main tube near the object-glass, and the right supporting the
eye-tube--gently and gradually push in the eye-piece till distinct
vision be obtained. In Gregorian reflecting telescopes this adjustment
is made by means of a screw connected with the small speculum; and in
large achromatics, by means of a rack and pinion connected with the
eye-tube. When the magnifying power of a telescope is comparatively
small, the eye-tube requires to be altered only a very little.

There is another adjustment requisite to be attended to, in order to
adapt the telescope to the eyes of different persons. Those whose eyes
are too convex, or who are short-sighted, require the eye-tube to be
pushed in, and those whose eyes are somewhat flattened, as old people,
require the tube to be drawn out. Indeed there are scarcely two persons
whose eyes do not require different adjustments in a slight degree.
In some cases I have found that the difference of adjustment for two
individuals, in order to produce distinct vision in each, amounted
to nearly half an inch. Hence the difficulty of exhibiting the sun,
moon, and planets through telescopes, and even terrestrial objects,
to a company of persons who are unacquainted with the mode of using
or adjusting such instruments--not one half of whom generally see
the object distinctly--for, upon the proper adjustment of a telescope
to the eye, the accuracy of vision, in all cases, depends; and no one
except the individual actually looking through the instrument, can
be certain that it is accurately adjusted to his eye, and even the
individual himself, from not being accustomed to the view of certain
objects, may be uncertain whether or not the adjustment be correct. I
have found by experience that when the magnifying powers are high, as
150 or 200, the difference of adjustment required for different eyes
is very slight; but when low powers are used, as 20, 30, or 40, the
difference of the requisite adjustments is sometimes very considerable,
amounting to 1/4 or 1/2 of an inch.

2. _State of the Atmosphere most proper for observing terrestrial
and celestial objects._ The atmosphere which is thrown around the
globe--while it is essentially requisite to the physical constitution
of our world, and the comfort of its inhabitants--is found in
many instances a serious obstruction to the accurate performance
of telescopes. Sometimes it is obscured by mists and exhalations,
sometimes it is thrown into violent undulations by the heat of the
sun and the process of evaporation, and even, in certain cases, where
there appears a pure unclouded azure, there is an agitation among its
particles and the substances incorporated with them, which prevents
the telescope from producing distinct vision either of terrestrial or
celestial objects. For viewing distant terrestrial objects, especially
with high powers, the best time is early in the morning, a little
after sun-rise, and, from that period till about 9 o’clock A.M., in
summer; and, in the evening about two or three hours before sun-set.
From about 10 o’clock A.M. till 4 or 5 in the afternoon, in
summer, if the sky be clear and the sun shining, there is generally
a considerable undulation in the atmosphere, occasioned by the solar
rays and the rapid evaporation, which prevents high powers from being
used with distinctness on any telescope, however excellent. The objects
at such times, when powers of 50, 70, or 100 are applied, appear to
undulate like the waves of the sea, and, notwithstanding every effort
to adjust the telescope, they appear confused and indistinct. Even
with very moderate magnifying powers this imperfection is perceptible.
In such circumstances, I have sometimes used a power of 200 times
on distant land objects, with good effect, a little before sun-set,
when, in the forenoon of the same day, I could not have applied a
power of 50 with any degree of distinctness. On days when the air is
clear, and the atmosphere covered with clouds, terrestrial objects
may be viewed with considerably high powers. When there has been a
long-continued drought, the atmosphere is then in a very unfit state
for enjoying distinct vision with high magnifying powers, on account of
the quantity of vapours with which the atmosphere is then surcharged,
and the undulations they produce. But, after copious showers of rain,
especially if accompanied with high winds, the air is purified, and
distant objects appear with greater brilliancy and distinctness than at
any other seasons. In using telescopes, the objects at which we look
should, if possible, be nearly in a direction opposite to that of the
sun. When they are viewed nearly in the direction of the sun, their
shadows are turned towards us, and they consequently appear dim and
obscure. By not attending to this circumstance, some persons, in trying
telescopes, have pronounced a good instrument to be imperfect, which,
had it been tried on objects properly illuminated, would have been
found to be excellent. In our variable northerly climate the atmosphere
is not so clear and serene for telescopic observation as in Italy,
the South of France, and in many of the countries which lie within
the tropics. The undulations of the air, owing to the causes alluded
to above, constitute one of the principal reasons why a telescope
magnifying above a hundred times can seldom be used with any good
effect in viewing terrestrial objects--though I have sometimes used a
power of nearly 200 with considerable distinctness, in the stillness
of a summer or autumnal evening, when the rays of the declining sun
strongly illuminated distant objects.

The atmosphere is likewise frequently a great obstruction to the
distinct perception of _celestial_ objects. It is scarcely possible for
one who has not been accustomed to astronomical observations, to form
a conception of the very great difference there is in the appearance
of some of the heavenly bodies in different states of the atmosphere.
There are certain conditions of the atmosphere essentially requisite
for making accurate observations with powerful telescopes, and it
is but seldom, especially in our climate, that all the favourable
circumstances concur. The nights must be very clear and serene--the
moon absent--no twilight--no haziness--no violent wind--no sudden
change of temperature, as from thaw to frost--and no surcharge of the
atmosphere with aqueous vapour. I have frequently found that, on the
first and second nights after a thaw, when a strong frost had set in,
and when the heavens appeared very brilliant, and the stars vivid
and sparkling--the planets, when viewed with high powers, appeared
remarkably undefined and indistinct; their margins appeared waving and
jagged, and the belts of Jupiter, which at other times were remarkably
distinct, were so obscured and ill-defined, that they could with
difficulty be traced. This is probably owing to the quantity of aqueous
vapour, and perhaps icy particles, then floating in the air, and to
the undulations thereby produced. When a hard frost has continued a
considerable time, this impediment to distinct observation is in a
great measure removed. But I have never enjoyed more accurate and
distinct views of the heavenly bodies than in fresh serene evenings,
when there was no frost and no wind, and only a few fleecy clouds
occasionally hovering around. On such evenings, and on such alone, the
highest powers may be applied. I have used magnifying powers on such
occasions with good effect, which could not have been applied, so as to
ensure distinct vision, more frequently than two or three days in the
course of a year.

Sir William Herschel has observed, in reference to this point, ‘In
beautiful nights, when the outside of our telescopes is dropping with
moisture, discharged from the atmosphere, there are now and then
favourable _hours_ in which it is hardly possible to put a limit to
the magnifying powers. But such valuable opportunities are extremely
scarce, and with large instruments it will always be lost labour to
observe at other times. In order therefore, to calculate how long a
time it must take to sweep the heavens, as far as they are within
the reach of my forty-feet telescope, charged with a magnifying
power of 1000, I have had recourse to my journals to find how many
favourable hours we may annually hope for in this climate. And, under
all favourable circumstances, it appears, that a year which will
afford ninety, or at most, one hundred _hours_ is to be called very
productive.’ ‘In the equator, with my twenty feet telescope, I have
swept over zones of two degrees with a power of 157, but an allowance
of ten minutes in Polar distance must be made for lapping the sweeps
over one another where they join. As the breadth of the zones may be
increased towards the poles, the northern hemisphere may be swept in
about 40 zones; to these we must add 19 southern zones; then 59 zones
which, on account of the sweeps lapping over one another, about 5
minutes of time in right ascension, we must reckon of 25 hours each,
will give 1475 hours. And allowing 100 hours per year, we find that
with the 20 feet telescope, the heavens may be swept in about 14 years
and three quarters. Now the time of sweeping with different magnifying
powers will be as the squares of the powers; and putting _p_ and _t_
for the power and time in the 20 feet telescope, and P = 1000 for
the power in the 40 feet instrument, we shall have p^2 : t :: P^2 :
^{tP2}/_{p^2} = 59840. Then making the same allowance for 100 hours per
year, it appears that it will require not less than 598 years, to look
with the 40 feet reflector, charged with the above-mentioned power,
only one single moment into each point of space; and even then, so much
of the southern hemisphere will remain unexplored, as will take up 213
years more to examine.’[28]

From the above remarks of so eminent an observer, the reader will
perceive how difficult it is to explore the heavens with minuteness and
accuracy, and with how many disappointments, arising from the state
of the atmosphere, the astronomer must lay his account, when employed
in planetary or sidereal investigation. Besides the circumstances now
stated, it ought to be noticed that a star or a planet is only in
a situation for a high magnifying power, about half the time it is
above the horizon. The density of the atmosphere, and the quantity of
vapours with which it is charged near the horizon, prevent distinct
vision of celestial objects with high powers, till they have risen to
at least 15 or 20 degrees in altitude, and the highest magnifiers can
scarcely be applied with good effect, unless the object is near the
meridian, and at a considerable elevation above the horizon. If the
moon be viewed a little after her rising, and afterwards when she comes
to her highest elevation in autumn, the difference in her appearance
and distinctness will be strikingly perceptible. It is impossible to
guess whether a night be well adapted for celestial observations,
till we actually make the experiment, and instruments are frequently
condemned, when tried at improper seasons, when the atmosphere only is
in fault. A certain observer remarks,--‘I have never seen the face of
Saturn more distinctly than in a night when the air has been so hazy,
that with my naked eye, I could hardly discern a star of less than the
third magnitude.’ The degree of the transparency of the air is likewise
varying almost in the course of every minute, so that even in the
course of the same half hour, planets and stars will appear perfectly
defined, and the reverse. The vapours moving and undulating the
atmosphere, even when the sky appears clear to the naked eye, will in a
few instants destroy the distinctness of vision, and in a few seconds
more, the object will resume its clear and well-defined aspect.[29]

3. _On the magnifying powers requisite for observing the phenomena of
the different planets--comets--double stars, &c._

There are some objects connected with astronomy which cannot be
perceived without having recourse to instruments and to powers of
great magnitude. But it is a vulgar error to imagine that very large
and very expensive telescopes are absolutely necessary for viewing
the greater part of the more interesting scenery of the heavens. Most
of the phenomena of the planets, comets and double stars and other
objects, are visible with instruments of moderate dimensions, so that
every one who has a relish for celestial investigations, may, at a
comparatively small expense, procure a telescope, for occasional
observations, which will show the principal objects and phenomena
described in books on astronomy. Many persons have been misled by some
occasional remarks which Sir W. Herschel made, in reference to certain
very high powers which he sometimes put, by way of experiment, on some
of his telescopes, as if these were the powers requisite for viewing
the objects to which he refers. For example, it is stated that he once
put a power of 6450 times on his 7 feet Newtonian telescope of 6-3/10
inches aperture; but this was only for the purpose of an experiment,
and could be of no use whatever when applied to the moon, the planets
and most objects in the heavens. Herschel, through the whole course of
his writings, mentions his only having used it _twice_, namely on the
stars α Lyræ, and γ Leonis, which stars can be seen more distinctly and
sharply defined with a power of 420. To produce a power of 6450 on such
a telescope, would require a lens of only 1/77th of an inch in focal
distance, and it is questioned by some whether Herschel had lenses of
so small a size in his possession, or whether it is possible to form
them with accuracy.

_Powers requisite for observing the phenomena of the planets._--The
planet Mercury requires a considerable magnifying power, in order to
perceive its phases with distinctness. I have seldom viewed this planet
with a less power than 100 and 150, with which powers its half moon,
its gibbous, and its crescent phase, may be distinctly perceived. With
a power of 40, 50, or even 60 times, these phases can with difficulty
be seen, especially as it is generally at a low altitude, when such
observations are made. The phases of _Venus_ are much more easily
distinguished, especially the _crescent_ phase, which is seen to
the greatest advantage about a month before and after the inferior
conjunction. With a power not exceeding 25 or 30 times, this phase,
at such periods, may be easily perceived. It requires, however, much
higher powers to perceive distinctly the variations of the gibbous
phase; and if this planet be not viewed at a considerably high altitude
when in a half-moon or gibbous phase, the obscurity and undulations
of the atmosphere near the horizon, prevent such phases from being
accurately distinguished, even when high powers are applied. Although
certain phenomena of the planets may be seen with such low powers as
I have now stated, yet, in every instance, the highest magnifying
powers, consistent with distinctness, should be preferred, as the eye
is not then strained, and the object appears with a greater degree of
magnitude and splendour. The planet _Mars_ requires a considerable
degree of magnifying power, even when at its nearest distance from the
earth, in order to discern its spots and its gibbous phase. I have
never obtained a satisfactory view of the spots which mark the surface,
and their relative position, with a less power than 130, 160, or 200
times; and even with such powers, persons not much accustomed to look
through telescopes, find a difficulty in distinguishing them.

The strongest and most prominent _belts of Jupiter_, may be seen with
a power of about 45; which power may be put upon a 20-inch achromatic,
or a 1 foot reflector. But a satisfactory view of all the belts, and
the relative positions they occupy, cannot be obtained with much
lower powers than 80, 100, or 140. The most common positions of these
belts are--one dark and well-defined belt to the south of Jupiter’s
equator; another of nearly the same description to the north of it,
and one about his north and his south polar circles. These polar belts
are much more faint, and consequently not so easily distinguished as
the equatorial belts. The _moons_ of this planet, in a very clear
night, may sometimes be seen with a pocket 1 foot achromatic glass,
magnifying about 15 or 16 times. Some people have pretended that they
could see some of these satellites with their naked eye; but this is
very doubtful, and it is probable that such persons mistook certain
fixed stars which happened to be near Jupiter for his satellites. But,
in order to have a clear and interesting view of these, powers of at
least 80 or 100 times should be used. In order to perceive their
immersions into the shadow of Jupiter, and the exact moment of their
emersions from it, a telescope not less than a 44 inch achromatic, with
a power of 150 should be employed. When these satellites are viewed
through large telescopes with high magnifying powers, they appear
with well defined disks, like small planets. The planet Jupiter has
generally been considered as a good test by which to try telescopes
for celestial purposes. When it is near the meridian and at a high
altitude, if its general surface, its belts, and its margin appear
distinct and well-defined, it forms a strong presumptive evidence that
the instrument is a good one.

The planet _Saturn_ forms one of the most interesting objects for
telescopic observation. The _ring_ of Saturn may be _seen_ with a power
of 45; but it can only be contemplated with advantage when powers
of 100, 150, and 200 are applied to a 3 or a 5 feet achromatic. The
_belts of Saturn_ are not to be seen distinctly with an achromatic
of less than 2-3/4 inches aperture, or a Gregorian reflector of less
than 4 inches aperture, nor with a less magnifying power than 100
times. Sir W. Herschel has drawn this planet with five belts across
its disk; but it is seldom that above one or two of them can be seen
by moderate-sized telescopes and common observers. The _division_
of the double ring, when the planet is in a favorable position for
observation, and in a high altitude, may sometimes be perceived with
a 44-inch achromatic, with an aperture of 2-3/4 inches, and with
powers of 150 or 180, but higher powers and larger instruments are
generally requisite to perceive this phenomenon distinctly; and even
when a portion of it is seen at the extremities of the _ansæ_, the
division cannot, in every case, be traced along the whole of the
half-circumference of the ring which is presented to our eye. Mr.
Hadley’s engraving of Saturn, in the ‘Philosophical Transactions’
for 1723, though taken with a Newtonian reflector with a power of
228, represents the division of the ring as seen only on the ansæ or
extremities of the elliptic figure in which the ring appears. The
best period for observing this division is when the ring appears at
its utmost width. In this position it was seen in 1840, and it will
appear nearly in the same position in 1855. When the ring appears like
a very narrow ellipse, a short time previous to its disappearance, the
division, or dark space between the rings, cannot be seen by ordinary
instruments.

Sir W. Herschel very properly observes, ‘There is not perhaps another
object in the heavens that presents us with such a variety of
extraordinary phenomena as the planet Saturn; a magnificent globe,
encompassed by a stupendous double ring; attended by seven satellites;
ornamented with equatorial belts; compressed at the poles; turning upon
its axis; mutually eclipsing its ring and satellites, and eclipsed by
them; the most distant of the rings also turning upon its axis, and the
same taking place with the farthest of the satellites; all the parts
of the system of Saturn occasionally reflecting light on each other;
the rings and moons illuminating the nights of the Saturnian, the globe
and satellites enlightening the dark parts of the ring; and the planet
and rings throwing back the sun’s beams upon the moons, when they are
deprived of them at the time of their conjunctions.’ This illustrious
astronomer states, that with a new 7 feet mirror of extraordinary
distinctness he examined this planet, and found that the ring reflects
more light than the body, and with a power of 570 the colour of the
body becomes yellowish, while that of the ring remains more white. On
March 11, 1780, he tried the powers of 222, 332, and 440 successively,
and found the light of Saturn less intense than that of the ring; the
colour of the body turning, with the high powers, to a kind of yellow
white, while that of the ring still remained white.

Most of the _satellites_ of Saturn are difficult to be perceived
with ordinary telescopes, excepting the 4th, which may be seen with
powers of from 60 to 100 times. It was discovered by Huygens in 1655,
by means of a common refracting telescope 12 feet long, which might
magnify about 70 times. The next in brightness to this is the 5th
satellite, which Cassini discovered in 1671, by means of a 17 feet
refractor, which might carry a power of above 80 times. The 3rd was
discovered by the same astronomer in 1672, by a longer telescope; and
the 1st and 2nd, in 1684, by means of two excellent object-glasses of
100 and 136 feet, which might have magnified from 200 to 230 times.
They were afterwards seen by two other glasses of 70 and 90 feet, made
by Campani, and sent from Rome to the Royal Observatory at Paris, by
the King’s order, after the discovery of the 3rd and 5th satellites.
It is asserted, however, that all those 5 satellites were afterwards
seen with a telescope of 34 feet, with an aperture of 3-3/10 inches,
which would magnify about 120 times. These satellites, on the whole,
except the 4th and 5th, are not easily detected. Dr. Derham, who
frequently viewed Saturn through Huygens’ glass of 126 feet focal
length, declares, in the preface to his ‘Astro-Theology,’ that he could
never perceive above 3 of the satellites. Sir W. Herschel observes,
that the visibility of these minute and extremely faint objects,
depends more on the _penetrating_ than upon the _magnifying_ power
of our telescopes; and that with a 10 feet Newtonian, charged with
a magnifying power of only 60, he saw all the 5 old satellites; but
the 6th and 7th, which were discovered and were easily seen with his
40-feet telescope, and were also visible in his 20-feet instrument,
were not discernible in the 7 or the 10-feet telescopes, though all
that _magnifying power_ can do may be done as well with the 7-feet as
with any larger instrument. Speaking of the 7th satellite, he says,
‘Even in my 40-feet reflector it appears no bigger than a very small
lucid point. I see it, however, very well in the 20-feet reflector; to
which the exquisite figure of the speculum not a little contributes.’
A late observer asserts, that in 1825, with a 12-feet achromatic, of 7
inches aperture, made by Tulley, with a power of 150, the 7 satellites
were easily visible, but not so easily with a power of 200; and that
the planet appeared as bright as brilliantly burnished silver, and the
division in the ring and a belt were very plainly distinguished, with a
power of 200.

The planet _Uranus_, being generally invisible to the naked eye, is
seldom an object of attention to common observers. A considerable
magnifying power is requisite to make it appear in a planetary form
with a well-defined disk. The best periods for detecting it are, when
it is near its opposition to the sun, or when it happens to approximate
to any of the other planets, or to a well-known fixed star. When none
of these circumstances occur, its position requires to be pointed out
by an Equatorial Telescope. On the morning of the 25th January, 1841,
this planet happened to be in conjunction with Venus, at which time
it was only 4 minutes north of that planet. Several days before this
conjunction, I made observations on Uranus. On the evening of the
24th, about 8 hours before the conjunction, the two planets appeared
in the same field of the telescope, the one exceedingly splendid, and
the other more obscure, but distinct and well-defined. Uranus could
not be perceived, either with the naked eye, or with an opera glass;
but could be distinguished as a very small star by means of a pocket
achromatic telescope magnifying about 14 times. It is questionable
whether, under the most favourable circumstances, this planet can ever
be distinguished by the naked eye. With magnifying powers of 30 and 70,
it appeared as a moderately large star with a steady light, but without
any sensible disk. With powers of 120, 180, and 250, it presented a
round and pretty well-defined disk, but not so luminous and distinct as
it would have done in a higher altitude.

The _Double Stars_ require a great _variety_ of powers, in order to
distinguish the small stars that accompany the larger. Some of them
are distinguished with moderate powers, while others require pretty
large instruments, furnished with high magnifying eye-pieces. I shall
therefore select only a few as a specimen. The star _Castor_, or α
Geminorum, may be easily seen to be double with powers of from 70 to
100. I have sometimes seen these stars, which are nearly equal in size
and colour, with a terrestrial power of 44 on a 44-inch achromatic. The
appearance of this star with such powers is somewhat similar to that
of η Coronæ in a 7 feet achromatic, of 5 inches aperture, with a power
of 500. γ Andromedæ may be seen with a moderate power. In a 30-inch
achromatic of 2 inches aperture, and a power of 80, it appears like
ε Bootis, when seen in a 5-feet achromatic, with a power of 460. This
star is said to be visible even in a 1-foot achromatic with a power of
35. ε _Lyræ_, which is a quintuple star, but appears to the naked eye
as a single star,--may be seen to be double with a power of from 6 to
12 time. γ _Leonis_ is visible in a 44-inch achromatic, with a power of
180 or 200. _Rigel_ in a 3-1/2-feet achromatic, may be seen with powers
varying from 130 to 200. The small star, however, which accompanies
Rigel, is sometimes difficult to be perceived, even with such powers.
ε _Bootis_ is seldom distinctly defined with an achromatic of less
aperture than 3-1/4 inches, or a reflector of less than 5 inches, with
a power of at least 250.

These and similar stars are not to be expected to be seen equally well
at all times, even when the magnifying and illuminating powers are
properly proportioned; as much depends upon the state of the weather,
and the pureness of the atmosphere. In order to perceive the closest
of the double stars, Sir W. Herschel recommends, that the power of the
telescope should be adjusted upon a star known to be single, of nearly
the same altitude, magnitude, and colour with the double star which
is to be observed, or upon one star above and another below it. Thus,
the late Mr. Aubert, the astronomer, could not see the two stars of γ
Leonis, when the focus was adjusted upon that star itself; but he soon
observed the small star, after he had adjusted the focus upon Regulus.
An exact adjustment of the focus of the instrument is indispensably
requisite, in order to perceive such minute objects.

In viewing the _Nebulæ_, and the very small and immensely distant
fixed stars, which require much light to render them visible, a large
aperture of the object-glass or speculum, which admits of a great
quantity of light, is of more importance than high magnifying powers.
It is light chiefly, accompanied with a moderate magnifying power,
that enables us to _penetrate_ into the distant regions of space. Sir
W. Herschel, when sweeping the profundities of the Milky way, and the
Hand and Club of Orion, used a telescope of the Newtonian form, 20-feet
focal length, and 18-7/10 inches diameter, with a power of only 157.
On applying this telescope and power to a part of the _Via Lactea_, he
found that it completely resolved the whole whitish appearance into
stars, which his former telescopes had not light enough to effect; and
which smaller instruments with much higher magnifying powers would
not have effected. He tells us, that with this power, ‘the glorious
multitude of stars,’ in the vicinity of Orion, ‘of all possible sizes,
that presented themselves to view, was truly astonishing, and that
he had fields which contained 70, 90 and 110 stars, so that a belt
of 15 degrees long, and 2 degrees broad, which passed through the
field of the telescope in an hour, could not contain less than fifty
thousand stars that were large enough to be distinctly numbered.’ In
viewing the Milky way, the Nebulæ, and small clusters of stars, such
as _Præsepe_ in Cancer, I generally use a power of 55 times, on an
achromatic telescope 6 feet 6 inches in focal length, and 4 inches
diameter. The eye-piece, which produces this power--which I formed
for the purpose--consists of two convex lenses, the one next the eye
3 inches focal length, and 1-2/10 inch diameter, and that next the
object 3-1/2 inches focus, and 1-4/10 inch diameter, the deepest convex
surfaces being next each other, and their distance 1/4 inch. With this
eye-piece a very large and brilliant field of view is obtained; and I
find it preferable to any higher powers in viewing the nebulosities,
and clusters of stars. In certain spaces of the heavens, it sometimes
presents in one field, nearly a hundred stars. It likewise serves to
exhibit a very clear and interesting view of the full moon.

In observing _Comets_, a very small power should generally be used,
even on large instruments. These bodies possess so small a quantity
of light, and they are so frequently enveloped in a veil of dense
atmosphere, that _magnifying_ power sometimes renders them more
obscure; and therefore the _illuminating_ power of a large telescope,
with a small power, is in all cases to be preferred. A comet eye-piece
should be constructed with a very large and uniformly distinct field,
and should magnify only from 15 to 30 or 40 times, and the lenses of
such an eye-tube should be nearly two inches in diameter. The late
Rev. F. Wollaston recommended for observing comets, ‘a telescope with
an achromatic object-glass of 16 inches focal length, and 2 inches
aperture, with a Ramsden’s eye-glass magnifying about 25 times, mounted
on a very firm equatorial stand, the field of view taking in 2 degrees
of a great circle.’

In viewing the _moon_, various powers may be applied according to
circumstances. The best periods of the moon for inspecting the
inequalities on its surface, are either when it assumes a crescent or
a half-moon phase, or two or three days after the period of half-moon.
Several days after full-moon, and particularly about the third quarter,
when this orb is waning, and when the shadows of its mountains and
vales are thrown in a different direction from what they are when
on the increase,--the most prominent and interesting views may be
obtained. The most convenient season for obtaining such views is during
the autumnal months, when the moon, about the third quarter, sometimes
rises as early as 8 o’clock P.M., and may be viewed at a
considerably high altitude by ten or eleven. When in the positions now
alluded to, and at a high altitude, very high magnifying powers may
sometimes be applied with good effect, especially if the atmosphere be
clear and serene. I have sometimes applied a power, in such cases, of
350 times, on a 46-inch achromatic, with considerable distinctness;
but it is only two or three times in a year, and when the atmosphere
is remarkably favourable, that such a power can be used. The autumnal
evenings are generally best fitted for such observations. The _full
moon_ is an object which is never seen to advantage with high powers,
as no shadows or inequalities on its surface can then be perceived.
It forms, however, a very beautiful object, when magnifying powers
not higher than 40, 50, or 60 times are used. A power of 45 times, if
properly constructed, will show the _whole of the moon_ with a margin
around it, when the darker and brighter parts of its surface will
present a variegated aspect, and appear somewhat like a map to the eye
of the observer.

4. _Mode of exhibiting the Solar spots._

The solar spots may be contemplated with advantage by magnifying powers
varying from 60 to 180 times; about 90 times is a good medium power,
though they may sometimes be distinguished with very low powers, such
as those usually adapted to a one-foot telescope, or even by means of a
common opera-glass. The common astronomical eye-pieces given along with
achromatic telescopes, and the sun-glasses connected with them, are
generally ill-adapted for taking a pleasant and comprehensive view of
the solar spots. In the higher magnifying powers, the first eye-glass
is generally at too great a distance from the eye, and the sun-glass
which is screwed over it, removes it to a still greater distance from
the point to which the eye is applied, so that not above one third
of the field of view can be taken in. This circumstance renders it
difficult to point the instrument to any particular small spot on the
solar disk which we wish minutely to inspect; and besides, it prevents
us from taking a comprehensive view of the _relative positions_ of
all the spots that may at any time be traversing the disk. To obviate
this inconvenience, the sun-glass would require to be placed so near
to the glass next the eye as almost to touch it. But this is sometimes
difficult to be attained, and, in high powers, even the thickness of
the sun-glass itself is sufficient to prevent the eye from taking in
the whole field of view. For preventing the inconveniences to which
I now allude, I generally make use of a _terrestrial_ eye-piece of a
considerable power, with a large field, the sun-glass is fixed at the
end of a short tube which slides on the eye-piece, and permits the
 glass to approach within a line or two of the lens next the
eye, so that the whole field of the telescope is completely secured.
The eye-piece alluded to carries a magnifying power of 95 times for
a 46-inch telescope, and takes in about three fourths of the surface
of the sun, so that the relative positions of all the spots may
generally be perceived at one view. Such a power is, in most cases,
quite sufficient for ordinary observations; and I have seldom found any
good effect to arise from attempting very high powers, when minutely
examining the solar spots.

But, the most pleasant mode of viewing the solar spots--especially when
we wish to exhibit them to others--is to throw the image of the sun
upon a white screen, placed in a room which is considerably darkened.
It is difficult, however, when the sun is at a high altitude, to put
this method into practice, on account of the great obliquity with
which his rays then fall, which prevents a screen from being placed
at any considerable distance from the eye-end of the telescope. The
following plan, therefore, is that which I uniformly adopt as being
both the easiest and the most satisfactory. A telescope is placed in a
convenient position, so as to be directed to the sun. This telescope
is furnished with a _diagonal eye-piece_, such as that represented,
fig. 77, (p. 344.) The window-shutters of the apartment are all closed,
excepting a space sufficient to admit the solar rays; and, when the
telescope is properly adjusted, a beautiful image of the sun, with
all the spots which then happen to diversify his surface, is thrown
upon the _ceiling_ of the room. This image may be from 12 to 20, or 30
inches or more in diameter, according to the distance of the ceiling
from the diagonal eye-piece. The greater this distance is, the larger
the image. If the sun is at a very high altitude, the image will be
elliptical; if he be at no great distance from the horizon, the image
will appear circular or nearly so; but in either case the spots will be
distinctly depicted, provided the focus of the telescope be accurately
adjusted. In this exhibition, the apparent motion of the sun, produced
by the rotation of the earth, and the passage of thin fleeces of clouds
across the solar disk, exhibit a very pleasing appearance.

By this mode of viewing the solar spots we may easily ascertain their
diameter and magnitude, at least to a near approximation. We have only
to take a scale of inches, and measure the diameter of any well-defined
and remarkable spot, and then the diameter of the solar image; and,
comparing the one with the other, we can ascertain the number of miles
either lineal or square, comprehended in the dimensions of the spot.
For example, suppose a spot to measure one half-inch in diameter,
and the whole image of the sun 25 inches, the proportion between
the diameter of the spot and that of the sun will be as 1 to 50, in
other words, the _one fiftieth_ part of the sun’s diameter. Now, this
diameter being 880,000 miles, this number, divided by 50, produces a
quotient of 17,600 = the number of miles which its diameter measures.
Such a spot will therefore contain an area of 243,285,504, or more
than two hundred and forty-three millions of square miles, which is 46
millions of miles more than the whole superficies of the terraquous
globe. Again, suppose the diameter of a spot measures 3/10 inch, and
the solar image 23 inches, the proportion of the diameter of the spot
to that of the sun is as 3 to 230 = the number of tenths in 23 inches.
The number of miles in the spot’s diameter will therefore be found
by the following proportion: 230 : 880,000 :: 3 : 11,478; that is,
the diameter of such a spot measures eleven thousand four hundred and
seventy-eight miles. Spots of such sizes are not unfrequently seen to
transit the solar disk.

By this mode of viewing the image of the sun, his spots may be
exhibited to twenty or thirty individuals at once without the least
straining or injury to the eyes; and as no separate screen is
requisite, and as the ceilings of rooms are generally white, the
experiment may be performed in half a minute without any previous
preparation, except screwing on and adjusting the eye-piece. The manner
of exhibiting the solar spots, in this way, is represented in fig. 82.

[Illustration: _figure 82._]

5. _On the space-penetrating power of telescopes._--The power of
telescopes to penetrate into the profundity of space is the result of
the quantity of light they collect and send to the eye in a state fit
for vision. This property of telescopes is sometimes designated by the
expression _Illuminating Power_.

Sir W. Herschel appears to have been the first who made a distinction
between the _magnifying_ power, and the _space-penetrating_ power
of a telescope; and there are many examples which prove that such
a distinction ought to be made, especially in the case of large
instruments. For example, the small star, or speck of light, which
accompanies the pole-star, may be seen through a telescope of large
aperture, with a smaller magnifying power than with a telescope of a
small aperture furnished with a much higher power. If the magnifying
power is sufficient to show the small star completely separated from
the rays which surround the large one, this is sufficient in one point
of view; but in order that this effect may be produced, so as to render
the small star perfectly distinguishable, a certain quantity of light
must be admitted into the pupil of the eye--which quantity depends
upon the area of the object-glass or speculum of the instrument, or,
in other words, on the illuminating power. If we compare a telescope
of 2-3/4 inches aperture with one of 5 inches aperture, when the
magnifying power of each does not exceed 50 times for terrestrial
objects, the effect of illuminating power is not so evident; but
if we use a power of 100 for day objects, and 180 for the heavenly
bodies, the effects of illuminating power is so clearly perceptible,
that objects not only appear brighter, and more clearly visible, in
the larger telescope, but with the same magnifying power, they also
_appear_ larger, particularly when the satellites of Jupiter and small
stars are the objects we are viewing.

Sir W. Herschel remarks, that ‘objects are viewed in their greatest
perfection, when, in penetrating space, the magnifying power is so
low as only to be sufficient to show the object well--and when,
in magnifying objects, by way of examining them minutely, the
space-penetrating power is no higher than what will suffice for the
purpose; for in the use of either power, the injudicious overcharge
of the other will prove hurtful to vision.’ When illuminating power is
in too high a degree, the eye is offended by the extreme brightness of
the object. When it is in too low a degree, the eye is distressed by
its endeavours to see what is beyond its reach; and therefore it is
desirable, when we wish to give the eye all the assistance possible, to
have the illuminating and the magnifying powers in due proportion. What
this proportion is, depends, in a certain degree, upon the brightness
of the object. In proportion to its brightness or luminosity, the
magnifying power may, to a certain extent, be increased. Sir W.
Herschel remarks, in reference to α Lyræ, ‘This star, I surmise, has
light enough to bear being magnified, at least a hundred thousand
times, with no more than six inches of aperture.’ However beautifully
perfect any telescopes may appear, and however sharp their _defining_
power, their performance is limited by their illuminating powers--which
are as the squares of the diameters of the apertures of the respective
instruments. Thus, a telescope whose object-glass is 4 inches diameter
will have four times the quantity of light, or illuminating power,
possessed by a telescope whose aperture is only 2 inches, or in the
proportion of 16 to 4,--the square of 4 being 16, and the square of 2
being 4.

The nature of the _space-penetrating power_, to which we are
adverting, and the distinction between it, and magnifying power,
may be illustrated from a few examples taken from Sir W. Herschel’s
observations.

The first observation which I shall notice refers to the _nebula_
between η and ζ Ophiuchi, discovered by Messier in 1764. The
observation was made with a 10 feet reflector, having a magnifying
power of 250, and a space-penetrating power of 28.67. His note is dated
May 3, 1783. ‘I see several stars in it, and make no doubt a higher
power and more light will resolve it all into stars. This seems to me
a good nebula for the purpose of establishing the connection between
nebulæ and clusters of stars in general.’--‘June 18, 1784. The same
nebula viewed with a Newtonian 20 feet reflector; penetrating power 61,
and a magnifying power of 157; a very large and a very bright cluster
of excessively compressed stars. The stars are but just visible, and
are of unequal magnitudes. The large stars are red, the cluster is a
miniature of that near Flamstead’s forty-second Comæ Berenices; Right
ascension 17^h 6^m 32^s Polar distance 108° 18´´’ In this case, a
penetrating power of about 28, with a magnifying power of 250, barely
shewed a few stars; when in the second instrument the illuminating
power of 60 with the magnifying power of only 157 showed them
completely.

Subsequently to the date of the latter observation, the 20 feet
Newtonian telescope was converted into an Herschelian instrument, by
taking away the small speculum, and giving the large one the proper
inclination for obtaining the front view; by which alteration the
illuminating power was increased from 61 to 75, and the advantage
derived from the alteration was evident in the discovery of the
satellites of Uranus by the altered telescope, which before was
incompetent in the point of penetration, or illuminating power. ‘March
14, 1798, I viewed the Georgian planet (or Uranus) with a new 25 feet
reflector. Its penetrating power is 95.85, and having just before
also viewed it with my 20 feet instrument, I found that with an equal
magnifying power of 300, the 25 feet telescope had considerably the
advantage of the former.’ The aperture of the 20 feet instrument was
18.8 inches, and that of the 25 feet telescope, 24 inches, so that
the superior effect of the latter instrument must have been owing
to its greater illuminating power. The following observations show
the superior power of the 40 feet telescope as compared with the 20
feet.--‘Feb. 24, 1786, I viewed the nebula near Flamstead’s fifth
Serpentis, with my 20 feet reflector, magnifying power 157. The most
beautiful extremely compressed cluster of small stars; the greatest
part of them gathered together into one brilliant nucleus, evidently
consisting of stars, surrounded with many detached gathering stars of
the same size and colour. R.A. 15^h 7^m 12^s. P.D. 87° 8´´’--‘May 27,
1791, I viewed the same object with my 40 feet telescope, penetrating
power 191.69, magnifying power 370. A beautiful cluster of stars. I
counted about 200 of them. The middle of it is so compressed, that
it is impossible to distinguish the stars.’--‘Nov. 5, 1791, I viewed
Saturn with the 20 and 40 feet telescopes. _Twenty feet._ The fifth
satellite of Saturn is very small. The first, second, third, fourth and
fifth, and the new sixth satellites are in their calculated places.
_Forty feet._ I see the new sixth satellite much better with this
instrument than with the 20 feet. The fifth is also much larger here
than in the 20 feet, in which it was nearly the same size as a small
fixed star, but here it is considerably larger than that star.’

These examples, and many others of a similar kind, explain sufficiently
the nature and extent of that species of power that one telescope
possesses over another, in consequence of its enlarged aperture; but
the exact quantity of this power is in some degree uncertain. To
ascertain practically the illuminating power of telescopes, we must
try them with equal powers on such objects as the following,--the
small stars near the pole-star, and near Rigel and ε Bootis--the
division in the ring of Saturn--and distant objects in the twilight or
towards the evening. These objects are distinctly seen with a 5 feet
achromatic of 3-8/10 inches aperture, and an illuminating power of
144, while they are scarcely visible in a 3-1/2 feet with an aperture
of 2-3/4 inches, and an illuminating power of 72, supposing the same
magnifying power to be applied. The illuminating power of a telescope
is best estimated, in regard to land objects, when it is tried on
minute objects, and such as are badly lighted up; and the advantage
of a telescope with a large aperture will be most obvious, when it is
compared with another of inferior size in the close of the evening,
when looking at a printed bill composed of letters of various sizes. As
darkness comes on, the use of illuminating power becomes more evident.
In a 5 feet telescope some small letters will be legible, which are
hardly discernible in the 3-1/2 feet, and in the 2-1/2 feet are quite
undefinable, though the magnifying powers be equal. Sir W. Herschel
informs us, that in the year 1776, when he had erected a telescope of
20 feet focal length of the Newtonian construction, one of its effects
by trial was, that when towards evening, on account of darkness, the
natural eye could not penetrate far into space, the telescope possessed
that power sufficiently to show, by the dial of a distant church
steeple, what o’clock it was, notwithstanding the naked eye could no
longer see the steeple itself.

In order to convey an idea of the _numbers_ by which the degree of
space-penetrating power is expressed, and the general grounds on which
they rest, the following statements may be made. The depth to which the
naked eye can penetrate into the spaces of the heavens, is considered
as extending to the twelfth order of distances--in other words, it can
perceive a star at a distance 12 times farther than those luminaries,
such as Sirius, Arcturus or Capella, which, from their vivid light, we
presume to be nearest to us. It has been stated above, that Herschel
calculated his 10 feet telescope to have a space-penetrating power of
28.67, that is, it could enable us to descry a star 28 times farther
distant than the naked eye can reach. His 20 feet Newtonian was
considered as having a similar power of 61; his 25 feet, nearly 96, and
his 40 feet instrument, a power of 191.69. If each of these numbers
be multiplied by 12, the product will indicate how much farther these
telescopes will penetrate into space than the nearest range of the
fixed stars, such as those of the first magnitude. For instance, the
penetrating power of the 40 feet reflector being 191.69, this number
multiplied by 12, gives a product of 2,300, which shows, that were
there a series of two thousand three hundred stars extended in a line
beyond Sirius, Capella and similar stars--each star separated from the
one beyond it, by a space equal to the distance of Sirius from the
earth--they might be all seen through the 40 feet telescope. In short,
the penetrating power of telescopes is a circumstance which requires
to be particularly attended to in our observations of celestial
phenomena, and in many cases, is of more importance than _magnifying_
power. It is the effect produced by illuminating power that renders
telescopes, furnished with comparatively small magnifying powers, much
more efficient in observing comets and certain nebulæ and clusters of
stars, than when high powers are attempted. Every telescope may be so
adjusted, as to produce different space-penetrating powers. If we wish
to diminish such a power, we have only to contract the object-glass or
speculum, by placing circular rims, or apertures of different degrees
of breadth, across the mouth of the great tube of the instrument. But
we cannot increase this illuminating power beyond a certain extent,
which is limited by the diameter of the object-glass. When we wish
illuminating power beyond this limit, we must be furnished with an
object-glass or speculum of a larger size; and hence, the rapid advance
in price of instruments which have large apertures, and consequently
high illuminating powers. Mr. Tulley’s 3-1/2 feet achromatics of 2-3/4
inches aperture, sell at £26 5s. When the aperture is 3-1/4 inches, the
price is £42. When 3-3/4 inches, £68 5s. The following table contains a
statement of the ‘comparative lengths, apertures, illuminating powers,
and prices, of Achromatic Refractors, and Gregorian Reflectors,’
according to Dr. Kitchener.

  ---------------------------------------------------+
                ACHROMATIC REFRACTORS.               |
  -----------+-----------+--------------+------------+
    Length   | Diameter  |              |            |
   and name  |    of     | Illuminating |   Price.   |
   they are  | aperture. |    power.    |            |
  called by. |           |              |            |
  -----------+-----------+--------------+------------+
     Feet.   | In.  Th.  |              |   £  s.    |
     2       |  1.   6   |      25      |   4   4    |
     2-1/2   |  2        |      40      |  12  12    |
     3-1/2   |  2.   7   |      72      |  21 to  42 |
     5       |  3.   8   |     144      | 105 to 150 |
     7       |  5        |     250      | 250        |
     7       |  6        |     360      | 360        |

  ----------------------------------------------------
  |             GREGORIAN &c. REFLECTORS.
  +--------------+-----------+--------------+---------
  |  Length and  | Diameter  |              |
  |  name they   |    of     | Illuminating | Price.
  |     are      | aperture. |    power.    |
  |  known by.   |           |              |
  +--------------+-----------+--------------+---------
  | Feet.        | In.  Th.  |              |   £   s.
  |  1           |  2.   5   |       62     |    7   7
  |  1-1/2       |  3        |       90     |   12  12
  |  2           |  4.   5   |      202     |   20
  |  3           |  5.   5   |      302     |   50
  |  4           |  7        |      490     |  105
  |  7 Newtonian |  7        |      490     |  126
  |  5 Gregorian |  9        |      810     |  200
  | 10 Newtonian | 10        |     1000     |  315

The illuminating powers stated in the above table are only comparative.
Fixing on the number 25 as the illuminating power of a 2 feet
telescope, 1-6/10 inch aperture, that of a 2-1/2 feet 2 inches inches
aperture, will be 40, of a 5 feet 3-8/10 inch aperture, 144, &c. If the
illuminating power of a Gregorian 1-1/2 foot, and 3 inches aperture, be
90, a 5 feet, with 9 inches aperture, will be 810, &c.

6. _On choosing Telescopes, and ascertaining their properties._

It is an object of considerable importance, to every astronomical
observer, that he should be enabled to form a judgment of the qualities
of his telescope, and of any instruments of this description which
he may intend to purchase. The following directions may perhaps be
useful to the reader in directing him in the choice of an achromatic
refracting telescope.

Supposing that an achromatic telescope of 3-1/2 feet focal length, and
3-1/4 inches aperture were offered for sale, and that it were required
to ascertain whether the object-glass, on which its excellence chiefly
depends--is a good one and duly adjusted;--some opinion may be formed
by laying the tube of the telescope in a horizontal position, on a
firm support, about the height of the eye,--and by placing a printed
card or a watch glass vertically, but in an inverted position, against
some wall or pillar, at 40 or 50 yards distant, so as to be exposed
to a clear sky. When the telescope is directed to this object, and
accurately adjusted to the eye--should the letters on the card, or the
strokes and dots on the watch-glass appear clearly and sharply defined,
without any mistiness or coloration, and if very small points appear
well defined--great hopes may be entertained that the glass will turn
out a good one. But a telescope may appear a good one, when viewing
common terrestrial objects, to eyes unaccustomed to discriminate
deviations from perfect vision, while it may turn out to be an
indifferent one, when directed to certain celestial objects. Instead
therefore of a printed card, fix a black board, or one half of a sheet
of black paper, in a vertical position at the same distance, and a
circular disk of white writing paper, about 1/4 of an inch in diameter,
on the centre of the black ground. Then having directed the telescope
to this object, and adjusted for the place of distinct vision, mark
with a black-lead pencil the sliding eye-tube, at the end of the main
tube, so that this position can always be known; and if this sliding
tube be gradually drawn out, or pushed in, while the eye beholds the
disk, it will gradually enlarge and lose its colour, till its edges
cease to be well-defined. Now, if the enlarged misty circle is observed
to be concentric with the disk itself, the object-glass is properly
centered, as it has reference to the tube; but if the misty circle goes
to one side of the disk, the cell of the object-glass is not at right
angles to the tube, and must have its screws removed and its holes
elongated, by a rattailed file, small enough to enter the holes. When
this has been done, the cell may be replaced, and the disk examined a
second time, and a slight stroke on one edge of the cell, by a wooden
mallet, will show by the alteration made in the position of the misty
portion of the disk, how the adjustment is to be effected, which is
known to be right when a motion in the sliding tube will make the
diluted disk enlarge in a circle concentric with the disk itself. When
the disk will enlarge so as to make a ring of diluted white light round
its circumference, as the sliding tube holding the eye-piece is pushed
in or drawn out, the cell may be finally fixed by the screws passing
through its elongated holes.

When the object-glass is thus adjusted, it may then be ascertained
whether the curves of the respective lenses composing the object-glass
are well-formed and suitable for each other. If a small motion of the
sliding tube of about 1/10th of an inch in a 3-1/2 feet telescope, from
the point of distinct vision, will dilute the light of the disk and
render the appearance confused, the figure of the object-glass is good;
particularly if the same effect will take place at equal distances
from the point of distinct vision, when the tube is alternately drawn
out and pushed in. A telescope that will admit of much motion in the
sliding tube without sensibly affecting the distinctness of vision,
will not define an object well at any point of adjustment, and must
be considered as having an imperfect object-glass, inasmuch as the
spherical aberration of the transmitted rays is not duly corrected. The
due adjustment of the convex lens, or lenses, to the concave one, will
be judged of by the absence of coloration round the enlarged disk, and
is a property distinct from the spherical aberration; the achromatism
depending on the relative focal distances of the convex and concave
lenses, is regulated by the relative dispersive powers of the pieces
of glass made use of; but the distinctness of vision depends on a good
figure of the computed curves that limit the focal distances. When an
object-glass is free from imperfection in both these respects, it may
be called a good glass for terrestrial purposes.

It still, however, remains to be determined how far such an
object-glass may be good for viewing a star or a planet, and can only
be known by actual observations on the heavenly bodies. When a good
telescope is directed to the moon or to Jupiter, the achromatism
may be judged of, by alternately pushing in, and drawing out the
eye-piece, from the place of distinct vision. In the former case, a
ring of purple will be formed round the edge; and in the latter, a
ring of light green, which is the central colour of the prismatic
spectrum; for these appearances show, that the extreme colours red
and violet are corrected. Again, if one part of a lens employed have
a different refractive power from another part of it, that is, if the
flint-glass particularly is not homogeneous, a star of the first and
even of the second magnitude will point out the natural defect by the
exhibition of an irradiation, or what is called _a wing_, at one side,
which no perfection of figure or of adjustment will banish, and the
greater the aperture the more liable is the evil to happen. Hence caps
with different apertures are usually supplied with large telescopes,
that the extreme parts of the glass may be cut off, in observations
requiring a round and well-defined image of the body observed.

Another method of determining the figure and quality of an object-glass
is by first covering its centre by a circular piece of paper, as much
as one half of its diameter, and adjusting it for distinct vision of a
given object, such as the disk above mentioned, when the central rays
are intercepted--and then trying if the focal length remains unaltered
when the paper is taken away, and an aperture of the same size applied,
so that the extreme rays may in their turn be cut off. If the vision
remains equally distinct in both cases, without any new adjustment
for focal distance, the figure is good, and the spherical aberration
cured, and it may be seen by viewing a star of the first magnitude
successively in both cases, whether the irradiation is produced more
by the extreme or by the central parts of the glass. Or, in case the
one half be faulty and the other good, a semicircular aperture, by
being turned gradually round in trial, will detect what semicircle
contains the defective portion of the glass; and if such portion should
be covered, the only inconvenience that would ensue, would be the loss
of so much light as is thus excluded. When an object-glass produces
radiations in a large star, it is unfit for the nicer observations of
astronomy, such as viewing double stars of the first class. The smaller
a large star appears in any telescope, the better is the figure of the
object-glass, but if the image of the star be free from wings, the size
of its disk is not an objection in practical observations.[30]

Some opticians are in the habit of inserting a diaphragm into the
body of the large tube, to cut off the extreme rays coming from the
object-glass when the figure is not good, instead of lessening the
aperture by a cap. When this is the case, a deficiency of light will
be the consequence beyond what the apparent aperture warrants. It
is therefore proper to examine that the diaphragm be not placed too
near the object-glass, so as to intercept any of the useful rays.
Sometimes a portion of the object-glass is cut off by the stop in
the eye-tube. To ascertain this, adjust the telescope to distinct
vision, then take out the eye-glasses, and put your finger on some
other object on the edge of the outside of the object-glass, and look
down the tube; if you can see the tip of your finger, or any object
in its place, just peeping over the edge of the object-glass, no part
is cut off. I once had a 3-1/2 feet telescope whose object-glass
measured 3 inches diameter, which was neither so bright, nor did it
perform in other respects nearly so well as another of the same length
whose object-glass was only 2-3/4 inches diameter; but I found that a
diaphragm was placed about a foot within the end of the large tube,
which reduced the aperture of the object-glass to less than 2-1/2
inches; and when it was removed the telescope was less distinct than
before. The powers given along with this instrument were much lower
than usual--none of them exceeding 100 times. This is a trick not
uncommon with some opticians.

Dr. Pearson mentions that an old Dollond’s telescope of 63 inches focal
length, and 3-3/4 inches aperture, supposed to be an excellent one,
was brought to Mr. Tulley, when he was present, and the result of the
examination was that its achromatism was not perfect. The imperfection
was thus determined by experiment. A small glass globe was placed at 40
yards distance from the object-end of the telescope when the sun was
shining, and the speck of light seen reflected from this globe formed a
good substitute for a large star, as an object to be viewed. When the
focal length of the object-glass was adjusted to this luminous object,
no judgment could be formed of its prismatic aberrations, till the
eye-piece had been pushed in beyond the place of correct vision; but
when the telescope was shortened a little, the luminous disk occasioned
by such shortening was strongly tinged with _red_ rays at its
circumference. On the contrary, when the eye-piece was drawn out, so as
to lengthen the telescope too much, the disk thus produced was tinged
with a small circle of _red_ at its _centre_, thereby denoting that
the convex lens had too short a focal length; and Mr. Tulley observed,
that if one or both of the curves of the convex lens were flattened
till the total focal length should be about 4 inches increased, it
would render the telescope quite achromatic, provided in doing this the
aberration should not be increased.

The following general remarks may be added. 1. To make anything
like an accurate comparison of telescopes, they must be tried not
only at the same place, but as nearly as possible at the same time,
and, if the instruments are of the same length and construction,
if possible, with the same eye-piece. 2. A difference of 8 or 10
times in the magnifying power, will sometimes, on certain objects,
give quite a different character to a telescope. It has been found
by various experiments that object-glasses of two or three inches
longer focus will produce different vision with the same eye-piece.
3. Care must be taken to ascertain that the eye-glasses are perfectly
clean and free from defects. The defects of glass are either from
veins--specks--scratches--colour, or an incorrect figure. To discover
veins in an eye or an object-glass, place a candle at the distance of
4 or 5 yards; then look through the glass, and move it from your eye
till it appear full of light--you will then see every vein, or other
imperfection in it which may distort the objects and render vision
imperfect. Specks or scratches, especially in object-glasses, are not
so injurious as veins, for they do not distort the object, but only
intercept a portion of the light. 4. We cannot judge accurately of the
excellence of any telescope by observing objects with which we are
not familiarly acquainted. Opticians generally try an instrument at
their own marks, such as the dial-plate of a watch, a finely engraved
card, a weather-cock, or the moon and the planet Jupiter, when near
the meridian. Of several telescopes of the same length, aperture and
magnifying power, that one is generally considered the best with which
we can read a given print at the greatest distance, especially if the
print consists of _figures_, such as a table of logarithms, where the
eye is not apt to be deceived by the imagination, in _guessing_ at the
sense of a passage, when two or three words are distinguished.

There is a circumstance which I have frequently noticed, in reference
to achromatic telescopes, particularly those of a small size, and which
I have never seen noticed by any optical writer. It is this,--if the
telescope, when we are viewing objects, be gradually turned round its
axis, there is a certain position in which the objects will appear
distinct and accurately defined; and if it be turned round exactly
a semicircle from this point, the same degree of distinctness is
perceived; but in all other positions, there is an evident want of
clearness and defining power. This I find to be the case in more than
ten 1 foot and 2 feet telescopes now in my possession; and therefore
I have put marks upon the object-end of each of them, to indicate the
positions in which they should be used for distinct observation.--This
is a circumstance which requires, in many cases, to be attended to
in the choice and the use of telescopical instruments, and in fixing
and adjusting them on their pedestals. In some telescopes this defect
is very striking, but it is in some measure perceptible in the great
majority of instruments which I have had occasion to inspect. Even in
large and expensive achromatic telescopes this defect is sometimes
observable. I have an achromatic whose object-glass is 4-1/10 inches
diameter, which was much improved in its defining power, by being
unscrewed from its original position, or turned round its axis--about
one-eighth part of its circumference. This defect is best detected by
looking at a large printed bill, or a sign-post at a distance, when, on
turning round the telescope or object-glass, the letters will appear
much better defined in one position than in another. The position in
which the object appears least distinct is when the upper part of the
telescope is a quadrant of a circle different from the two positions
above-stated, or at an equal distance from each of them.

7. _On the mode of determining the magnifying power of Telescopes._

In regard to refracting telescopes, we have already shown that, when a
single eye-glass is used, the magnifying power may be found by dividing
the focal distance of the object-glass by that of the eye-glass. But
when a Huygenian eye-piece, or a four-glass terrestrial eye-piece such
as is now common in achromatic telescopes, is used, the magnifying
power cannot be ascertained in this manner; and in some of the delicate
observations of practical astronomy, it is of the utmost importance
to know the exact magnifying power of the instrument with which the
observations are made, particularly when micrometrical measurements
are employed to obtain the desired results.--The following is a
general method of finding the magnifying powers of telescopes when the
instrument called a _dynameter_ is not employed; and it answers for
refracting and reflecting telescopes of every description.

Having put up a small circle of paper, an inch or two in diameter, at
the distance of about 100 yards, draw upon a card 2 black parallel
lines, whose distance from each other is equal to the diameter of the
paper circle. Then view through the telescope the paper circle with one
eye, and the parallel lines with the other; and let the parallel lines
be moved nearer to or further from, the eye, till they seem exactly
to cover the small circle viewed through the telescope. The quotient
obtained by dividing the distance of the paper circle by the distance
of the parallel lines from the eye, will be the magnifying power of the
telescope. It requires a little practice before this experiment can
be performed with accuracy. The one eye must be accustomed to look at
an object near at hand, while the other is looking at a more distant
object through the telescope. Both eyes must be open at the same time,
and the image of the object seen through the telescope must be brought
into apparent contact with the real object near at hand. But a little
practice will soon enable any observer to perform the experiment with
ease and correctness, if the telescope be mounted on a firm stand, and
its elevation or depression produced by rack-work.

The following is another method, founded on the same
principle:--Measure the space occupied by a number of the courses, or
rows of bricks in a modern building--which, upon an average, is found
to have 8 courses in 2 feet, so that each course or row, is 3 inches.
Then cut a piece of paper 3 inches in height, and of the length of a
brick--which is about 9 inches--so that it may represent a brick, and
fixing the paper against the brick wall, place the telescope to be
examined at the distance of about 80 or 100 yards from it. Now, looking
through the telescope at the paper with one eye, and at the same time,
with the other eye, looking past the telescope, observe what extent of
wall the magnified image of the paper appears to cover, then count the
courses of bricks in that extent, and it will give the magnifying power
of the telescope. It is to be observed, however, that the magnifying
power determined in this way, will be a fraction greater than for very
distant objects, as the focal distance of the telescope is necessarily
lengthened in order to obtain distinct vision of near objects.

In comparing the magnifying powers of two telescopes, or of the same
telescope, when different magnifying powers are employed, I generally
use the following simple method. The telescopes are placed at 8 or 10
feet distant from a window, with their eye-ends parallel to each other,
or at the same distance from the window. Looking at a distant object,
I fix upon a portion of it whose magnified image will appear to fill
exactly two or three panes of the window. Then putting on a different
power, or looking through another telescope, I observe the same
object, and mark exactly the extent of its image on the window-panes,
and compare the extent of the one image with the other. Suppose for
example, that the one telescope has been previously found to magnify
90 times, and that the image of the object fixed upon exactly fills
three panes of the window, and that with the other power or the other
telescope, the image fills exactly two panes, then the magnifying power
is equal to two thirds of the former, or 60 times; and were it to
fill only one pane, the power would be about 30 times. A more correct
method is to place at one side of the window, a narrow board, two or
three feet long, divided into 15 or 20 equal parts, and observe how
many of these parts appear to be covered by the respective images, of
the different telescopes. Suppose, in the one case, 10 divisions to be
covered by the image, in a telescope magnifying 90 times, and that the
image of the same object in another telescope, measures 6 divisions,
then its power is found by the following proportion, 10 : 90 : 6 : 54 :
that is, this telescope magnifies 54 times.

Another mode which I have used for determining, to a near
approximation, the powers of telescopes, is as follows:--Endeavour
to find the focus of a single lens which is exactly equivalent to
the magnifying power of the eye-piece, whether the Huygenian or the
common terrestrial eye-piece. This may be done by taking a small lens,
and using it as an object-glass to the eye-piece. Looking through
the eye-piece to a window and holding the lens at a proper distance,
observe whether the image of one of the panes exactly coincides with
the pane, as seen by the naked eye; if it does, then the magnifying
power of the eye-piece is equal to that of the lens. If the lens be
1/2 inch focal length, the eye-piece will produce the same magnifying
power, as a single lens when used as an eye-glass to the telescope, and
the magnifying power will then be found by dividing the focal distance
of the object-glass by that of the eye-glass. But if the image of
the pane of glass does not exactly coincide with the pane as seen by
the other eye, then proportional parts may be taken by observing the
divisions of such a board as described above, or we may try lenses of
different focal distances. Suppose, for example, that a lens 2 inches
focal length had been used, and that the image of a pane covered
exactly the space of two panes, the power of the eye-piece is then
equal to that of a single lens 1 inch focal distance.

The following is another mode depending on the same general principle.
If a slip of writing-paper one inch long, or a disk of the same
material of one inch diameter, be placed on a black ground at from
30 to 50 yards distance from the object-end of the telescope, and a
staff painted white, and divided into inches and parts by strong black
lines, be placed vertically near the said paper or disk; the eye that
is directed through the telescope when adjusted for vision, will see
the magnified disk, and the other eye, looking along the outside of
the telescope, will observe the number of inches and parts that the
disk projected on it will just cover, and as many inches as are thus
covered will indicate the magnifying power of the telescope--at the
distance for which it is adjusted for distinct vision. The solar power,
or powers for very distant objects, may be obtained by the following
proportion:--As the terrestrial focal length, at the given distance:
is to the solar focal length :: so is the terrestrial power, to the
solar power. For example, a disk of white paper one inch in diameter,
was placed on a black board, and suspended on a wall contiguous to a
vertical black staff that was graduated into inches by strong white
lines, at a distance of 33 yards 2-1/2 feet, and when the adjustment
for vision was made with a 42 inch telescope, the left eye of the
observer viewed the disk projected on the staff, while the right eye
observed that the enlarged image of the disk covered just 58-1/2 inches
on the staff, which number was the measure of the magnifying power,
at the distance answering to 33 yards 2-1/2 feet--which in this case
exceeded the solar focus by an inch and a half. Then according to the
above analogy, we have, as 43.5 : 42 :: 58.5 : 56.5 nearly. Hence the
magnifying power due to the solar focal length of the telescope in
question is 56.5, and the distance 33 yards 2-1/2 feet, is that which
corresponds to an elongation of the solar focal distance an inch and a
half.[31] If we multiply the terrestrial and the solar focal distances
together, and divide the product by their difference, we shall again
obtain the distance of the terrestrial object from the telescope. Thus,
(43.5 + 42)/1.5 = 1218 inches = 101.5 feet, or 33 yards 2-1/2 feet.

The magnifying power of a telescope is also determined, by measuring
the image which the object-glass or the large speculum of a telescope
forms at its solar focus. This is accomplished by means of an
instrument called a _Dynameter_. This apparatus consists of a strip of
mother-of-pearl, marked with equal divisions, from the 1/100th to the
1/1000th of an inch apart, according to the accuracy required. This
measure is attached to a magnifying lens in its focus, in order to
make the small divisions more apparent. When the power of a telescope
is required, the person must measure the clear aperture of the
object-glass, then holding the pearl _dynameter_ next the eye-glass,
let him observe how many divisions the small circle of light occupies,
when the instrument is directed to a bright object. Then by dividing
the diameter of the object-glass by the diameter of this circle of
light, the power will be obtained.[32] The most accurate instrument
of this kind is the _Double Image Dynameter_ invented by Ramsden,
and another on the same principle now made by Dollond, a particular
description of which may be found in Dr. Pearson’s ‘Introduction to
Practical Astronomy.’ The advantage attending these dynameters is that
they do not require any knowledge of the thickness and focal lengths
of any of the lenses employed in a telescope, nor yet of their number
or relative positions; neither does it make any difference whether
the construction be refracting or reflecting, direct or inverting.
One operation includes the result arising from the most complicated
construction.

I shall only mention farther the following method of discovering the
magnifying power, which is founded on the same general principle as
alluded to above. Let the telescope be placed in such a position
opposite the sun, that the rays of light may fall perpendicularly on
the object-glass; and the pencil of rays may be received on a piece of
paper, and its diameter measured. Then, as the diameter of the pencil
of rays is to that of the object-glass, so is the magnifying power of
the telescope.

8.--_On cleaning the lenses of telescopes._--

It is necessary, in order to distinct vision, that the glasses,
particularly the eye-glasses of telescopes be kept perfectly clean,
free of damp, dust, or whatever may impede the transmission of the
rays of light. But great caution ought to be exercised in the wiping
of them, as they are apt to be scratched, or otherwise injured by
a rough and incautious mode of cleaning them. They should never be
attempted to be wiped unless they really require it; and, in this
case, they should be wiped carefully and gently with a piece of new
and soft lamb’s-skin leather. If this be not at hand, a piece of fine
silk paper, or fine clean linen may be used as a substitute. The lens
which requires to be most particularly attended to is the second glass
from the eye, or the field-glass; for if any dust or other impediment
be found upon this glass, it is always distinctly seen, being magnified
by the glass next the eye. The next glass which requires attention is
the fourth from the eye, or that which is next the object. Unless the
glass next the eye be very dusty, a few small spots or grains of dust
are seldom perceptible. The object-glass of an achromatic should seldom
be touched, unless damp adheres to it. Care should be taken never to
use pocket handkerchiefs or dirty rags for wiping lenses. From the
frequent use of such articles, the glasses of seaman’s telescopes get
dimmed and scratched in in the course of a few years. If the glasses be
exceedingly dirty, and if greasy substances are attached to them, they
may be soaked in spirits and water, and afterwards carefully wiped.
In replacing the glasses in their socket, care should be taken not
to touch the surfaces with the fingers, as they would be dimmed with
the perspiration: they should be taken hold of by the edges only, and
carefully screwed into the same cells from which they were taken.


ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.

It appears to have been almost overlooked by opticians and others, that
telescopes may be constructed so as to exhibit a beautiful and minute
view of very near objects, and to produce even a microscopic effect,
without the least alteration in the _arrangement_ of the lenses of
which they are composed. This object is effected simply by making the
eye-tube of a telescope of such a length as to be capable of being
drawn out 12 or 13 inches beyond the point of distinct vision for
distant objects. The telescope is then rendered capable of exhibiting
with distinctness all kinds of objects, from the most distant to those
which are placed within 3 or 4 feet of the instrument--or not nearer
than double the focal distance of the object-glass. Our telescopes,
however, are seldom or never fitted with tubes that slide farther than
an inch or two beyond the point of distinct vision for distant objects,
although a tube of a longer size than usual, or an additional tube
would cost but a very trifling expence.

The following, among many others, are some of the objects on which I
have tried many amusing experiments with telescopes fitted up with
the long tubes to which I allude. The telescope to which I shall
more particularly advert is an achromatic, mounted on a pedestal,
having an object-glass about 19 inches focal length, and 1-5/8 inch
diameter, with magnifying powers for distant objects of 13 and 20
times. When this instrument is directed to a miniature portrait,
3-1/2 inches in length, placed in a good light, at the distance of
about 8 or 10 feet, it appears as large as an oil-painting four or
five feet long, and represents the individual as large as life. The
features of the face appear to stand out in bold relief: and perhaps
there is no representation of the human figure that more resembles
the living prototype, than in this exhibition, provided the miniature
is finely executed. In this case the tube requires to be pulled out
four or five inches from the point of distinct vision for distant
objects, and consequently the magnifying power is proportionally
increased. Another class of objects to which such a telescope may be
applied is _Perspective prints_, either of public buildings, streets
or landscapes. When viewed in this way they present a panoramic
appearance, and seem nearly as natural as life--just in the same manner
as they appear in the Optical Diagonal Machine, or when reflected in
a large concave mirror--with this advantage, that, while in these
instruments the left hand side of the print appears where the right
should be,--the objects seen through the telescope appear exactly in
their natural position. In this case, however, the telescope should
have a small magnifying power, not exceeding 5 or 6 times, so as to
take in the whole of the landscape. If an astronomical eye-piece be
used, the print will require to be inverted.

Other kinds of objects which may be viewed with this instrument, are
trees, flowers, and other objects in gardens immediately adjacent to
the apartment in which we make our observations. In this way we may
obtain a distinct view of a variety of rural objects, which we cannot
easily approach, such as the buds and blossoms on the tops of trees,
and the insects with which they may be infested. There are certain
objects on which the telescope may be made to produce a powerful
microscopical effect, such as the more delicate and beautiful kinds
of flowers, the leaves of trees, and similar objects. In viewing
such objects, the telescope may be brought within little more than
double the focal distance of the object-glass from the objects to be
viewed, and then the magnifying power is very considerably increased.
A nosegay composed of a variety of delicate flowers, and even a
single flower, such as the sea-pink, makes a splendid appearance in
this way. A peacock’s feather, or even the fibres on a common quill,
appear very beautiful, when placed in a proper light. The leaves of
trees, particularly the leaf of the plane-tree, when placed against a
window-pane, so that the light may shine through them--appear, in all
their internal ramifications, more distinct, beautiful and interesting,
than when viewed in any other way; and in such views a large portion
of the object is at once exhibited to the eye. In this case, the
eye-piece of such a telescope as that alluded to requires to be drawn
out 12 or 14 inches beyond the point of distinct vision for objects
at a distance--and the distance between these near objects and the
object-end of the telescope, is only about 3-1/2 feet.

A telescope having a diagonal eye-piece presents a very pleasant view
of near objects in this manner. With an instrument of this kind, I have
frequently viewed the larger kind of small objects alluded to above,
such as the leaves of shrubs and trees, flowers consisting of a variety
of parts, the fibres of a peacock’s feather and similar objects. In
this case the object-glass of the instrument, which is 10-1/2 inches
focal length, was brought within 22 inches of the object, and the eye
looked down upon it, in the same manner, as when we view objects in
a compound microscope. A common pocket achromatic telescope may be
used for the purposes now stated, provided the tube in the eye-piece
containing the two lenses next the object, be taken out, in which case
the two glasses next the eye form an astronomical eye-piece, and the
tubes may be drawn out 5 or 6 inches beyond the focal point for distant
objects, and will produce distinct vision for objects not farther
distant than about 20 or 24 inches. But, in this case, the objects to
be viewed must be inverted, in order that they may be seen in their
natural positions when viewed through the instrument. Telescopes of
a large size and high magnifying powers may likewise be used with
advantage for viewing very near objects in gardens adjacent to the room
in which the instruments are placed, provided the sliding-tube next the
eye has a range of two or three inches beyond the point of vision for
distant objects. In this case, a magnifying power of 100 times on a
3-1/2 or a 5 feet achromatic produces a very pleasant effect. In making
the observations to which I have now alluded, it is requisite in order
to distinct vision, and to obtain a pleasing view of the objects, that
the instrument should be placed on a pedestal, and capable of a motion
in every direction. The adjustment for distinct vision may be made
either by the sliding-tube, or by removing the telescope nearer to or
farther from the object.


REFLECTIONS ON LIGHT AND VISION--AND ON THE NATURE AND UTILITY OF
TELESCOPES.

Light is one of the most wonderful and beneficial, and at the same time
one of the most mysterious agents in the material creation. Though the
sun from which it flows to this part of our system is nearly a hundred
millions of miles from our globe, yet we perceive it as evidently, and
feel its influence as powerfully, as if it emanated from no higher a
region than the clouds. It supplies life and comfort to our physical
system, and without its influence and operations on the various objects
around us, we could scarcely subsist and participate of enjoyment for a
single hour. It is diffused around us on every hand from its fountain
the sun; and even the stars, though at a distance hundreds of thousands
of times greater than that of the solar orb, transmit to our distant
region a portion of this element. It gives beauty and fertility to the
earth, it supports the vegetable and animal tribes, and is connected
with the various motions which are going forward throughout the
system of the universe. It unfolds to us the whole scenery of external
nature--the lofty mountains and the expansive plains, the majestic
rivers and the mighty ocean; the trees, the flowers, the crystal
streams, and the vast canopy of the sky adorned with ten thousands of
shining orbs. In short there is scarcely an object within the range of
our contemplation, but what is exhibited to our understanding through
the medium of light, or at least bears a certain relation to this
enlivening and universal agent. When we consider the extreme minuteness
of the rays of light, their inconceivable velocity, the invariable laws
by which they act upon all bodies, the multifarious phenomena produced
by their inflections, refractions and reflections, while their original
properties remain the same; the endless variety of colours they
produce on every part of our terrestrial creation, and the facility
with which millions of rays pass through the smallest apertures, and
pervade substances of great density, while every ray passes forward in
the crowd without disturbing another, and produces its own specific
impression--we cannot but regard this element as the most wonderful,
astonishing and delightful part of the material creation. When we
consider the admirable beauties and the exquisite pleasures of which
light is the essential source, and how much its nature is still
involved in mystery, notwithstanding the profound investigations of
modern philosophers, we may well exclaim with the Poet:--

    “How then shall I attempt to sing of HIM
    Who, light himself, in uncreated light
    Invested deep, dwells awfully retired
    From mortal eye or angel’s purer ken;
    Whose single smile has, from the first of time,
    Filled, overflowing, all yon lamps of heaven,
    That beam for ever through the boundless sky.”--THOMSON.

The eye is the instrument by which we perceive the beautiful and
multifarious effects of this universal agent. Its delicate and
complicated structure, its diversified muscles, its coats and
membranes, its different humours possessed of different refractive
powers, and the various contrivances for performing and regulating
its external and internal motions, so as to accomplish the ends
intended--clearly demonstrate this organ to be a master-piece of
Divine mechanism--the workmanship of Him whose intelligence surpasses
conception, and whose Wisdom is unsearchable. ‘Our sight (says Addison)
is the most perfect and delightful of all our senses. It fills the
mind with the largest variety of ideas, converses with its objects at
the greatest distance, and continues the longest in action, without
being tired or satiated with its proper enjoyments. The sense of
feeling can indeed give us a notion of extension, shape, and all other
ideas that enter the eye, except colours; but at the same time it is
very much strained, and confined in its operation to the number, bulk
and distance of its particular objects. Our sight seems designed to
supply all these defects, and may be considered as a more delicate and
diffusive kind of touch that spreads itself over an infinite multitude
of bodies, comprehends the largest figures, and brings into our reach
some of the more remote parts of the universe.’

Could we suppose an order of beings endued with every human faculty but
that of sight, it would appear incredible to such beings--accustomed
only to the slow information of touch--that by the addition of an
organ consisting of a ball and socket, of an inch diameter, they might
be enabled, in an instant of time, without changing their place, to
perceive the disposition of a whole army, the order of a battle, the
figure of a magnificent palace, or all the variety of a landscape. If
a man were by feeling to find out the figure of the Peak of Teneriffe,
or even of St. Peter’s church at Rome, it would be the work of a
lifetime. It would appear still more incredible to such beings as we
have supposed, if they were informed of the discoveries which may
be made by this little organ in things far beyond the reach of any
other sense--that, by means of it we can find our way in the pathless
ocean--that we can traverse the globe of the earth, determine its
figure and dimensions, and delineate every region of it--yea, that we
can measure the planetary orbs, and make discoveries in the sphere of
the fixed stars. And, if they were farther informed that, by means of
this same organ, we can perceive the tempers and dispositions, the
passions and affections of our fellow-creatures, even when they want
most to conceal them--that when the tongue is taught most artfully to
lie and dissemble, the hypocrisy should appear in the countenance to a
discerning eye--and that by this organ we can often perceive what is
straight and what is crooked in the mind as well as in the body--would
it not appear still more astonishing to beings such as we have now
supposed?[33]

Notwithstanding these wonderful properties of the organ of vision, the
eye, when unassisted by art, is comparatively limited in the range
of its powers. It cannot ascertain the existence of certain objects
at the distance of three or four miles, nor perceive what is going
forward in nature or art beyond such a limit. By its natural powers
we perceive the moon to be a globe about half a degree in diameter,
and diversified with two or three dusky spots, and that the sun is
a luminous body of apparently the same size--that the planets are
luminous points, and that about a thousand stars exist in the visible
canopy of the sky. But the ten thousandth part of those luminaries,
which are within the reach of human vision, can never be seen by the
unassisted eye. Here the TELESCOPE interposes, and adds a new
power to the organ of vision, by which it is enabled to extend its
views to regions of space immeasurably distant, and to objects, the
number and magnitude of which could never otherwise have been surmised
by the human imagination. By its aid we obtain a sensible demonstration
that space is boundless--that the universe is replenished with
innumerable suns and worlds--that the remotest regions of immensity,
immeasurably beyond the limits of unassisted vision, display the
energies of Creating Power, and that the Empire of the Creator extends
far beyond what eye hath seen or the human imagination can conceive.

The telescope is an instrument of a much more wonderful nature than
what most people are apt to imagine. However popular such instruments
now are, and however common a circumstance it is to contemplate objects
at a great distance which the naked eye cannot discern, yet, prior
to their invention and improvement, it would have appeared a thing
most mysterious, if not impossible, that objects at the distance of
ten miles could be made to appear as if within a few yards of us, and
that some of the heavenly bodies could be seen as distinctly as if we
had been transported by some superior power, hundreds of millions of
miles beyond the bounds of our terrestrial habitation. Who could ever
have imagined--reasoning _a priori_--that the refraction of light in
glass--the same power by which a straight rod appears crooked in water,
by which vision is variously distorted, and by which we are liable to
innumerable deceptions--that that same power, or law of nature, by the
operation of which the objects in a landscape appear distorted when
seen through certain panes of glass in our windows, that that power
should ever be so modified and directed as to extend the boundaries
of vision, and enable us clearly to distinguish scenes and objects at
a distance a thousand times beyond the natural limits of our visual
organs? Yet such are the discoveries which science has achieved,
such the powers it has brought to light, that by glasses ground into
different forms, and properly adapted to each other, we are enabled as
it were to contract the boundaries of space, to penetrate into the most
distant regions, and to bring within the reach of our knowledge the
most sublime objects in the universe.

When Pliny declared in reference to Hipparchus, the ancient astronomer,
‘_Ausus rem Deo improbam annumerare posteris stellas_,’--that ‘he dared
to enumerate the stars for posterity, an undertaking forbidden by God,’
what would that natural historian have said, had it been foretold that
in less than 1600 years afterwards, a man would arise who should enable
posterity to perceive, and to enumerate ten times more new stars than
Hipparchus ever beheld--who should point out higher mountains on the
moon than on the earth, who should discover dark spots, as large as our
globe, in the sun, the fountain of light--who should descry four moons
revolving in different periods of time around the planet Jupiter, and
could show to surrounding senators the varying phases of Venus? and
that another would soon after arise who should point out a double ring
of six hundred thousand miles in circumference, revolving around the
planet Saturn, and ten hundreds of thousands of stars which neither
Hipparchus nor any of the ancient astronomers could ever descry? Yet
these are only a small portion of the discoveries made by Galileo
and Herschel, by means of the telescope. Had any one prophetically
informed Archimedes, the celebrated geometrician of Syracuse, that
vision would, in after ages, be thus wonderfully assisted by art--and
further, that one manner of improving vision would be to place a dark
_opake_ body directly between the object and the eye--and that another
method would be, not to look at the object, but to keep the eye quite
in a different, and even in an _opposite_ direction, or to stand with
the back directly opposed to it, and to behold all the parts of it,
invisible to the naked eye, most distinctly in this way--he would,
doubtless have considered the prophet as an enthusiastic fool or a
raving madman. Yet these things have been realized in modern times in
the fullest extent. In the Gregorian reflecting telescope an opake
body, namely the small speculum near the end of the tube, interposes
_directly_ between the eye and the object. In the Newtonian Reflector,
and in the diagonal eye-pieces formerly described, the eye is directed
in a line at right angles to the object, or a deviation of 90 degrees
from the direct line of vision. In Herschel’s’ large telescopes, and in
the _Aerial Reflector_ formerly described (in pp. 311-325) the back is
turned to the object, and the eye in an opposite direction.

These circumstances should teach us humility and a becoming diffidence
in our own powers; and they should admonish us not to be too dogmatical
or peremptory in affirming what is possible or impossible in regard
either to nature or art, or to the operations of the Divine Being.
Art has accomplished, in modern times, achievements, in regard to
locomotion, marine and aërial navigation, the improvement of vision,
the separation and combinations of invisible gases, and numerous
other objects, of which the men of former ages could not have formed
the least conception. And even yet, we can set no boundaries to the
future discoveries of science and the improvements of art; but have
every reason to indulge the hope that, in the ages to come, scenes of
Divine mechanism in the system of nature will be unfolded, and the
effects of chemical and mechanical powers displayed, of which the human
mind, in its present state of progress, cannot form the most imperfect
idea. Such circumstances likewise should teach us not to reject any
intimations which have been made to us in relation to the character,
attributes, and dispensations of the Divine Being, and the moral
revelations of his will given in the Sacred Records, because we are
unable to comprehend every truth and to remove every difficulty, which
relates to the moral government of the Great Ruler of the universe.
For, if we meet with many circumstances in secular science, and even in
the common operations of nature, which are difficult to comprehend--if
even the construction of such telescopes as we now use, would have
appeared an incomprehensible mystery to ancient philosophers--we must
expect to find difficulties almost insurmountable to such limited
minds as ours, in the eternal plans and moral arrangements of the
“King Immortal and Invisible,” as delineated only in their outlines,
in the Sacred Oracles--particularly those which relate to the origin
of physical and moral evil, the ultimate destiny of man, and the
invisible realities of a future world.

The UTILITY of the telescope may be considered in relation to
the following circumstances.

In the first place, it may be considered as an instrument or machine
which virtually transports us to the distant regions of space. When
we look at the moon through a telescope which magnifies 200 times,
and survey its extensive plains, its lofty peaks, its circular ranges
of mountains, throwing their deep shadows over the vales, its deep
and rugged caverns, and all the other varieties which appear on the
Lunar surface, we behold such objects in the same manner as if we were
standing at a point 238,800 miles from the earth in the direction of
the moon, or only twelve hundred miles from that orb, reckoning its
distance to be 240,000 miles. When we view the planet Saturn with a
similar instrument, and obtain a view of its belts, and satellites, and
its magnificent rings, we are transported, as it were, through regions
of space, to a point in the heavens more than _nine hundred millions
of miles_ from the surface of our globe, and contemplate those august
objects, as if we were placed within five millions of miles of the
surface of that planet.[34] Although a supernatural power, sufficient
to carry us in such a celestial journey, a thousand miles every day,
were exerted--it would require more than two thousand four hundred and
sixty years, before we could arrive at such a distant position; yet
the telescope, in a few moments, transports our visual powers to that
far distant point of space. When we view, with such an instrument, the
minute and very distant clusters of stars in the Milky Way, we are
carried in effect through the regions of space to the distance of _five
hundred thousand millions of miles_ from the earth; for we behold those
luminaries through the telescope nearly as if they were actually viewed
from such a distant point in the spaces of the firmament. These stars
cannot be conceived as less than _a hundred billions_ of miles from
our globe, and the instrument we have supposed brings them within the
two hundredth part of this distance. Suppose we were carried forward
by a rapid motion towards this point, at the rate of a thousand miles
_every hour_, it would require more than _fifty-seven thousand years_,
before we could reach that very distant station in space to which the
telescope, _in effect_, transports us. So that this instrument is far
more efficient in opening to our view the scenes of the universe than
if we were invested with powers of locomotion to carry us through the
regions of space, with the rapidity of a cannon ball at its utmost
velocity; and all the while we may sit at ease in our terrestrial
apartments.

In the next place, the telescope has been _the means of enlarging
our views of the sublime scenes of creation_, more than any other
instrument which art has contrived. Before the invention of this
instrument the universe was generally conceived as circumscribed within
very narrow limits. The earth was considered as among the largest
bodies in creation; the planets were viewed as bodies of a far less
size than what they are now found to be; no bodies similar to our moon
were suspected as revolving around any of them; and the stars were
supposed to be little more than a number of brilliant lamps hung up
to emit a few glimmering rays, and to adorn the canopy of our earthly
habitation. Such a wonderful phenomenon as the Ring of Saturn was
never once suspected, and the sun was considered as only a large ball
of fire. It was suspected, indeed, that the moon was diversified with
mountains and vales, and that it might possibly be a habitable world;
but nothing certainly could be determined on this point, on account of
the limited nature of unassisted vision. But the telescope has been
the means of expanding our views of the august scenes of creation to
an almost unlimited extent. It has withdrawn the veil which formerly
interposed to intercept our view of the distant glories of the sky.
It has brought to light five new planetary bodies, unknown to former
astronomers, one of which is more than eighty times larger than the
earth--and seventeen _secondary_ planets which revolve around the
primary. It has expanded the dimensions of the solar system to double
the extent which was formerly supposed. It has enabled us to descry
hundreds of comets which would otherwise have escaped our unassisted
vision, and to determine some of their trajectories and periods of
revolution.

It has explored the profundities of the Milky Way, and enabled us to
perceive hundreds of thousands of those splendid orbs, where scarcely
one is visible to the naked eye. It has laid open to our view thousands
of _Nebulæ_, of various descriptions, dispersed through different
regions of the firmament--many of them containing thousands of separate
stars. It has directed our investigations to thousands of double,
treble and multiple stars--suns revolving around suns, and systems
around systems, and has enabled us to determine some of the periods of
their revolutions. It has demonstrated the immense distances of the
starry orbs from our globe, and their consequent magnitudes; since
it shows us that, having brought them nearer to our view by several
hundreds or thousands of times, they still appear only as so many
shining _points_. It has enabled us to perceive that mighty changes are
going forward throughout the regions of immensity--new stars appearing,
and others removed from our view, and motions of incomprehensible
velocity carrying forward those magnificent orbs through the spaces of
the firmament. In short, it has opened a vista to regions of space so
immeasurably distant, that a cannon ball impelled with its greatest
velocity, would not reach tracts of creation so remote in two thousand
millions of years, and even light itself, the swiftest body in nature,
would require more than a thousand years before it could traverse this
mighty interval. It has thus laid a foundation for our acquiring an
approximate idea of the infinity of space, and for obtaining a glimpse
of the far distant scenes of creation, and the immense extent of the
universe.

Again, the telescope, in consequence of the discoveries it has enabled
us to make, has tended _to amplify our conceptions of the attributes
and the Empire of the Deity_. The amplitude of our conceptions of
the Divine Being bears a certain proportion to the expansion of our
views in regard to his works of creation, and the operations he is
incessantly carrying forward throughout the universe. If our views of
the works of God, and of the manifestations he has given of himself to
his intelligent creatures, be circumscribed to a narrow sphere, as to
a parish, a province, a kingdom, or a single world, our conceptions of
that Great Being, will be proportionably limited. For it is chiefly
from the manifestation of God in the material creation that our
ideas of his Power, his Wisdom, and his other natural attributes,
are derived. But in proportion to the ample range of prospect we
are enabled to take of the operations of the Most High, will be
our conceptions of his character, attributes, and agency. Now, the
telescope--more than any other invention of man--has tended to open to
our view the most magnificent and extensive prospects of the works of
God. It has led us to ascertain that, within the limits of the solar
system, there are bodies which, taken together, comprise a mass of
matter nearly two thousand five hundred times greater than that of the
earth--that these bodies are all constituted and arranged in such a
manner as to fit them for being habitable worlds--and that the sun, the
centre of this system, is five hundred times larger than the whole.
But, far beyond the limits of this system, it has presented to our view
a universe beyond the grasp of finite intelligences, and to which human
imagination can assign no boundaries. It has enabled us to descry suns
clustering behind suns, rising to view in boundless perspective, in
proportion to the extent of its magnifying and illuminating powers--the
numbers of which are to be estimated, not merely by thousands, and
tens of thousands, and hundreds of thousands, but by scores of
_millions_--leaving us no room to doubt that hundreds of millions more,
beyond the utmost limits of human vision, even when assisted by art,
lie hid from mortal view’s in the unexplored and unexplorable regions
of immensity.

Here, then, we are presented with a scene which gives us a display
of _Omnipotent Power_ which no other objects can unfold, and which,
without the aid of the telescope, we should never have beheld--a
scene which expands our conceptions of the Divine Being, to an extent
which the men of former generations could never have anticipated--a
scene which enables us to form an approximate idea of Him who is the
“King Eternal, Immortal, and Invisible,” who “created all worlds, and
for whose pleasure they are, and were created.” Here we behold the
operations of a Being whose power is illimitable and uncontrollable,
and which far transcends the comprehension of the highest created
intelligences--a power, displayed not only in the vast extension of
material existence, and the countless number of mighty globes which
the universe contains--but in the astonishingly _rapid motions_ with
which myriads of them are carried along through the immeasurable spaces
of creation,--some of those magnificent orbs moving with a velocity
of one hundred and seventy thousand miles an hour. Here, likewise, we
have a display of the infinite _Wisdom_ and Intelligence of the Divine
Mind, in the harmony and order with which all the mighty movements of
the universe are conducted--in proportionating the magnitudes, motions
and distances of the planetary worlds--in the nice adjustment of the
projectile velocity to the attractive power--in the constant proportion
between the times of the periodical revolution of the planets and
the cubes of their mean distances--in the _distances_ of the several
planets from the central body of the system, compared with their
respective _densities_--and in the constancy and regularity of their
motions, and the exactness with which they accomplish their destined
rounds--all which circumstances evidently show that He who contrived
the universe is “the only Wise God,” who is “wonderful in counsel
and excellent in working.” Here, in fine, is a display of _boundless
benevolence_. For we cannot suppose, for a moment, that so many myriads
of magnificent globes, fitted to be the centres of a countless number
of mighty worlds, should be nothing else than barren wastes, without
the least relation to intelligent existence. And if they are peopled
with intellectual beings of various orders--how vast must be their
numbers, and how overflowing that Divine Beneficence which has provided
for them all, every thing requisite to their existence and happiness!

In these discoveries of the telescope, we obtain a glimpse of the
grandeur and the unlimited extent of God’s universal empire. To this
empire no boundaries can be perceived. The larger, and the more
powerful our telescopes are, the further are we enabled to penetrate
into those distant and unknown regions; and however far we penetrate
into the abyss of space, new objects of wonder and magnificence still
continue rising to our view--affording the strongest presumption,
that were we to penetrate ten thousand times farther into those
remote spaces of immensity, new suns, and systems, and worlds would
be disclosed to our view. Over all this vast assemblage of material
existence, and over all the sensitive and intellectual beings it
contains, God eternally and unchangably presides; and the minutest
movements, either of the physical or the intelligent system, throughout
every department of those vast dominions, are at every moment “naked
and open” to his Omniscient eye. What _boundless Intelligence_ is
implied in the _Superintendence_ and _arrangement_ of the affairs of
such an unlimited empire! and what a lofty and expansive idea does it
convey of Him who sits on the throne of Universal Nature, and whose
greatness is unsearchable! But without the aids of the telescopic tube,
we could not have formed such ample conceptions of the greatness,
either of the Eternal Creator himself, or of the universe which he hath
brought into existence.

Besides the above, the following uses of the telescope, in relation to
science and common life, may be shortly noticed:--

In the business of astronomy, scarcely any thing can be done with
accuracy without the assistance of the telescope. 1. It enables the
astronomer to determine with precision _the transits of the planets
and stars_, across the meridian; and on the accuracy with which
these transits are obtained, a variety of important conclusions and
calculations depend. The computation of astronomical and nautical
tables for aiding the navigator in his voyages round the globe, and
facilitating his calculations of latitude and longitude, is derived
from observations made by the telescope, without the use of which
instrument, they cannot be made with precision. 2. The _apparent
diameters of the planets_ can only be measured by means of this
instrument, furnished with a micrometer. By the naked eye no accurate
measurements of the diameters of these bodies can be taken; and
without knowing their apparent diameters, in minutes or seconds, their
real bulk cannot be determined, even although their exact distances
be known. The differences, too, between their polar and equatorial
diameters cannot be ascertained without observations made by powerful
telescopes. For example, the equatorial diameter of Jupiter is found
to be in proportion to the polar as 14 to 13, that is, the equatorial
is more than 6000 miles longer than the polar diameter, which could
never have been determined by observations made by the naked eye.
3. The _parallaxes_ of the heavenly bodies can only be accurately
ascertained by the telescope; and it is only from the knowledge of
their parallaxes, that their distances from the earth or from the
sun can be determined. In the case of the fixed stars, nothing of
the nature of a parallax could ever be expected to be found without
the aid of a telescope. It was by searching for the parallax of a
certain fixed star, that the important fact of the _Aberration of
light_ was discovered. The observations, for this purpose, were made
by means of a telescope 24 feet long, fixed in a certain position. 4.
The motions and revolutionary periods of _Sidereal systems_, can only
be determined by observations made by telescopes of great magnifying
and illuminating powers. Without a telescope the small stars which
accompany double or treble stars cannot be perceived, and much less
their motions or variation of their relative positions. Before the
invention of the telescope such phenomena--now deemed so wonderful
and interesting--could never have been surmised. 5. The accurate
determination of the longitude of places on the earth’s surface is
ascertained by the telescope, by observing with this instrument the
immersions and emersions of the satellites of Jupiter. From such
observations, with the aid of a chronometer, and having the time at any
known place, the situation of any unknown place is easily determined.
But the eclipses of Jupiter’s moons can be perceived only by telescopic
instruments of considerable power. 6. By means of a telescope, with
cross hairs in the focus of the eye-glass, and attached to a Quadrant,
the altitude of the sun or of a star, particularly the pole-star, may
be most accurately taken; and, from such observations, the _latitude_
of the place may be readily and accurately deduced.

Again, in the _Surveying of land_, the telescope is particularly
useful; and for this purpose it is mounted on a stand with a horizontal
and vertical motion, pointing out by divisions the degrees and minutes
of inclination of the instrument. For the more accurate reading
of these divisions, the two limbs are furnished with a Nonius, or
_Vernier’s scale_. The object here is to take the angular distances
between distant objects on a plane truly horizontal; or else the
angular elevation or depression of objects above or below the plane
of the horizon. In order to obtain either of those kinds of angles to
a requisite degree of exactness, it is necessary that the surveyor
should have as clear and distinct a view as possible of the objects,
or station-staves, which he fixes up for his purpose, that he may
with the greater certainty determine the point of the object which
exactly corresponds with the line he is taking. Now, as such objects
are generally at too great a distance for the surveyor to be able
to distinguish with the naked eye, he takes the assistance of the
telescope, by which he obtains, 1. A distinct view of the object to
which his attention is directed, and 2. he is enabled to determine the
precise point of the object aimed at, by means of the cross hairs in
the focus of the eye-glass. A telescope mounted for this purpose is
called a _Theodolite_, which is derived from two Greek words θεομαι _to
see_, and οδος, _the way_ or _distance_.

In the next place, the telescope is an instrument of special
importance, in the conducting of _Telegraphs_, and in the conveyance
of _signals_ of all descriptions. Without its assistance telegraphic
dispatches could not be conveyed with accuracy to any considerable
distance, nor in quadruple the time in which they are now communicated,
and the different stations would need to be exceedingly numerous. But
by the assistance of the telescope information may be communicated, by
a series of telegraphs, with great rapidity. Twenty-seven telegraphs
convey information from Paris to Calais--a distance of 160 miles--in 3
minutes; twenty-two from Paris to Lisle in 2 minutes; forty-six from
Strasburg to Paris in 4-1/2 minutes; and eighty from Paris to Brest
in 10 minutes. In many other cases which occur both on land and on
sea, the telescope is essentially requisite for descrying signals.
The _Bell-Rock Light House_, for example, is situated 12 miles from
Arbroath, and from every other portion of land, so that the naked eye
could not discern any signal which the keepers of that light could have
it in their power to make; but by means of a large telescope in the
station-house in Arbroath, the hoisting of a ball every morning at 9
A.M.--which indicates that ‘All is well’--may be distinctly
recognised.

Many other uses of this instrument, in the ordinary transactions of
life, will readily occur to the reader; and therefore I shall only
mention the following purpose to which it may be applied, namely,--

_To measure the distance of an object from one station._ This depends
upon the increase of the focal distance of the telescope in the case
of near objects. Look through a telescope at the object whose distance
is required, and adjust the focus till it appear quite distinct;
then slide in the drawer, till the object begins to be obscure, and
mark that place of the tube precisely. Next draw out the tube till
the object begins to be again obscured, and then make another mark
as before. Then take the middle point between these two marks, and
that will be the point where the image of the object is formed most
distinctly; which is to be nicely measured from the object lens, and
compared with the solar focus of the lens or telescope, so as to
ascertain their difference. And the rule for finding the distance
is,--‘As the difference between the focal distance of the object, and
the solar focal distance : Is to the solar focal distance :: So is the
focal distance of the object : To its true distance from the object
lens.’ An example will render this matter more perspicuous.

[Illustration: _figure 84._]

Let AB (fig. 84.) be the object lens, EY the eye-glass, FC the radius,
or focus of the lens AB, and C_f_ the focal distance of the object OB,
whose distance is to be measured. Now suppose CF = 48 inches, or 4
feet, and that we find by the above method that C_f_ is 50 inches, then
F_f_ is 2 inches; and the analogy is:--As F_f_ = 2, is to CF = 48, so
is C_f_ = 50, to CQ = 1200 inches, or 100 feet. Again, suppose C_f_ =
49 inches, then will F_f_ = 1 inch; and the proportion is, 1 : 48 :: 49
: 2352 = QC, or 196 feet. A telescope of this focal length, however,
will measure only small distances. But, suppose AB a lens whose solar
focus is 12 feet, or 144 inches; and that we find, by the above method,
that C_f_, or the focal distance of the object, is 146 inches; then
will F_f_ be 2 inches, and the proportion will be, as 2 : 144 :: 146
: 21024 inches, or 1752 feet = the distance QC. If with such a large
telescope, we view an object OB, and find F_f_ but 1/10th of an inch,
this will give the distance of the object as 17292 feet or nearly 3-1/3
miles.

Since the difference between the radius of the object lens and the
focal distance of the object is so considerable as 2 inches in a tube
of 4 feet, and more than 12 inches in one of 12 feet, a method might
be contrived for determining the distance of near objects by the
former, and more distant objects by the latter, by inspection only.
This may be done by adjusting or drawing a spiral line round the drawer
or tube, through the _two inch space_ in the small telescope, and by
calculation, graduate it for every 100 feet, and the intermediate
inches, and then, at the same time we view an object, we may see its
distance on the tube. In making such experiments, a common object-glass
of a long focal length, and a single eye-glass, are all that is
requisite; since the inverted appearance of the object can cause no
great inconveniency.




CHAPTER VII.


ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA.

I originally intended to enter into particular details on this subject,
for the purpose of gratifying those mechanics and others who wish
to amuse themselves by constructing telescopes and other optical
instruments for their own use; but, having dwelt so long on the subject
of telescopes, in the preceding pages, I am constrained to confine
myself to a very general sketch.

1. _To grind and polish lenses for eye-glasses, microscopes, &c._

First provide an upright spindle, at the bottom of which a pulley is
fixed, which must be turned by a wheel by means of a cord and handle.
At the top of the spindle make a screw the same as a lathe-spindle, on
which you may screw chocks of different sizes, to which the brass tool
in which the lens is to be ground, may be fixed. Having fixed upon the
breadth and focal length of the lens, and whether it is to be a plano,
or a double convex--take a piece of tin-plate or sheet copper, and,
with a pair of compasses, draw an arch upon its surface, near one of
its extremities, with a radius equal to the focal distance of the lens,
if intended to be double convex, or with half that distance, if it is
to be plano-convex. Remove with a file that part of the copper which
is without the circular arch, and then a _convex_ gage is formed. With
the same radius strike another arch, and having removed that part of
the copper which is _within_ it, a _concave_ gage will be obtained. The
brass tool, in which the glass is to be ground, is then to be fixed
upon a turning-lathe, and turned into a portion of a concave sphere,
so as to correspond to the convex gage. In order to obtain an accurate
figure to the concave tool, a convex tool of exactly the same radius
is generally formed, and they are ground one upon another with flour
emery; and when they exactly coincide, they are fit for use. The convex
tool will serve for grinding _concave_ glasses of the same radius--and
it should be occasionally ground in the concave tool to prevent it from
altering its figure.

The next thing to be attended to is, to prepare the piece of glass
which is to be ground, by chipping it in a circular shape, by means
of a large pair of scissors, and removing the roughness from its
edges by a common grind-stone. The faces of the glass near the edges
should likewise be ground on the grind-stone, till they nearly fit
the concave gage, by which the labour of grinding in the tool will be
considerably saved. The next thing required is to prepare the emery for
grinding, which is done in the following manner. Provide four or five
clean earthen vessels; fill one of them with water, and put into it a
pound or half a pound of fine emery, and stir it about with a stick;
after which let it stand 3 or 4 seconds, and then pour it into another
vessel, which may stand about 10 seconds; then pour it off again into
the several vessels till the water is quite clear; and by this means,
emery of different degrees of fineness is obtained, which must be kept
separate from each other, and worked in their proper order, beginning
at the first, and working off all the marks of the grind-stone; then
take of the second, next of the third, &c.,--holding the glass upon
the pan or tool with a light hand, when it comes to be nearly fit
for polishing. The glass in this operation should be cemented to a
wooden handle, by means of pitch or other strong cement. After the
finest emery has been used, the roughness which remains may be taken
away, and a slight polish given by grinding the glass with pounded
pumice-stone. Before proceeding to the polishing, the glass should be
ground as smooth as possible, and all the scratches erased, otherwise
the polishing will become a tedious process. The polishing is performed
as follows: Tie a piece of linen rag or of fine cloth about the tool,
and with fine putty, (calcined tin), or colcothar of vitriol (a very
fine powder, sometimes called the red oxide of iron) moistened with
water, continue the grinding motion, and in a short time there will be
an excellent polish.

In order to grind lenses very accurately for the finest optical
purposes, particularly object-glasses for telescopes--the concave tool
is firmly fixed to a table or bench, and the glass wrought upon it by
the hand with circular strokes so that its centre may never go beyond
the edges of the tool. For every 6 or 7 circular strokes, the glass
should receive 2 or 3 cross ones along the diameter of the tool, and in
different directions; and while the operation is going on, the convex
tool should, at the end of five minutes, be wrought upon the concave
one for a few seconds, in order to preserve the same curvature to the
tools and to the glass. The finest polish is generally given in the
following way. Cover the concave tool with a layer of pitch hardened
by the addition of a little rosin, to the thickness of 1/15th of an
inch. Then, having taken a piece of thin writing paper, press it upon
the surface of the pitch with the convex tool, and pull the paper
quickly from the pitch before it has adhered to it; and if the surface
of the pitch is marked every where with the lines of the paper, it
will be truly spherical. If any paper remains on the surface of the
pitch, it may be rubbed off by soap and water, and if the marks of
the paper should not appear on any part of it, the operation must be
repeated, till the polisher or bed of pitch is accurately spherical.
The glass is then to be wrought on the polisher by circular and cross
strokes with the putty or colcothar, till it has received a complete
polish. When one side is finished, the glass must be separated from its
handle, by inserting the point of a knife between it and the pitch,
and giving it a gentle stroke. The pitch which remains upon the glass
may be removed by rubbing it with a little oil or spirits of wine. The
operation of polishing on cloth is slower, and the polish less perfect
than on pitch; but it is a mode best fitted for those who have little
experience, and who would be apt, in the first instance, to injure the
figure of the lens by polishing it on a bed of pitch.

2. _On the method of casting and grinding the Specula of Reflecting
Telescopes._

The first thing to be considered in the formation of reflecting
telescopes, is the _composition_ of the metal of which the specula
are made. The qualities required are--a sound uniform metal, free
from all microscopic pores--not liable to tarnish by absorption of
moisture from the atmosphere--not so hard as to be incapable of taking
a good figure and polish--nor so soft as to be easily scratched, and
possessing a high reflecting power. Various compositions have been
used for this purpose, of which the following are specimens:--Take
good Swedish copper 32 ounces, and when melted, add 14-1/2 ounces of
grain tin to it; then, having taken off the scoria, cast it into an
ingot. This metal must be a second time melted to cast a speculum; but
it will fuse in this compound state with a small heat, and therefore
will not calcine the tin to putty. It should be poured off as soon as
it is melted, giving it no more heat than is absolutely necessary. The
best method for giving the melted metal a good surface is this: the
moment before it is poured off, throw into the crucible a spoonful of
charcoal-dust; immediately after which the metal must be stirred with
a wooden spatula and poured into the moulds.--The following is another
composition somewhat similar. Take 2 parts copper as pure as it is
possible to procure; this must be melted in a crucible by itself. Then
put, in another crucible, 1 part of pure grain tin. When they are both
melted, mix and stir them with a wooden spatula, keeping a good flux
on the melted surface to prevent oxidation, and then pour the metal
quickly into the moulds, which may be made of founder’s loam.

The composition suggested, more than half a century ago, by the Rev.
Mr. Edwards, has often been referred to with peculiar approbation. This
gentleman took a great deal of pains to discover the best composition,
and to give his metals a fine polish and the true parabolical figure.
His telescopes were tried by Dr. Maskelyne, the Astronomer Royal, who
found them greatly to excel in brightness, and to equal in other
respects those made by the best artists. They showed a white object
perfectly white, and all objects of their proper colour. He found,
after trying various combinations, the following to be the best: namely
32 ounces of copper, with 15 or 16 ounces of grain tin, (according to
the purity of the copper) with the addition of one ounce of brass,
one of silver, and one ounce of arsenic. This, he affirms, will form
a metal capable, when polished in a proper manner, of reflecting more
light than any other metal yet made public.

The Rev. J. Little, in his observations on this subject in the ‘Irish
Transactions,’ proposes the following composition, which he found to
answer the purpose better than any he had tried, namely--32 parts
of best bar copper, previously fluxed with the black flux, of two
parts tartar and one of nitre, 4 parts brass, 16 parts tin, and 1-1/4
arsenic. If the metal be granulated, by pouring it, when first melted,
into water, and then fused a second time, it will be less porous than
at first. In this process, the chief object is, to hit on the exact
point of the saturation of the copper, &c., by the tin. For, if the
latter be added in too great quantity, the metal will be dull 
and soft; if too little, it will not attain the most perfect whiteness,
and will certainly tarnish.[35]

When the metal is cast, and prepared by the common grind-stone for
receiving its proper figure--the gages and grinding-tools are to be
formed in the same manner as formerly described for lenses, with this
difference, that the radius of the gages must always be _double_
the focal length of the speculum, as the focus of parallel rays by
reflection is at one half the radius of concavity. In addition to the
concave and convex tools--which should be only a little broader than
the metal itself--a convex elliptical tool of lead and tin should
be formed with the same radius, so that its transverse should be to
its conjugate diameter as 10 to 9, the latter being exactly equal to
the diameter of the metal. The grinding of the speculum is then to
be commenced, on this tool, with coarse emery powder and water, when
the roughness is taken off, by moving the speculum across the tool,
in different directions, walking round the post on which the tool
is fixed, holding the speculum by the wooden handle to which it is
cemented. It is then to be wrought with great care on the convex brass
tool, with circular and cross strokes, and with emery of different
degrees--the concave tool being sometimes ground upon the convex one,
to keep them all of the same radius, and when every scratch is removed
from its surface, it will be fit for receiving the final polish.

When the metal is ready for polishing, the elliptical tool is to
be covered with black pitch about 1/20th of an inch thick, and the
polisher formed in the same way as in the case of lenses, either with
the concave brass tool or with the metal itself. The colcothar of
vitriol should then be triturated between two surfaces of glass, and
a considerable quantity of it applied at first to the surface of the
polisher. The speculum is then to be wrought, in the usual way, upon
the polishing tool, till it has received a brilliant lustre, taking
care to use no more of the colcothar, if it can be avoided, and only a
small quantity of it, if it should be found necessary. When the metal
moves stiffly on the polisher, and the colcothar assumes a dark muddy
hue, the polish advances with great rapidity. The tool will then grow
warm, and would probably stick to the speculum, if its motion were
discontinued for a moment. At this stage of the process, therefore, we
must proceed with great caution, breathing continually on the polisher,
till the friction is so great, as to <DW44> the motion of the speculum.
When this happens, the metal is to be slipped off the tool at one side,
cleaned with soft leather, and placed in a tube for the purpose of
trying its performance; and if the polishing has been conducted with
care, it will be found to have a true _parabolic_ figure.[36]

It was formerly the practice, before the speculum was brought to the
polisher, to smooth it on a _bed of hones_, or a convex tool made of
the best blue stone, such as clockmakers use in polishing their work,
which was made one fourth part larger than the metal which was to be
ground upon it, and turned as true as possible to a gage. But this tool
is not generally considered as absolutely necessary, except when silver
and brass enter into the composition of the metal, in order to remove
the roughness which remains after grinding with the emery.

_To try the figure of the metal._--In order to this, the speculum must
be placed in the tube of the telescope for which it is intended; and,
at about 20 or 30 yards distant, there should be put up a watch-paper,
or similar object, on which there are some very fine strokes of an
engraver. An annular kind of diagram should be made with card-paper,
so as to cover a circular portion of the middle part of the speculum,
between the hole and the circumference, equal in breadth to about
1/8 of its diameter. This paper ring should be fixed in the mouth
of the telescope, and remain so during the whole experiment. There
must likewise be two other circular pieces of card-paper cut out, of
such sizes, that one may cover the centre of the metal, by completely
filling the hole in the annular piece now described: and the other such
a round piece as shall exactly fill the tube, and so broad as that
the inner edge just touches the outward circumference of the middle
annular piece. All these pieces together will completely shut up the
mouth of the telescope. Let the round piece which covers the centre of
the metal be removed, and adjust the instrument so that the image may
be as sharp and distinct as possible. Then replace the central piece,
and remove the outside annular one, by which means the circumference
only of the speculum will be exposed; and the image now formed will be
from the rays reflected from the exterior side of the metal. If the two
images formed by these two portions of the metal be perfectly sharp and
equally distinct, the speculum is perfect and of the true parabolic
curve. If, on the contrary, the image from the outside of the metal
should not be distinct and that it should be necessary to bring the
little speculum _nearer_ by the screw, the metal is not yet brought to
the parabolic figure; but if, in order to procure distinctness, we be
obliged to move the small speculum farther off, then the figure of the
great speculum has been carried beyond the parabolic, and has assumed
the hyperbolic form.

_To adjust the eye-hole of Gregorian Reflectors._--If there is only
one eye-glass, then the distance of the small hole should be as nearly
as possible equal to its focal length. But in the compound Huygenian
eye-piece, the distance of the eye-hole may be thus found:--Multiply
the difference between the focal distance of the glass next to the
speculum, and the distance of the two eye-glasses, by the focal
distance of the glass nearest the eye; divide the product by the sum
of the focal distances of the two lenses, lessened by their distance,
and the quotient will be the compound focal distance required. Thus,
if the focal distance of the lens next the speculum be 3 inches, that
of the lens next the eye 1 inch, and their distance 2 inches, then the
compound focal distance from the eye-glass will be (3 - 2 × 1)/(3 × 1
- 2) = 1/2 inch.--The _diameter_ of the eye-hole is always equal to
the quotient obtained by dividing the diameter of the great speculum
by the magnifying power of the telescope. It is generally from 1/25th
to 1/50th of an inch in diameter. It is necessary, in many cases, to
obtain, _from direct experiment_, an accurate determination of the
place and size of the eye-hole, as on this circumstance depends, in a
certain degree, the accurate performance of the instrument.

_To center the two specula of Gregorian Reflectors._--Extend two fine
threads or wires across the aperture of the tube at right angles, so as
to intersect each other, exactly in the axis of the telescope. Before
the arm is finally fastened to the slider, place it in the tube, and
through the eye-piece (without glasses) the intersection of the cross
wires must be seen exactly in the centre of the hole of the arm. When
this exactness is obtained, let the arm be firmly riveted and soldered
to the slider.

_To centre lenses._--The centering of lenses is of great importance,
more especially for the object-glasses of achromatic instruments. The
following is reckoned a good method:--Let the lens to be centered be
cemented on a brass chuck, having the middle turned away so as not to
touch the lens, but near the edge, which will be hid when mounted. This
rim is very accurately turned flat where it is to touch the glass. When
the chuck and cement is warm it is made to revolve rapidly: while in
motion a lighted candle is brought before it, and its reflected image
attentively watched. If this image has any motion, the lens is not
flat or central; a piece of soft wood must therefore be applied to it
in the manner of a turning tool, till such time as the light becomes
stationary. When the whole has cooled, the edges of the lens must be
turned by a diamond, or ground with emery.

For more particular details in reference to grinding and polishing
specula and lenses, the reader is referred to Smith’s ‘Complete system
of Optics’--Imison’s ‘School of Arts’--_Huygenii Opera_--Brewster’s
Appendix to ‘Ferguson’s Lectures’--‘Irish Transactions,’ vol. X.,
or ‘Nicholson’s Journal,’ vol. XVI., Nos. 65, 66, for January and
February, 1807.




PART III.

ON VARIOUS ASTRONOMICAL INSTRUMENTS.




CHAPTER I.


ON MICROMETERS.

A micrometer is an instrument attached to a telescope, in order to
measure small spaces in the heavens, such as the spaces between two
stars, and the diameters of the sun, moon and planets--and by the
help of which the _apparent magnitude_ of all objects viewed through
telescopes may be measured with great exactness.

There are various descriptions of these instruments, constructed with
different substances, and in various forms, of which the following
constitute the principal variety. The _Wire_ micrometer--the _Spider’s
line_ micrometer--the Polymetric reticle--Divided _object glass_
micrometer--Divided _eye-glass_ micrometer--Ramsden’s _Catoptric_
micrometer--Rochon’s _crystal_ micrometer--Maskelyne’s _Prismatic_
micrometer--Brewster’s _micrometrical telescope_--Sir W. Herschel’s
_Lamp_ micrometer--Cavallo’s _Mother of Pearl_ micrometer, and several
others. But, instead of attempting even a general description of
these instruments, I shall confine myself merely to a very brief
description of _Cavallo’s Micrometer_, as its construction will be
easily understood by the general reader, as it is one of the most
simple of these instruments, and is so cheap as to be procured for
a few shillings; while some of the instruments now mentioned are so
expensive, as to cost nearly as much as a tolerably good telescope.[37]

This micrometer consists of a thin and narrow slip of mother of pearl
finely divided, which is placed in the focus of the eye-glass of a
telescope, just where the image of the object is formed; and it may be
applied either to a reflecting or a refracting telescope, provided the
eye-glass be a convex lens. It is about the 20th part of an inch broad,
and of the thickness of common writing paper, divided into equal parts
by parallel lines, every fifth and tenth of which is a little longer
than the rest. The simplest way of fixing it is to stick it upon the
diaphragm which generally stands within the tube, and in the focus of
the eye-glass. When thus fixed, if you look through the eye-glass, the
divisions of the micrometrical scale will appear very distinct, unless
the diaphragm is not exactly in the focus of the eye-glass, in which
case it must be moved to the proper place;--or, the micrometer may be
placed exactly in the focus of the eye-lens by the interposition of a
circular piece of paper, card, or by means of wax. If a person should
not like to see always the micrometer in the field of the telescope,
then the micrometrical scale, instead of being fixed to the diaphragm,
may be fitted to a circular perforated plate of brass, of wood, or even
of paper, which may be occasionally placed upon the said diaphragm. One
of these micrometers, in my possession, which contains 600 divisions in
an inch, is fitted up in a separate eye-tube, with a glass peculiar to
itself, which slides into the eye-piece of the telescope, when its own
proper glass is taken out.

_To ascertain the value of the divisions of this micrometer._--Direct
the telescope to the sun, and observe how many divisions of the
micrometer measure its diameter exactly. Then take out of the Nautical
Almanack the diameter of the sun for the day on which the observation
is made. Divide it by the above-mentioned number of divisions, and the
quotient is the value of one division of the micrometer. Thus, suppose
that 26-1/2 divisions of the micrometer measure the diameter of the
sun, and that the Nautical Almanack gives for the measure of the same
diameter 31´: 22´´, or 1882´´. Divide 1882 by 26.5, and the quotient is
71´´ or 1´: 11´´, which is the value of one division of the micrometer;
the double of which is the value of two divisions, and so on. The value
of the divisions may likewise be ascertained by the passage of an
equatorial star over a certain number of divisions in a certain time.
The stars best situated for this purpose are such as the following--δ
in the Whale, R. A. 37°: 3-1/3´, Dec. 37´: 50´´ S; δ in Orion, R. A.
80°: 11´: 42´´, Dec. 28´: 40´´ S; υ in the Lion, R. A. 171°: 25´: 21´´,
Dec. 23´: 22´´ N.; η in Virgo R. A. 182°: 10´, Dec. 33´: 27´´ N. But
the following is the most easy and accurate method of determining the
value of the divisions:--

Mark upon a wall or other place the length of _six inches_, which may
be done by making two dots or lines six inches asunder, or by fixing
a six inch ruler upon a stand. Then place the telescope before it,
so that the ruler or six-inch length may be at right angles with the
direction of the telescope, and just 57 feet 3-1/2 inches distant
from the object-glass of the telescope; this done, look through the
telescope at the ruler, or other extension of six inches, and observe
how many divisions of the micrometer are equal to it, and that same
number of divisions is equal to half a degree, or 30´; and this is all
that is necessary for the required determination. The reason of which
is, because an extension of six inches subtends an angle of 30´, at the
distance of 57 feet, 3-1/2 inches, as may be easily calculated from the
rules of plane Trigonometry.

[Illustration: _figure 85._]

Fig. 85, exhibits this micrometer scale, but shows it four times larger
than the real size of one which was adapted to a 3 feet achromatic
telescope magnifying 84 times. The divisions upon it are the 200ths of
an inch, which reach from one edge of the scale to about the middle of
it, excepting every fifth and tenth division, which are longer. Two
divisions of this scale are very nearly equal to one minute; and as a
quarter of one of these divisions may be distinguished by estimation,
therefore an angle of 1/8 of a minute, or of 7-1/2´´ may be measured
with it. When a telescope magnifies more, the divisions of the
micrometer must be more minute. When the focus of the eye-glass of the
telescope is shorter than half an inch, the micrometer may be divided
with the 500ths of an inch; by means of which, and the telescope
magnifying about 200 times, one may easily and accurately measure an
angle smaller than half a second. On the other hand, when the telescope
does not magnify above 30 times, the divisions need not be so minute.
In one of Dollond’s pocket telescopes, which, when drawn out for use
is only 14 inches long, a micrometer with the hundredths of an inch
is quite sufficient, and one of its divisions is equal to little less
than 3 minutes, so that an angle of a minute may be measured by it.
Supposing 11-1/2 of those divisions equal to 30´ or 23 to a degree--any
other angle measured by any other number of divisions, is determined
by proportion. Thus, suppose the diameter of the sun, seen through the
same telescope, be found equal to 12 divisions, say As 11-1/2 divisions
: are to 30 minutes :: so are 12 divisions : to ((12 × 30)/11.5) 31.3,
which is the required diameter of the sun.

_Practical uses of this Micrometer._--This micrometer may be applied
to the following purposes:--1. For measuring the apparent diameters of
the sun, moon, and planets. 2. For measuring the apparent distances of
the satellites from their primaries. 3. For measuring the cusps of the
moon in eclipses. 4. For measuring the apparent distances between two
contiguous stars--between a star and a planet--between a star and the
moon--or between a comet and the contiguous stars, so as to determine
its path. 5. For finding the difference of declination of contiguous
stars, when they have nearly the same R. Ascension. 6. For measuring
the small elevations or depressions of objects above and below the
horizon. 7. For measuring the proportional parts of buildings, and
other objects in perspective drawing. 8. For ascertaining whether a
ship at sea, or any moving object is coming nearer or going farther
off; for if the angle subtended by the object appears to increase,
it shows that the object is coming nearer, and if the angle appears
to decrease, it indicates that the object is receding from us. 9.
For ascertaining the real distances of objects of known extension,
and hence to measure heights, depths, and horizontal distances. 10.
For measuring the real extensions of objects when their distances
are known. 11. For measuring the distance and size of an object when
neither of them is known.

When the micrometer is adapted to those telescopes which have four
glasses in the eye-tube--and _when the eye-tube only is used_, it
may be applied to the following purposes:--1. For measuring the
real or lineal dimensions of small objects, instead of the angles.
For if the tube be unscrewed from the rest of the telescope, and
applied to small objects, it will serve for a microscope, having a
considerable magnifying power, as we have already shown, (p. 348);
and the micrometer, in that case, will measure the lineal dimensions
of the object, as the diameter of a hair, the length of a flea, or
the limbs of an insect. In order to find the value of the divisions
for this purpose, we need only apply a ruler, divided into tenths
of an inch, to the end of the tube, and, looking through the tube,
observe how many divisions of the micrometer measure one tenth of an
inch on the ruler, which will give the required value. Thus, if 30
divisions are equal to 1/10th of an inch, 300 of them must be equal
to 1 inch, and one division is equal to the 300dth part of an inch.
2. For measuring the magnifying power of other telescopes. This is
done by measuring the diameter of the pencil of light at the eye-end
of the telescope in question. For, if we divide the diameter of the
object lens by the diameter of this pencil of light, the quotient will
express how many times that telescope magnifies in diameter. Thus,
suppose that 300 divisions of the micrometer are equal to the apparent
extension of 1 inch--that the pencil of light is measured by 4 of these
divisions--and that the diameter of the object lens measures 1 inch
and 2 tenths:--Multiply 1.2 by 300, and the product 360, divided by 4,
gives 90 for the magnifying power of the telescope.

_Problems which may be solved by this micrometer._ I. The angle--not
exceeding one degree--which is subtended by an extension of 1 foot,
being given, to find its distance from the place of observation:--Rule
1. If the angle be expressed in minutes, say, as the given angle :
is to 60 :: so is 687.55 : to a fourth proportional, which gives the
answer in inches. 2. If the angle be expressed in seconds, say, As the
given angle : is to 3600 :: so is 687.55 to a fourth proportional,
which expresses the answer in inches. 3. If the angle be expressed in
minutes and seconds, turn it all into seconds, and proceed as above.
Example, at what distance is a globe of 1 foot in diameter, when it
subtends an angle of 2 seconds? 2 : 3600 :: 687.55 : (3600 × 687.55)/2
= 1237596 inches, or 103132-1/2 feet = the answer required. II. The
angle which is subtended by any known extension being given, to find
its distance from the place of observation. Rule, Proceed as if the
extension were of one foot, by Problem I, and call the answer B; then
if the extension in question be expressed in inches, say, as 12 inches
: are to that extension :: so is B : to a fourth proportional, which
is the answer in inches. But if the extension in question be expressed
in feet, then we need only multiply it by B, and the product is the
answer in inches.--Example, At what distance is a man 6 feet high, when
he appears to subtend an angle of 30´´? By Problem I, if the man were
1 foot high, the distance would be 82506 inches; but as he is 6 feet
high, therefore multiply 82506 by 6, and the product is the required
distance, namely 495036 inches, or 41253 feet.

For greater conveniency, especially in travelling, when one has not
the opportunity of making such calculations, the following two tables
have been calculated; the first of which shows the distance answering
to any angle from one minute to one degree, which is subtended by a
man whose height is considered an extension of 6 feet, because at a
mean, such is the height of a man when dressed with hat and shoes on.
These tables may be transcribed on a card, and may be kept always ready
with a pocket telescope furnished with a micrometer. Their use is to
ascertain distances without any calculations; and they are calculated
only to minutes, because with a pocket telescope and micrometer, it
is not possible to measure an angle more accurately than to a minute.
Thus, if we want to measure the extension of a street, let a foot ruler
be placed at the end of the street; measure the angular appearance of
it, which suppose to be 36´, and in the table we have the required
distance against 36´, which is 95-1/2 feet. Thus also a man who appears
to be 49´ high, is at the distance of 421 feet. Again, Suppose the
trunk of a tree which is known to be 3 feet in diameter be observed to
subtend an angle of 9´-1/2. Take the number answering to 9´ out of the
table, namely 382, and subtract from it a proportional part for the
half minute, namely 19.1, which subtracted from 382, leaves 362.9. This
multiplied by 3, the diameter of the tree, produces 1087.7 feet = the
distance from the object end of the telescope.

  +-----------------------------------++-----------------------------------+
  |       Angles subtended by an      ||       Angles subtended by an      |
  |     extension of _one foot_ at    ||     extension of _six feet_ at    |
  |        different distances.       ||        different distances.       |
  +-------+---------+-------+---------++-------+---------+-------+---------+
  |Angles |Distances|Angles |Distances||Angles |Distances|Angles |Distances|
  |Minutes|in feet. |Minutes|in feet. ||Minutes|in feet. |Minutes|in feet. |
  +-------+---------+-------+---------++-------+---------+-------+---------+
  |   1   |  3438   |   31  |  110.9  ||   1   | 20626.8 |   31  |  665.4  |
  |   2   |  1719   |   32  |  107.4  ||   2   | 10313.  |   32  |  644.5  |
  |   3   |  1146   |   33  |  104.2  ||   3   |  6875.4 |   33  |  625.   |
  |   4   |   859.4 |   34  |  101.1  ||   4   |  5156.5 |   34  |  606.6  |
  |   5   |   687.5 |   35  |  98.2   ||   5   |  4125.2 |   35  |  589.3  |
  |   6   |   572.9 |   36  |  95.5   ||   6   |  3437.7 |   36  |  572.9  |
  |   7   |   491.1 |   37  |  92.9   ||   7   |  2946.6 |   37  |  557.5  |
  |   8   |   429.7 |   38  |  90.4   ||   8   |  2578.2 |   38  |  542.8  |
  |   9   |   382   |   39  |  88.1   ||   9   |  2291.8 |   39  |  528.9  |
  |  10   |   343.7 |   40  |  85.9   ||  10   |  2062.6 |   40  |  515.6  |
  |  11   |   312.5 |   41  |  83.8   ||  11   |  1875.2 |   41  |  503.1  |
  |  12   |   286.5 |   42  |  81.8   ||  12   |  1718.8 |   42  |  491.1  |
  |  13   |   264.4 |   43  |  79.9   ||  13   |  1586.7 |   43  |  479.7  |
  |  14   |   245.5 |   44  |  78.1   ||  14   |  1473.3 |   44  |  468.8  |
  |  15   |   229.2 |   45  |  76.4   ||  15   |  1375.  |   45  |  458.4  |
  |  16   |   214.8 |   46  |  74.7   ||  16   |  1298.1 |   46  |  448.4  |
  |  17   |   202.2 |   47  |  73.1   ||  17   |  1213.3 |   47  |  438.9  |
  |  18   |   191   |   48  |  71.6   ||  18   |  1145.9 |   48  |  429.7  |
  |  19   |   181   |   49  |  70.1   ||  19   |  1085.6 |   49  |  421.   |
  |  20   |   171.8 |   50  |  68.7   ||  20   |  1031.4 |   50  |  412.5  |
  |  21   |   162.7 |   51  |  67.4   ||  21   |   982.2 |   51  |  404.4  |
  |  22   |   156.2 |   52  |  66.1   ||  22   |   937.6 |   52  |  396.7  |
  |  23   |   149.4 |   53  |  64.8   ||  23   |   896.8 |   53  |  389.2  |
  |  24   |   143.2 |   54  |  63.6   ||  24   |   859.4 |   54  |  381.9  |
  |  25   |   137.5 |   55  |  62.5   ||  25   |   825.  |   55  |  375.   |
  |  26   |   132.2 |   56  |  61.4   ||  26   |   793.3 |   56  |  368.3  |
  |  27   |   127.3 |   57  |  60.3   ||  27   |   763.9 |   57  |  361.9  |
  |  28   |   122.7 |   58  |  59.1   ||  28   |   736.6 |   58  |  355.6  |
  |  29   |   118.5 |   59  |  58.2   ||  29   |   711.3 |   59  |  349.6  |
  |  30   |   114.6 |   60  |  57.3   ||  30   |   687.5 |   60  |  343.7  |
  +-------+---------+-------+---------++-------+---------+-------+---------+

In this way the distance of a considerably remote object, as a town
or building at 10 or 12 miles distant, may be very nearly determined;
provided we have the lineal dimensions of a house or other object that
stands at right angles to the line of vision. The breadth of a river,
of an arm of the sea, or the distance of a light house, whose elevation
above the sea or any other point, is known, may likewise in this manner
be easily determined.




CHAPTER II.


ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY.

The equatorial instrument is intended to answer a number of useful
purposes in Practical Astronomy, independently of any particular
observatory. Besides answering the general purpose of a Quadrant, a
Transit instrument, a Theodolite, and an Azimuth instrument--it is
almost the only instrument adapted for viewing the stars and planets
_in the day-time_, and for following them in their apparent diurnal
motions. It may be made use of in any steady room or place, and
performs most of the useful problems in astronomical science.

The basis of all equatorial instruments is a revolving axis, placed
parallel to the axis of the earth, by which an attached telescope is
made to follow a star or other celestial body in the arc of its diurnal
revolution, without the trouble of repeated adjustments for changes of
elevation, which quadrants and circles with vertical and horizontal
axes require. Such an instrument is not only convenient for many useful
and interesting purposes in celestial observations, but is essentially
requisite in certain cases, particularly in examining and measuring the
relative positions of two contiguous bodies, or in determining the
diameters of the planets, when the spider’s-line micrometer is used.

Christopher Scheiner is supposed to have been the first astronomer who,
in the year 1620, made use of a polar axis, but without any appendage
of graduated circles. It was not, however, till the middle of the
last century, that any instruments of this description, worthy of the
name, were attempted to be constructed. In 1741, Mr. Henry Hindley, a
clock-maker in York, added to the polar axis, an equatorial plate, a
quadrant of altitude, and declination semicircle; but when this piece
of mechanism was sent to London for sale in 1748, it remained unsold
for the space of 13 years. Mr. Short, the optician, published in the
Philosophical Transactions, for 1750, a ‘description of an equatorial
telescope,’ which was of the reflecting kind, and was mounted over a
combination of circles and semicircles, which were strong enough to
support a tube, and a speculum of the Gregorian construction 18 inches
in focal length. This instrument consisted of a somewhat cumbersome
and expensive piece of machinery--a representation of which may be
seen in volume III of Martin’s ‘_Philosophia Britannica_, or system of
the Newtonian philosophy.’ Various modifications of this instrument
have since been made by Nairne, Dollond, Ramsden, Troughton, and
other artists; but even at the present period, it has never come into
very general use, though it is one of the most pleasant and useful
instruments connected with astronomical observations.

As many of these instruments are somewhat complicated, and very
expensive, I shall direct the attention of the reader solely to
one which I consider as the most simple--which may be purchased
at a moderate expence, and is sufficiently accurate for _general_
observations.

[Illustration: _figure 86._]

This instrument consists of the following parts: A _horizontal circle_
EF (fig. 86.) divided into four quadrants of 90 degrees each. There is
a fixed nonius at N; and the circle is capable of being turned round
on an axis. In the centre of the horizontal circle is fixed a strong
upright pillar, which supports the centre of a vertical semicircle
AB, divided into two quadrants of 90 degrees each. This is called the
_semicircle of altitude_, and may, at any time, serve the purpose of
a quadrant in measuring either altitudes or depressions. It has a
nonius plate at K. At right angles to the plane of this semicircle, the
_equatorial circle_ MN is firmly fixed. It represents the equator, and
is divided into twice 12 hours, every hour being divided into 12 parts
of 5 minutes each. Upon the equatorial circle moves another circle,
with a chamfered edge, carrying a nonius by which the divisions on
the equatorial may be read off to single minutes; and at right angles
to this moveable circle is fixed the _semicircle of declination_ D,
divided into two quadrants of 90 degrees each. The telescope PO, is
surmounted above this circle, and is fixed to an index moveable on the
semicircle of declination, and carries a nonius opposite to Q. The
telescope is furnished with 2 or 3 Huygenian eye-pieces, and likewise
with a diagonal eye-piece for viewing objects near the zenith. Lastly,
there are 2 spirit levels fixed on the horizontal circle, at right
angles to each other, by means of which this circle is made perfectly
level when observations are to be made.

_To adjust the equatorial for observation._ Set the instrument
on a firm support. Then _to adjust the levels and the horizontal
circle_:--Turn the horizontal circle till the beginning O of the
divisions coincides with the middle stroke of the nonius, or near it.
In this situation one of the levels will be found to lie either in a
right line joining the 2 foot screws which are nearest the nonius, or
else parallel to such a right line. By means of the 2 last screws,
cause the bubble in the level to become stationary in the middle of
the glass; then turn the horizontal circle half round, by bringing
the other O to the nonius; and if the bubble remains in the middle,
as before, the level is well-adjusted; if it does not, correct the
position of the level, by turning one or both of the screws which
pass through its ends, till the bubble has moved half the distance it
ought to come to reach the middle, and cause it to move the other half
by turning the foot-screws already mentioned. Return the horizontal
circle to its first position, and if the adjustments have been well
made, the bubble will remain in the middle: if otherwise, the process
must be repeated till it bears this proof of its accuracy. Then turn
the horizontal circle till 90° stands opposite to the nonius; and by
the foot-screw,immediately opposite the other 90°, cause the bubble of
the same level to stand in the middle of the glass. Lastly, by its own
proper screws set the other level so that its bubble may occupy the
middle of its glass.

_To adjust the line of sight._ Set the nonius on the _declination_
semicircle at O, the nonius on the horary circle at VI, and the nonius
on the semicircle of altitude at 90. Look through the telescope towards
some part of the horizon, where there is a diversity of remote objects.
Level the horizontal circle, and then observe what object appears in
the centre of the cross-wires, or in the centre of the field of view,
if there be no wires. Reverse the semicircle of altitude, so that the
other 90° may apply to the nonius; taking care, at the same time, that
the other three noniuses continue at the same parts of their respective
graduations as before. If the remote object continues to be seen on the
centre of the cross-wires, the line of sight is truly adjusted.

_To find the correction to be applied to observations by the
semicircle of altitude._ Set the nonius on the declination-semicircle
to 0, and the nonius on the horary circle to XII; direct the telescope
to any fixed and distant object, by moving the horizontal circle and
semicircle of altitude, and nothing else; note the degree and minute
of altitude or depression; reverse the declination-semicircle, by
directing the nonius on the horary circle to the opposite XII; direct
the telescope again to the same object, by means of the horizontal
circle and semicircle of altitude, as before. If its altitude or
depression be the same as was observed in the other position, no
correction will be required; but, if otherwise, half the difference of
the two angles is the correction to be added to all observations made
with that quadrant, or half of the semicircle which shows the least
angle, or to be subtracted from all the observations made with the
other quadrant, or half of the semicircle. When the levels and other
adjustments are once truly made, they will be preserved in order for a
length of time, if not deranged by violence; and the correction to be
applied to the semicircle of altitude is a constant quantity.

_Description of the nonius._ The nonius--sometimes called the
_vernier_--is a name given to a device for subdividing the arcs of
quadrants and other astronomical instruments. It depends on the
simple circumstance, that if any line be divided into equal parts,
the length of each part will be greater, the fewer the divisions; and
contrariwise, it will be less in proportion as those divisions are
more numerous. Thus, in the equatorial now described, the distance
between the two extreme strokes on the nonius is exactly equal to 11
degrees on the limb, but that it is divided into 12 equal parts. Each
of these last parts will therefore be shorter than the _degree_ on the
limb in the proportion of 11 to 12, that is to say, it will be 1/12th
part, or 5 minutes shorter. Consequently, if the middle stroke be set
precisely opposite to any degree, the relative positions of the nonius
and the limb must be altered 5 minutes of a degree, before either of
the two adjacent strokes next the middle on the nonius, can be brought
to coincide with the nearest stroke of a degree; and so likewise the
second stroke on the nonius will require a change of 10 minutes, the
third of 15, and so on to 30, when the middle line of the nonius will
be seen to be equi-distant between 2 of the strokes on the limb; after
which the lines on the opposite side of the nonius will coincide in
succession with the strokes on the limb. It is clear from this, that
whenever the middle stroke of the nonius does not stand precisely
opposite to any degree, the odd minutes--or distance between it and
the degree immediately preceding--may be known by the _number_ of the
stroke marked on the nonius, which coincides with any of the strokes on
the limb.[38] In some instruments the nonius-plate has its divisions
fewer than the number of parts on the limb to which it is equal; but
when once a clear idea of the principle of any nonius is obtained, it
will be easy to transfer it to any other mode in which this instrument
is contrived.

_To find by this equatorial the_ MERIDIAN LINE, and the time,
FROM ONE OBSERVATION OF THE SUN. In order to this it is
requisite that the sun’s declination, and the latitude of the place
be known. The declination of the sun may be found, for every day,
in the Nautical Almanack, or any other astronomical Ephemeris; and
the latitude of the place may be found by means of the semicircle of
altitude, when the telescope is directed to the sun or a known fixed
star. It is likewise requisite to make the observation when the azimuth
and altitude of the sun alter quickly; and this is generally the case,
the farther that luminary is from the meridian:--Therefore, at the
distance of 3 or 4 hours, either before or after noon, (in summer)
adjust the horizontal circle; set the semicircle of altitude, so that
its nonius may stand at the co-latitude of the place; lay the plane
of the last-mentioned semicircle in the meridian, by estimation, its
0 being directed towards the depressed pole; place the nonius of the
declination semicircle to the declination, whether north or south. Then
direct the telescope towards the sun, partly by moving the declination
semicircle on the axis of the equatorial circle, and partly by moving
the horizontal circle on its own axis. There is but one position of
these which will admit of the sun being seen exactly in the middle
of the field of view. When this position is obtained, the nonius on
the equatorial circle shows _the apparent time_, and _the circle of
altitude is in the plane of the meridian_. When this position is
ascertained, the meridian may be settled by a land-mark at a distance.

With an equatorial instrument, nearly similar to that now described, I
formerly made a series of ‘_day observations_ on the celestial bodies,’
which were originally published in vol. 36 of ‘Nicholson’s Journal of
Natural Philosophy,’ and which occupy twenty pages of that journal.
Some of these observations I shall lay before the reader, after having
explained the manner in which they are made.

The instrument was made by Messrs. W. and S. Jones, opticians,
Holborn, London. The telescope which originally accompanied the
instrument was an achromatic refractor, its object-glass being 8-1/2
inches focal distance, and one inch diameter. This telescope, not
admitting sufficiently high magnifying powers for the observations
intended, was afterwards thrown aside for another telescope, having an
object-glass 20 inches focal length, and 1-3/4 inch diameter, which was
attached to the equatorial machinery in place of the small telescope.
It was furnished with magnifying powers of 15, 30, 45, 60, and 100
times. The instrument was placed on a firm pedestal about three feet
high. The feet of this pedestal had short iron pikes, which slipped
into corresponding holes in the floor of the apartment adjacent to a
south window, so that when the direction of the meridian was found, and
the circles properly adjusted, the instrument was in no danger of being
shifted from this position. Though this instrument generally stood
fronting the southern part of the heavens, yet the equatorial part,
along with the telescope, could occasionally be removed to another
position fronting the north and north-west, for observing the stars in
those quarters.

_Manner of observing stars and planets in the day-time by the
equatorial._ Before such observations can be made, the semicircle of
altitude must be placed in the meridian, and the degree and minute
pointed out by the nonius on the horizontal circle, when in this
position, noted down in a book, so that it may be placed again in the
same position, should any derangement afterwards happen. The semicircle
of altitude must be set to the co-latitude of the place; that is, to
what the latitude wants of 90°. Suppose the latitude of the place
of observation be 52° 30´ north, this latitude subtracted from 90°,
leaves 37° 30´ for the co-latitude; and therefore, the semicircle of
altitude--on which the equatorial circle is fixed--must be elevated to
37° 30´, and then the equatorial circle on the instrument coincides
with the equator in the heavens. Lastly, the telescope must be adjusted
on the declination semicircle, so as exactly to correspond with the
declination of the heavenly body to be viewed. If the body is in the
equator, the telescope is set by the index at 0 on the semicircle of
declination, or at the middle point between the two quadrants, and
then when the telescope, along with the semicircle of declination, is
moved from right to left, or the contrary, it describes an arc of the
equator. If the declination of the body be north, the telescope is
elevated to the northern division of the semicircle; if south, to the
southern part of it.

These adjustments being made, take the difference between the Right
Ascension of the sun and the body to be observed; and if the Right
Ascension of the body be greater than that of the sun, subtract the
difference from the time of observation; if not, add to the time of
observation.[39] The remainder in one case, or the sum in the other,
will be the hour and minute to which the nonius on the equatorial
circle is to be set; which being done, the telescope will point to the
star or planet to whose declination the instrument is adjusted. When
the heavenly body is thus found, it may be followed, in its diurnal
course, for hours, or as long as it remains above the horizon. For as
the diurnal motion of a star is parallel to the equator, the motion of
the telescope on the equatorial circle, will always be in the star’s
diurnal arc; and should it have left the field of the telescope for any
considerable time, it may be again recovered, by moving the telescope
onward according to the time which elapsed since it was visible in the
field of view. We may illustrate what has been now stated by an example
or two. Suppose on the 30th April, 1841, at 1 o’clock, P.M. we
wished to see the star _Aldebaran_. The Right Ascension of this star
is 4^h 27^m; and the sun’s Right Ascension for that day at noon, as
found in ‘White’s Ephemeris,’ or the ‘Nautical Almanack,’ is 2^h 30^m.
Subtract this last number from 4^h 27^m, and the remainder 1^h 57^m,
shows that the star comes to the meridian on that day at 57 minutes
past 1 o’clock, P.M. And as the time of observation is 1
P.M., the nonius which moves on the equatorial circle must be
set to 3 minutes past XI, as the star is at that hour 57 minutes from
the meridian. The declination of Aldebaran is 16° 11´ north, to which
point on the semicircle of declination, the telescope must be adjusted,
and then the star will be visible in the field of view. Again, suppose
we wished to observe the planet Venus on the 1st January, 1842, at
12 o’clock noon. The sun’s Right Ascension on that day is 18^h 46^m,
and that of Venus 17^h 41^m, from which the sun’s Right Ascension
being subtracted, the remainder is 22^h 55^m, or 55 minutes past 10,
A.M. Here, as the Right Ascension of Venus is too small to
have the sun’s Right Ascension taken from it, we borrow 24 hours, and
reckon the remainder from XII at noon. As the planet at 12 noon, is 1
hour 5 minutes past the meridian, the nonius on the equatorial circle
must be set to that point, and the telescope adjusted to 23° 6´ of
south declination, which is the declination of Venus for that day, when
this planet will appear in the field of view.


_Observations on the fixed stars and planets, made in the day-time by
the Equatorial._

For the purpose of illustrating the descriptions now given, and for
affording some information respecting celestial day observations,
I shall select a few of the observations above alluded to, which I
formerly published in Nicholson’s Journal, along with a few others
which have been since made. These observations were made with a view to
determine the following particulars:--1. What stars and planets may be
conveniently seen in the day-time, when the sun is above the horizon?
2. What degrees of magnifying power are requisite for distinguishing
them? 3. How near their conjunction with the sun they may be seen?
and 4. Whether the diminution of the aperture of the object-glass of
the telescope, or the increase of magnifying power, conduces most to
render a star or a planet visible in day-light. Having never seen such
observations recorded in books of astronomy or in scientific journals,
I was induced to continue them, almost every clear day for nearly a
year, in order to determine the points now specified. Some of the
results are stated in the following pages.

_Observations on fixed stars of the first magnitude._ April 23, 1813,
at 10^h 15^m, A.M., the sun being 5-1/2 hours above the
horizon. Saw the star _Vega_, or α Lyræ, very distinctly with a power
of 30 times. Having contracted the aperture of the object-glass to
9/10 of an inch, saw it on a darker ground, but not more plainly than
before. Having contracted the aperture still farther, to half an inch,
I perceived the star, but not so distinctly as before. The sky being
very clear, and the star in a quarter of the heavens nearly opposite
to the sun, I diminished the magnifying power to 15, and could still
perceive the star, but indistinctly; it was just perceptible. August
23, at 0^h 12^m, P.M., saw the star _Capella_, or α _Aurigæ_,
with a power of 60, and immediately afterwards with a power of 30;
the aperture undiminished. With this last power it appeared extremely
distinct, but not so brilliant and splendid as with the former power.
Having diminished the aperture to 9/10 of an inch, it appeared on a
darker ground, though in the former case, it was equally perceptible.
A few minutes afterwards, could distinguish it with a power of 15, the
aperture being contracted to half an inch. It appeared very small;
it was with difficulty the eye could fix upon it in the field of the
telescope; but when it was once perceived, its motion across the field
of view could be readily followed. It could not be perceived, when the
diminished aperture was removed. The sun was then shining in meridian
splendour.

August 10th, 9^h 30^m, A.M. Saw the star Sirius with a power
of 60, the aperture contracted to 9/10 inch. Saw it likewise when
the aperture was diminished to half an inch, but not so distinctly
as through the aperture of inch. Having put on a power of 30, could
distinguish it distinctly enough through each of the former apertures,
and likewise when they were removed; but somewhat more distinctly
with the apertures of nine-tenths and half an inch than without them.
At this time the star was 2^h 42^m in time of Right Ascension west
of the sun, having an elevation above the horizon of about 17° 10’;
the sun shining bright, and the sky very much enlightened in that
quarter of the heavens where the star appeared. There was also a
considerable undulation of the air, which is generally the case in
the hot mornings of summer--which renders a star more difficult to
be perceived than in the afternoon, especially when it is viewed at
a low altitude. June 4th, 1^h 30^m, P.M., saw Sirius with a
power of 30 with great distinctness, the aperture not contracted. The
star was then within 1^h 50^m, in time of Right Ascension east from
the sun. August 24th, 9^h 5^m, A.M., saw the star _Procyon_,
or α _Canis-Minoris_ distinctly with a power of 60, the aperture not
contracted. When diminished to 9/10 inch, it appeared rather more
distinct, as the ground on which it was seen was darker. With a power
of 30, and the aperture contracted to 9/10 inch, could perceive it, but
somewhat indistinctly. When the equatorial motion was performed, in
order to keep it in the field of view, it was sometime before the eye
could again fix upon it. When the aperture was diminished to half an
inch, it could not be perceived. Saw it when both the apertures were
removed, but rather more distinctly with the aperture of 9/10 inch. The
difference in the result of this observation, from that of Capella,
above stated, was owing to the star’s proximity to the sun, and the
consequent illumination of the sky in that quarter where it appeared.
Its difference in Right Ascension from that of the sun was then about
2^h 5^m of time, and its difference of declination about 4° 50´.[40]
This star may be considered as one of those which rank between the
first and second magnitudes.

Similar observations to the above were made and frequently repeated
on the stars Rigel, Aldebaran, Betelguese Cor-Leonis and other stars
of the first magnitude, which gave nearly the same results. The stars
Altares and Fomalhaut are not so easily distinguished, on account of
their great southern declination, and consequent low elevation above
the horizon. The following observation on _Arcturus_ may be added.
June 3rd, observed Arcturus very distinctly, a little before 7 in the
evening, the sun being about 1^h 40^m above the horizon, and shining
bright--with a power of 15; the aperture not contracted. It appeared
very small but distinct. This star is easily distinguishable at any
time of the day with a power of 30.

_Observations on stars of the second magnitude._ May 5, 1813, at 6^h,
P.M.; the sun being an hour and three quarters above the
horizon. Saw _Alphard_, or α Hydræ, a star of the second magnitude,
with a power of 60; the aperture diminished to 9/10 inch. A few minutes
afterwards could perceive it, but indistinctly, with a power of 30,
the aperture contracted as above. It could not be seen very distinctly
with this power, till about half an hour before sun-set. It was then
seen rather more distinctly when the aperture was contracted than
without the contraction. May 7th. Saw the star _Deneb_, or β _Leonis_,
distinctly with a power of 60, about an hour and a half before sun-set.
August 20th. Saw Ras Alkague, or α _Ophiuchi_, at 4^h 40^m, _P.M._,
with a power of 100, the sun being nearly 3 hours above the horizon,
and shining bright. Perceived it about an hour afterwards, with a
power of 60--with the aperture contracted to 9/10 inch, and also when
this contraction was removed. The star was seen nearly as distinctly
in the last case as in the first. August 27, 5^h, _P.M._, the same
star appeared quite distinct with a power of 60, the aperture not
contracted. It did not appear more distinct when the aperture was
contracted to 9/10 inch. The sun was then more than 2 hours above the
horizon. August 28th. Saw the star _Pollux_, or β _Gemini_, 2 hours
after sun-rise with a power of 60, aperture undiminished. November
12th, 1_{h} 30´, _P.M._ Saw the star _Altair_, or α _Aquilæ_, with an
8-1/2 inch telescope, 1 inch aperture, carrying a power of 45, the
aperture not contracted. Having contracted the aperture a little, it
appeared somewhat less distinct. This star is reckoned by some to
belong to the class of stars of the first magnitude; but in White’s
‘Ephemeris’ and other Almanacks, it is generally marked as being of the
second magnitude. It forms a kind of medium between stars of the 1st
and of the 2nd magnitude.

Similar observations, giving the same results, were made on the stars
Bellatrix, Orion’s Girdle, α Andromedæ, α Pegasi, Alioth, Benetnasch,
North Crown, or α Coronæ Borealis, and various other stars of the same
magnitude.

From the above and several hundreds of similar observations, _the
following conclusions_ are deduced.

1. That a magnifying power of 30 times is sufficient for distinguishing
a fixed star of the first magnitude, even at noon-day, at any season
of the year; provided it have a moderate degree of elevation above the
horizon, and be not within 30° or 40° of the sun’s body. Also, that, by
a magnifying power of 15, a star of this class may be distinguished,
when the sun is not above an hour and a half above the horizon. But,
in every case, higher powers are to be preferred. Powers of 45 or 60,
particularly the last, were found to answer best in most cases, as with
such powers the eye could fix on the star with ease, as soon as it
entered the field of the telescope.

2. That most of the stars of the 2nd magnitude may be seen with a power
of 60, when the sun is not much more than 2 hours above the horizon;
and, at any time of the day, the brightest stars of this class may be
seen with a power of 100, when the sky is serene, and the star not too
near the quarter in which the sun appears.

3. That, in every instance, an increase of magnifying power has
the principal effect in rendering a star easily perceptible. That
diminution of aperture, in most cases, produces a very slight effect;
in some cases, none at all; and, when the aperture is contracted beyond
a certain limit, it produces a hurtful effect. The cases in which a
moderate contraction is useful, are the two following:--1. When the
star appears in a bright part of the sky, not far from that quarter in
which the sun appears. 2. When an object-glass of a large aperture, and
a small degree of magnifying power, is used. In almost every instance
the contraction of the object-glass of the 8-1/2-inch telescope with
a power of 45, had a hurtful effect. But when the 20-inch telescope
carried a power of only 15, the contraction served to render the object
more perceptible.


_Observations on the Planets made in the day-time._

Some of the planets are not so easily distinguished in the day-time as
the fixed stars of the first magnitude. The one which is most easily
distinguished at all times, is the planet Venus.

1. _Observations on Venus._ My observations on this planet commenced
about the end of August, 1812, about three or four weeks after its
inferior conjunction. About that period, between ten and eleven in the
forenoon, with a power of 45, it appeared as a beautiful crescent,
quite distinct and well-defined, with a lustre similar to that of the
moon about sun-set, but of a whiter colour. The view of its surface and
phase was fully more distinct and satisfactory than what is obtained
in the evening after sun-set; for, being at a high elevation, the
undulation near the horizon did not affect the distinctness of vision.
The planet was then very distinctly seen with a power of 7 times, when
it appeared like a star of the first or second magnitude. I traced the
variation of its phases, almost every clear day, till the month of May,
1813. As at that time, it was not far from its superior conjunction
with the sun, I wished to ascertain how near its conjunction with that
luminary it might be seen; and particularly whether it might not be
possible, in certain cases, to see it at the moment of its conjunction.

The expressions of all astronomical writers previous to this period,
when describing the phases of Venus, either directly assert, or,
at least imply, that it is _impossible_ to see that planet, in any
instance, at the time of its superior conjunction. This is the language
of Dr. Long, Dr. Gregory, Dr. Brewster, Ferguson, Adams, B. Martin, and
most other writers on the science of astronomy. How far such language
is correct will appear from the following observations and remarks.

April 24, 1813, 10^h 50´ A.M. Observed Venus with a power of
30, the aperture not contracted. She was then about 31 minutes, in
time, of right ascension, distant from the sun. Their difference of
declination 3° 59´. She appeared distinct and well-defined. With a
power of 100, could distinguish her gibbous phase. May 1st, 10^h 20^m,
A.M. Viewed this planet with a power of 60; the aperture not
contracted. It appeared distinct. Saw it about the same time with
a power of 15, the aperture being contracted to 9/10 inch. Having
contracted the aperture to 1/2 inch, saw it more distinctly. When the
contracted apertures were removed, the planet could with difficulty be
distinguished, on account of the direct rays of the sun striking on
the inside of the tube of the telescope. The sun was shining bright,
and the planet about 25´ of time in R.A. west of his centre, their
difference of declination being 3° 7´. May 7th, 10^h, A.M.
Saw Venus distinctly with a power of 60, the sun shining bright. It
was then about 19´ in time of R.A. and 4° 27´ in longitude west of the
sun; their difference of declination being 2° 18´. I found a diminution
of aperture particularly useful when viewing the planet at this time,
even when the higher powers were applied. This was the last observation
I had an opportunity of making prior to the conjunction of Venus with
the sun, which happened on May 25th, at 9^h 30^m, A.M. Its
geocentric latitude at that time being about 16´ south, the planet must
have passed almost close by the sun’s southern limb. Cloudy weather for
nearly a month after the last observation, prevented any further views
of the planet, when it was in that part of the heavens which was within
the range of the instrument. The first day that proved favourable after
it had passed the superior conjunction, was June 5th. The following is
the memorandum of the observation then taken.

June 5th, 9^h, A.M. Adjusted the Equatorial Telescope for
viewing the planet Venus, but it could not be perceived, on account
of the direct rays of the sun entering the tube of the telescope.
I contrived an apparatus for screening his rays, but could not get
it conveniently to move along with the telescope; and therefore
determined to wait till past eleven, when the top of the window of the
place of observation would intercept the solar rays. At 11^h 20^m,
A.M., just as the sun had passed the line of sight from the
eye to the top of the window, and his body was eclipsed by it, I was
gratified with a tolerably distinct view of the planet, with a power
of 60. The aperture being contracted to 9/10 inch. The distinctness
increased as the sun retired, till, in two or three minutes, the planet
appeared perfectly well-defined. Saw it immediately afterwards, with
a power of 30, the aperture contracted as before. Saw it also quite
distinctly with a power of 15; but it could not be distinguished with
this power, when the contracted aperture was removed. At this time
Venus was just 3° in longitude, or about 13´ in time of R.A. east of
the sun’s centre, and of course only about 2-3/4 degrees from his
eastern limb; the difference of their declination being 27´, and the
planet’s latitude 11´ north.

Several years afterwards, I obtained views of this planet, when
considerably nearer the sun’s margin than as stated in the above
observation, particularly on the 16th October, 1819, when Venus was
seen when only 6 days and 19 hours past the time of the superior
conjunction. At that time its distance from the sun’s eastern limb
was only 1° 28´ 42´´. A subsequent observation proved that Venus can
be seen when only 1° 27´, from the sun’s margin--which I consider as
approximating to the nearest distance from the sun at which this
planet is distinctly visible.--I shall only state farther the two or
three following observations.

June 7th, 1813, 10^h, A.M. Saw Venus with a power of 60, the
aperture being contracted to 9/10 inch--the direct rays of the sun
_not being intercepted by the top of the window_. The aperture having
being further contracted to 1/2 inch, could perceive her, but not quite
so distinctly. When the contractions were removed, she could scarcely
be seen. She was then 3° 33´ in longitude, and nearly 15 minutes in
time of R.A. distant from the sun’s centre. Some fleeces of clouds
having moved across the field of view, she was seen remarkably distinct
in the interstices--the sun at the same time, being partly obscured
by them.--August 19th, 1^h 10´, P.M. Viewed Venus with a
magnifying power of 100. Could perceive her surface and gibbous phase
almost as distinctly as when the sun is below the horizon. She appeared
bright, steady in her light, and well defined, without that glare and
tremulous appearance she exhibits in the evening when near the horizon.
She was then nearly on the meridian. On the whole, such a view of this
planet is as satisfactory, if not preferable, to those views we obtain
with an ordinary telescope in the evening, when it is visible to the
naked eye.

All the particulars above stated have been confirmed by many subsequent
observations continued throughout a series of years. I shall state only
two recent observations which show that Venus may be seen somewhat
nearer the sun than what is deduced from the preceding observations,
and at the point of its superior conjunction. March 10th, 1842,
observed the planet Venus, then very near the sun, at 19 minutes past
11, A.M. It had passed the point of its superior conjunction
with the sun, on the 5th March, at 1^h 19^m, P.M. The
difference of right ascension between the sun and the planet was then
about 6-1/2 minutes of time, or about 1° 37-1/2´, and it was only about
1° 21´ distant from the sun’s eastern limb. It appeared quite distinct
and well-defined, and might perhaps have been seen on the preceding
day, had the observation been then made.--The following observation
shows that Venus may be seen still nearer the sun than in the preceding
observations, and even _at the moment of its superior_ conjunction. On
the 2nd of October, 1843, this planet passed the point of its superior
conjunction with the sun, at 4^h 15^m, P.M. At two o’clock,
P.M.--only two hours before the conjunction, I perceived the
planet distinctly, and kept it in view for nearly ten minutes, till
some dense clouds intercepted the view. It appeared tolerably distinct
and well-defined, though not brilliant, and with a round full face,
and its apparent path was distinctly traced several times across the
field of view of the telescope. I perceived it afterwards, about half
past four, P.M., only a few minutes after it had passed the
point of conjunction, on which occasion it appeared less distinct than
in the preceding observation, owing to the low altitude of the planet,
being then only a few degrees above the horizon. The observations,
in this instance, were made not with an equatorial instrument, which
I generally use in such observations, but with a good achromatic
telescope 44-1/2 inches focal distance, mounted on a common tripod,
with a terrestrial power of 95 times. A conical tube about ten inches
long was fixed on the object-end of the telescope, at the extremity of
which an aperture, 1-1/2 inch diameter was placed, so as to intercept,
as much as possible, the direct ingress of the solar rays. The top of
the upper sash of the window of the place of observation was likewise
so adjusted as to intercept the greater part of the sun’s rays from
entering the tube of the telescope. The sun’s declination at that time
was 3° 26´ south, and that of Venus 2° 12´ south; consequently, the
difference of declination was 1° 14´ = the distance of Venus from the
sun’s centre; and as the sun’s diameter was about 16´, Venus was then
only 58´ from the sun’s northern limb, or 6´ less than two diameters of
the sun.

This is the nearest approximation to the sun at which I have ever
beheld this planet, and it demonstrates that Venus may be seen even
when within a degree of the sun’s margin; and it is perhaps the nearest
position to that luminary in which this planet can be distinctly
perceived. It shows that the light reflected from the surface of Venus
is far more brilliant than that reflected from the surface of our moon;
for no trace of this nocturnal luminary can be perceived, even when at
a much greater distance from the sun, nor is there any other celestial
body that can be seen within the limit now stated. This is the first
observation, so far as my information extends, of Venus having been
seen at the time of her superior conjunction.[41]

The practical conclusion from this observation is, that, at the
superior conjunction of this planet, when its distance from the sun’s
margin is not less than 58´, _its polar and equatorial diameter may be
measured_ by a micrometer, when it will be determined whether or not
Venus be of a _spheroidal_ figure. The Earth, Mars, Jupiter and Saturn
are found to be not spheres but _spheroids_, having their polar shorter
than their equatorial diameters. But the true figure of Venus has never
yet been ascertained, because it is only at the superior conjunction
that she presents a full enlightened hemisphere, and when both
diameters can be measured, except at the time when she transits the
sun’s disk, which happens only twice in the course of 120 years.[42]

The following conclusions are deduced from the observations made on
Venus.

1. That this planet may be seen distinctly, with a moderate degree of
magnifying power, _at the moment of its superior conjunction with the
sun_, when its geocentric latitude, either north or south, at the
time of conjunction, is not less than 1° 14´, or, when the planet is
about 58´ from the sun’s limb. This conclusion is deduced from the
observation of Oct. 2, 1843,[45] stated above.

2. Another conclusion is--that during the space of 583 days, or about
19 months--the time this planet takes in moving from one conjunction
with the sun to a like conjunction again--when its latitude at the
time of its superior conjunction exceeds 1° 14´, it may be seen
with an equatorial telescope every clear day without interruption,
except about the period of its _inferior_ conjunction, when its dark
hemisphere is turned towards the earth, and a short time before and
after it. When its geocentric latitude is less than 1° 14´, it will be
hid only about four days before, and the same time after its superior
conjunction. During the same period it will be invisible to the naked
eye, and consequently no observations can be made upon it with a common
telescope, for nearly six months, and sometimes more, according as its
declination is north or south, namely about two or three months before,
and the same time after its superior conjunction, except where there
is a very free and unconfined horizon. In regard to the time in which
this planet can be hid about the period of its _inferior_ conjunction,
I have ascertained from observation, that it can never be hid longer
than during a space of 2 days 22 hours; having seen Venus, about noon,
like a fine slender crescent, only 35 hours after she had passed the
point of her inferior conjunction; and in a late instance she was seen
when little more than a day from the period of conjunction. The longest
time, therefore, that this planet can be hid from view during a period
of 583 days, is only about 10 days; and when its latitude at the time
of the superior conjunction, equals or exceeds 1° 14´, it can be hid
little more than two days. This is a circumstance which cannot be
affirmed of any other celestial body, the sun only excepted.

3. That every variation of the phases of this planet--from a slender
crescent to a full enlightened hemisphere--may, on every clear day,
be conveniently exhibited by means of the equatorial telescope.
This circumstance renders this instrument peculiarly useful in the
instruction of the young in the principles of astronomy. For, if the
phase which Venus should exhibit at any particular time be known, the
equatorial telescope may be directed to the planet, and its actual
phase in the heavens be immediately exhibited to the astronomical pupil.

4. Since it is only at the period of the superior conjunction that
this planet presents a full enlightened hemisphere, and since it is
only when this phase is presented that both its diameters can be
measured--it is of some importance that observations be made on it at
the moment of conjunction, by means of powerful telescopes furnished
with micrometers, so as to determine the difference (if any) between
its polar and equatorial diameters.

5. Another conclusion from the observations on Venus, is, that a
moderate diminution of the aperture of the object-glass of the
telescope is useful, and even necessary in viewing this planet when
near the sun. Its effect is owing in part to the direct solar rays
being thereby more effectually excluded; for when these rays enter
directly into the tube of the telescope, it is very difficult, and
almost impossible to perceive this planet, or any other celestial body
when in the vicinity of the sun.


_Observations on Jupiter and other planets._

This planet is very easily distinguished in the day-time with a very
moderate magnifying power, when it is not within 30° or 35° of the sun.
The following extract from my memorandums may serve as a specimen. May
12, 1813, 1^h 40^m, P.M. Saw Jupiter with a power of 15 times,
the aperture not contracted. The planet appeared so distinct with this
power, that I have reason to believe, it would have been perceived with
a power of 6 or 7 times. When the aperture was contracted 9/10 inch,
and afterwards to half an inch, there was little perceptible difference
in its appearance. It was then about 58° in longitude, east of the sun.

Though Jupiter when at a considerable distance from the sun, and near
his opposition, appears to the naked eye with a brilliancy nearly equal
to that of Venus, yet there is a very striking difference between them,
in respect of lustre, when viewed in day-light. Jupiter, when viewed
with a high magnifying power, in the day-time, always exhibits a very
dull cloudy appearance; whereas Venus appears with a moderate degree of
splendour. About the end of June 1813, between 5 and 6 in the evening,
having viewed the planet Venus, then within 20° of the sun, and which
appeared with a moderate degree of lustre, I directed the telescope to
Jupiter, at that time more than 32° from the sun, when the contrast
between the two planets was very striking, Jupiter appearing so faint
as to be just discernible, though his apparent magnitude was nearly
double that of Venus. In this observation a power of 65 was used.
In his approach towards the sun, about the end of July, I could not
perceive him when he was within 16° or 17° of his conjunction with that
luminary.--_These circumstances furnish a sensible and popular proof_,
independently of astronomical calculations, _that the planet Jupiter
is placed at a much greater distance from the sun than Venus_; since
its light is so faint as to be scarcely perceptible when more than 20
degrees from the sun, while that of Venus is distinctly seen amidst
the full splendour of the solar rays, when only about a degree from
the margin of that luminary. With a power of 65 I have been enabled
to distinguish the _belts_ of Jupiter before sun-set, but could never
perceive any of his satellites till the sun was below the horizon.
There are no observations which so sensibly and strikingly indicate
the different degrees of light emitted by the different planets as
those which are made in the day-time. To a common observer, during
night, Jupiter and Venus appear, in a clear sky, nearly with equal
brilliancy, and even Mars, when about the point of his _opposition_ to
the sun, appears with a lustre somewhat similar, though tinged with a
ruddy hue; but when seen in day-light their aspect is very dissimilar.
This circumstance evidently indicates, 1. that these planets are
placed at different distances from the sun, and consequently are
furnished with different degrees of light proportional to the square
of their distances from that luminary;--and 2. that there are certain
circumstances connected with the surfaces and atmospheres of the
planetary bodies, which render the light they emit more or less
intense, independently of their different distances from the central
luminary. For Mars, though much nearer to the sun than Jupiter, is not
so easily distinguished in the day-time, and, even in the night-time,
appears with a less degree of lustre.

My observations on _Saturn_ in day-light, have not been so frequent as
those on Jupiter. I have been enabled to distinguish his ring several
times before sun-set, with a power of 65; but his great southern
declination, and consequent low altitude, at the periods when these
observations were made, were unfavourable for determining the degree
of his visibility in day-light; for a planet or a star is always more
distinctly perceptible in a _high_ than in a low altitude, on account
of the superior purity of the atmosphere through which a celestial
object is seen when at a high elevation above the horizon. This planet,
however, is not nearly so distinctly visible in day-light as Jupiter,
and I have chiefly seen it, when the sun was not more than an hour or
two above the horizon, but never at noon-day; although it is probable
that with powerful instruments it may be seen even at that period
of the day. The planet _Mars_ is seldom distinctly visible in the
day-time, except when at no great distance from its opposition to the
sun. The following is a memorandum of an observation on Mars, when in a
favourable position. October 24, 1836. Saw the planet Mars distinctly
with a power of about 60, at 40 minutes past 9 A.M., the
sun having been above the horizon nearly three hours. It appeared
tolerably distinct, but scarcely so brilliant as a fixed star of the
first magnitude, but with apparently as much light as Jupiter generally
exhibits when viewed in day-light. It could not be traced longer at the
time, so as to ascertain if it could be seen at mid-day; on account
of the interposition of the western side of the window of the place
of observation. The ruddy aspect of this planet--doubtless caused by
a dense atmosphere with which it is environed--is one of the causes
which prevents its appearing with brilliancy in the day-time. With
respect to the planet _Mercury_, I have had opportunities of observing
it several times after sun-rise, and before sun-set, about 10 or 12
days before and after its greatest elongation from the sun, with a
power of 45. I have several times searched for this planet about noon,
but could not perceive it. The air, however, at the times alluded to,
was not very clear, and I was not certain that it was within the field
of the telescope; and therefore, I am not convinced but that, with a
moderately high power, it may be seen even at noon-day.

Such are some specimens of the observations I have made on the heavenly
bodies in the day-time, and the conclusions which may be deduced from
them. I have been induced to communicate them, from the consideration,
that the most minute facts, in relation to any science, are worthy of
being known, and may possibly be useful. They may at least gratify the
astronomical tyro with some information which he will not find in the
common treatises on astronomy, and may perhaps excite him to prosecute
a train of similar observations for confirming or correcting those
which have been noted above.

Besides the deductions already stated, the following general
conclusions may be noted.--1. That a celestial body may be as easily
distinguished at noon-day, as at any time between the hours of nine in
the morning and three in the afternoon, except during the short days
in winter. 2. They are more easily distinguished at a high than at
a low altitude--in the afternoon than in the morning, especially if
their altitudes be low--and in the northern region of the heavens than
in the southern. The difficulty of perceiving them at a low altitude
is obviously owing to the thick vapours near the horizon. Their being
less easily distinguished in the morning than in the afternoon is owing
to the undulations of the atmosphere, which are generally greater in
the morning than in the afternoon. This may be evidently perceived
by looking at distant land-objects at those times, in a hot day,
through a telescope which magnifies about 40 or 50 times, when they
will be found to appear tremulous and distorted in consequence of these
undulations, especially if the sun be shining bright. In consequence
of this circumstance, we can seldom use a high terrestrial power with
effect on land objects, except early in the morning, and a short
time before sun-set. Their being more easily distinguished in the
northern region of the heavens is owing to that part of the sky being
of a deeper azure, on account of its being less enlightened than the
southern with the splendour of the solar rays.


_Utility of Celestial Day Observations._

The observations on the heavenly bodies in the day-time, to which I
have now directed the attention of the reader, are not to be considered
as merely gratifications of a rational curiosity, but may be rendered
subservient to the promotion of astronomical science. As to the planet
Venus--when I consider the degree of brilliancy it exhibits, even in
day-light, I am convinced that useful observations might frequently
be made on its surface in the day-time, to determine some of its
physical peculiarities and phenomena. Such observations might set
at rest any disputes which may still exist respecting the period of
rotation of this planet. Cassini, from observations on a bright spot,
which advanced 20° in 24^h 34^m determined the time of its rotation to
be 23 hours, 20 minutes. On the other hand, Bianchini, from similar
observations, concluded that its diurnal period was 24 days and 8
hours. The difficulty of deciding between these two opinions, arises
from the short time in which observations can be made on this planet,
either before sun-rise, or after sun-set, which prevents us from
tracing, with accuracy, the progressive motion of its spots for a
sufficient length of time. And, although an observer should mark the
motion of the spots at the same hour, on two succeeding evenings, and
find they had moved forward about 15° in 24 hours, he would still be
at a loss to determine, whether they had moved only 15°, in all, since
the preceding observation, or had finished a revolution and 15° more.
If, therefore, any spots could be perceived on the surface of Venus
in the day-time, their motion might be traced, when she is in north
declination, for 12 hours or more, which would completely settle the
period of rotation. That it is not improbable that spots, fitted for
this purpose, may be discovered on her disk in the day-time, appears
from some of the observations of Cassini, who saw one of her spots when
the sun was more than eight degrees above the horizon.[46] The most
distinct and satisfactory views I have ever had of this planet were
those which I obtained in the day-time, in summer, when it was viewed
at a high altitude, with a 44-1/2 inch achromatic telescope, carrying a
power of 150. I have at such times distinctly perceived the distinction
between the shade and colour of its margin, and the superior lustre of
its central parts, and some spots have occasionally been seen, though
not so distinctly marked as to determine its rotation. Such distinct
views are seldom to be obtained in the evening after sun-set, on
account of the undulations of the atmosphere, and the dense mass of
vapours through which the celestial bodies are viewed when near the
horizon.

Nor do I consider it altogether improbable that its _satellite_ (if
it have one, as some have supposed) may be detected in the day time,
when this planet is in a favourable position for such an observation;
particularly when a pretty large portion of its enlightened surface
is turned towards the earth, and when its satellite, of course, must
present a similar phase. About the period of its greatest elongation
from the sun, and soon after it assumes a crescent phase, in its
approach to the inferior conjunction, may be considered as the most
eligible times for prosecuting such observations. If this supposed
satellite be about one third or one fourth of the diameter of its
Primary, as Cassini, Short, Baudouin, Montbarron, Montaigne, and other
astronomers supposed, it must be nearly as large as Mercury, which has
been frequently seen in day-light. If such a satellite have a real
existence, and yet undistinguishable in day-light, its surface must be
of a very different quality for reflecting the rays of light from that
of its primary; for it is obvious to every one who has seen Venus with
a high power, in the day-time, that a body of equal brilliancy--though
four times less in diameter--would be quite perceptible, and exhibit a
visible disk. Such observations, however, would be made, with a much
greater effect in Italy and other Southern countries, and particularly
in Tropical climates, such as the southern parts of Asia and America,
and in the West India Islands, where the sky is more clear and serene,
and where the planet may be viewed at higher altitudes, and for a
greater length of time, without the interruption of clouds, than in our
island.

Again, the apparent magnitudes of the fixed stars--the quantity of
light they respectively emit--and the precise class of magnitude which
should be assigned to them--might be more accurately determined by
day observations, than by their appearance in the nocturnal sky. All
the stars which are reckoned to belong to the _first magnitude_ are
not equally distinguishable in day-light. For example, the stars
_Aldebaran_ and _Procyon_ are not so easily distinguished, nor do they
appear with the same degree of lustre by day, as the stars α _Lyræ_
and _Capella_. In like manner the stars _Altair_, _Alphard_, _Deneb
Ras Alkague_, considered as belonging to the _second_ magnitude,
are not equally distinguishable by the same aperture and magnifying
power--which seems to indicate, that a different quantity of light
is emitted by these stars, arising from a difference either in their
magnitude, their distance, or the quality of the light with which they
are irradiated.

The following are likewise practical purposes to which celestial day
observations may be applied. In accurately adjusting Circular and
Transit instruments, it is useful, and even necessary, for determining
the exact position of the meridian, to take observations of certain
stars, which differ greatly in zenith distance, and which transit the
meridian nearly at the same time. But as the stars best situated for
this purpose, cannot, at every season, be seen in the evenings, we
must, in certain cases, wait for several months till such observations
can be made, unless we make them in the day-time, which can very easily
be done, if the instrument have a telescope adapted to it, furnished
with such powers as those above stated, or higher powers if required. I
have likewise made use of observations on the stars in the day time for
adjusting a clock or watch to meantime, when the sun was in a situation
beyond the range of the instrument, or obscured by clouds, and when
I did not choose to wait till the evening. This may, at first view,
appear to some as paradoxical; since the finding of a star in day-light
depends on our knowing its right Ascension from the sun, and this last
circumstance depends, in some measure, on our knowing the true time.
But if a watch or clock is known not to have varied above seven or
eight minutes from the time, a star of the first magnitude may easily
be found, by moving the telescope a little backwards or forwards, till
the star appear; and when it is once found, the exact variation of the
movement is then ascertained, by comparing the calculations which were
previously necessary, with the time pointed out by the nonius on the
Equatorial circle--or, in other words, by ascertaining the difference
between the time assumed, and the time indicated by the instrument,
when the star appears in the centre of the field of view. All this may
be accomplished in five or six minutes.

Besides the practical purposes now stated, the Equatorial telescope is
perhaps the best instrument for instructing a learner in the various
operations of practical astronomy, and particularly for enabling him
to distinguish the names and positions of the principal stars. For,
when the right Ascension and Declination of any star is known, from
astronomical tables, the telescope may be immediately adjusted to
point to it, which will infallibly prevent his mistaking one star for
another. In this way, likewise, the precise position of the planet
_Mercury_, _Uranus_, _Vesta_, _Juno_, _Ceres_, _Pallas_--a small
comet, a nebula, a double star, or any other celestial body not easily
distinguishable by the naked eye, may be readily pointed out, when its
right Ascension and Declination are known to a near approximation.

In conclusion, I cannot but express my surprise, that the Equatorial
telescope is so little known, even by many of the lovers of
astronomical science. In several respectable academies in this part of
Britain, and, if I am not misinformed, in most of our universities,
this instrument is entirely unknown. This is the more unaccountable,
as a small equatorial may be purchased for a moderate sum; and as
there is no single instrument so well adapted for illustrating all the
operations of Practical Astronomy. Where very great accuracy is not
required, it may occasionally be made to serve the general purposes
of a _transit instrument_ for observing the passages of the sun and
stars across the meridian. It may likewise be made to serve as a
_theodolite_ for surveying land and taking horizontal angles--as a
_Quadrant_ for taking angles of altitude--as a _level_--as an _equal
altitude instrument_--an _azimuth instrument_ for ascertaining the
sun’s distance from the north or south points of the horizon--and
as an accurate Universal Sun Dial, for finding the exact _mean_ or
_true_ time, on any occasion when the sun is visible. The manner of
applying it to these different purposes will be obvious to every one
who is in the least acquainted with the nature and construction of this
instrument.

The price of a small Equatorial instrument, such as that described
p. 454, is about 16 guineas, exclusive of some of the eye-pieces,
which were afterwards added for the purpose of making particular
observations. Instruments of a larger size, and with more complicated
machinery, sell from 50 to 100 guineas and upwards. Messrs. W. and S.
Jones, Holborn, London, construct such instruments.


ON THE QUADRANT.

[Illustration: _figure 87._]

Every circle being supposed to be divided into 360 equal parts, or
degrees,--it is evident, that 90 degrees, or the fourth part of a
circle, will be sufficient to measure all angles, between the horizon
of any place and the line perpendicular to it which goes up to the
zenith. Thus, in fig. 87, the line CB represents the plane of the
horizon. ACBH, the quadrant, AC the perpendicular to the horizon,
and A the zenith point. If the lines BC and CA represent a pair of
compasses with the legs standing perpendicular to each other, and
the curved lines AB, DE and FG, the quarter of as many circles of
different sizes--it is evident that although each of these differs
from the others in size, yet that each contains the same portion of a
circle, namely a quadrant or fourth part; and thus it would be from the
smallest to the largest quadrant that could be formed,--they would all
contain exactly 90 degrees each. By the application of this principle
the comparative measure of angles may be extended to an indefinite
distance. By means of an instrument constructed in the form of a
quadrant of a circle, with its curved edge divided into 90 equal parts,
the altitude of any object in the heavens can at any time be determined.

There are various constructions of this instrument, some of them
extremely simple, and others considerably complex and expensive,
according to the degree of accuracy which the observations require. The
following is a description of the _Pillar Quadrant_, as it was made by
Mr. Bird, for the observatory of Greenwich, and several continental
observatories.

This instrument consists of a quadrant E E H G L (fig. 88.) mounted on
a pillar B, which is supported by a tripod AA, resting on three foot
screws. The quadrant, the pillar, and the horizontal circle all revolve
round a vertical axis. A telescope H is placed on the horizontal
radius, and is directed to a meridian mark previously made on some
distant object for placing the plane of the instrument in the meridian,
and also for setting the zero, or beginning of the scale truly
horizontal. This is sometimes done by a level instead of a telescope,
and sometimes by a plumb-line G, suspended from near the centre, and
brought to bisect a fine dot made on the limb, where a microscope is
placed to examine the bisection. The weight or plummet at the end of
the plumb-line is suspended in the cistern of water _b_, which keeps it
from being agitated by the air. A similar dot is made for the upper end
of the plumb-line upon a piece of brass, adjustable by a screw _d_, in
order that the line may be exactly at right angles to the telescope,
when it is placed at O. The quadrant is screwed by the centre
of its frame, against a piece of brass _e_ with three screws, and this
piece is screwed to the top of the pillar B with other three screws.
By means of the first three screws, the plane of the quadrant can be
placed exactly parallel to the vertical axis, and by the other screws
the telescope H can be placed exactly perpendicular to it. The nut of
the delicate screw L is attached to the end of the telescope F, by a
universal joint. The collar for the other end is jointed in the same
manner to a clamp which can be fastened to any part of the limb.
A similar clamp-screw and slow motion is seen at _n_ for the lower
circle, which is intended to hold the circle fast, and adjust its
motion. The divisions of the lower, or horizontal circle, are read by
verniers, or noniuses, fixed to the arms of the tripod at _l_ and _m_,
and, in some cases three are used to obtain greater accuracy.

[Illustration: _figure 88._]

In using this quadrant, the axis of the telescope H is adjusted to a
horizontal line, and the plane of the quadrant to a vertical line, by
the means already stated. The screw of the champ L is then loosened,
and the telescope directed to the star, or other object, whose altitude
is required. The clamp screw being fixed, the observer looks through
the telescope, and with the nut of the screw L he brings the telescope
into a position where the star is bisected by the intersection of the
wires in the field of the telescope. The divisions are then to be read
off upon the vernier, and the altitude of the star will be obtained.
By means of the horizontal circle D, all angles in the plane of the
horizon may be accurately measured--such as the amplitudes and azimuths
of the celestial bodies.

Quadrants of a more simple construction than the above, may be
occasionally used, such as Gunter’s, Cole’s, Sutton’s and others; but
none of these are furnished with telescopes, or telescopic sights, and
therefore an altitude cannot be obtained by them with the same degree
of accuracy as with that which has been now described.

By means of the Quadrant, not only the altitudes of the heavenly bodies
may be determined, but also the distances of objects on the earth by
observations made at two stations--the altitude of fireballs and other
meteors in the atmosphere--the height of a cloud, by observation on
its altitude and velocity--and numerous other problems, the solution of
which depends upon angular measurements. A _Mural Quadrant_ is the name
given to this instrument when it is fixed upon a wall of stone, and in
the plane of the meridian, such as the quadrant which was erected by
Flamstead in the Observatory at Greenwich. Although the quadrant was
formerly much used in astronomical observations, yet it may be proper
to state, that its use has now been almost completely superseded by
the recent introduction of _Astronomical Circles_, of which we shall
now give the reader a very short description, chiefly taken from
Troughton’s account of the instrument he constructed, as found in Sir
D. Brewster’s Supplement to Ferguson’s Astronomy.


THE ASTRONOMICAL CIRCLE.

[Illustration: _figure 89._]

An astronomical circle is a _complete circle_ substituted in place
of the quadrant, and differs from it only in the superior accuracy
with which it enables the astronomer to make his observations. The
large vertical or declination circle CC (fig. 89.) is composed of two
complete circles strengthened by an edge bar on their inside, and
firmly united at their extreme borders by a number of short braces or
bars which stand perpendicular between them, and which keep them at
such a distance as to admit the achromatic telescope TT. This double
circle is supported by 16 conical bars, firmly united along with the
telescope, to a horizontal axis. The exterior limb of each circle is
divided into degrees and parts of a degree, and these divisions are
divided into seconds by means of the micrometer microscopes _mm_,
which read off the angle on opposite sides of each circle. The cross
wires in each microscope may be moved over the limb till they coincide
with the nearest division of the limb, by means of the micrometer
screws _cc_, and the space moved through is ascertained by the
divisions on the graduated head above _c_, assisted by a scale within
the microscope. The microscopes are supported by two arms proceeding
from a small circle concentric with the horizontal axis, and fixed
to the vertical columns. This circle is the centre upon which they
can turn round nearly a quadrant for the purpose of employing a new
portion of the divisions of the circle, when it is reckoned prudent to
repeat any delicate observations upon any part of the limb. At _h_ is
represented a level for placing the axis in a true horizontal line, and
at _k_ is fixed another level parallel to the telescope, for bringing
the zero of the divisions to a horizontal position. The horizontal axis
to which the vertical circle and the telescope are fixed, is equal in
length to the distance between the vertical pillars, and its pivots are
supported by semicircular bearings, placed at the top of each pillar.
These two vertical pillars are firmly united at their bases to a cross
bar _f_. To this cross bar is also fixed a vertical axis about three
feet long, the lower end of which terminating in an obtuse point, rests
in a brass conical socket firmly fastened at the bottom of the hollow
in the stone pedestal D, which receives the vertical axis. This socket
supports the whole weight of the moveable part of the instrument. The
upper part of the vertical axis is supported by two pieces of brass,
one of which is seen at _e_, screwed to the ring _i_, and containing
a right angle, or Y. At each side of the ring, opposite to the points
of contact, is placed a tube containing a heliacal spring, which, by
a constant pressure on the axis, keeps it against its bearings, and
permits it to turn, in these four points of contact, with an easy and
steady motion. The two bearings are fixed upon two rings capable of
a lateral adjustment; the lower one by the screw _d_ to incline the
axis to the east or west, while the screw _b_ gives the upper one _i_
a motion in the plane of the meridian. By this means the axis may be
adjusted to a perpendicular position as exactly as by the usual method
of the tripod with feet screws. These rings are attached to the centre
piece _s_, which is firmly connected with the upper surface of the
stone by six conical Tubes A, A, A, &c., and brass standards at every
angle of the pedestal. Below this frame lies the azimuth circle EE
consisting of a circular limb, strengthened by ten hollow cones firmly
united with the vertical axis, and consequently turning freely along
with it. The azimuth circle EE is divided and read off in the same
manner as the vertical circle. The arms of the microscopes BB project
from the ring _i_, and the microscopes themselves are adjustable by
screws, to bring them to zero and to the diameter of the circle. A
little above the ring _i_ is fixed an arm L which embraces and holds
fast the vertical axis with the aid of a champ screw. The arm L is
connected at the extremity with one of the arms A, by means of the
screw _a_, so that by turning this screw, a slow motion is communicated
to the vertical axis and the azimuth circle.

In order to place the instrument in a true vertical position, a
plumb-line, made of fine silver wire, is suspended from a small hook
at the top of the vertical tube _n_, connected by braces with one of
the large pillars. The plumb-line passes through an angle in which it
rests, and by means of a screw may be brought into the axis of the
tube. The plummet at the lower end of the line is immersed in a cistern
of water _t_, in order to check its oscillations, and is supported on
a shelf proceeding from one of the pillars. At the lower end of the
tube _n_ are fixed two microscopes _o_ and _p_, at right angles to
one another, and opposite to each is placed a small tube containing a
lucid point. The plumb-line is then brought into such a position by
the screws _d_, _b_, and by altering the suspension of the plumb-line
itself, that the image of the luminous point, like the disk of a
planet, is formed on the plumb-line, and accurately bisected by it. The
vertical axis is then turned round, and the plumb-line examined in some
other position. If it still bisects the luminous point, the instrument
is truly vertical; but if it does not, one half of the deviation must
be corrected by the screws _d_ _b_, and the other half by altering the
suspension of the line till the bisection of the circular image is
perfect in every position of the instrument.

It is not many years since Circular Repeating instruments came into
general use. The principle on which the construction of a repeating
circle is founded appears to have been first suggested by Professor
Mayer of Gottingen, in 1758; but the first person who applied this
principle to measure round the limb of a divided instrument, was Borda,
who about the year 1789, caused a repeating circle to be constructed
that would measure with equal facility horizontal and vertical angles.
Afterwards, Mr. Troughton greatly improved the construction of Borda’s
instrument by the introduction of several contrivances which ensure,
at the same time, its superior accuracy and convenience in use; and
his instruments have been introduced into numerous observatories.
Circular instruments, on a large scale, have been placed in the Royal
Observatory of Greenwich, and in most of the principal observatories
on the continent of Europe. Although it is agreed on all hands that
greater accuracy may be obtained by a repeating circle, than by any
other having the same radius, yet there are some objections to its use
which do not apply to the altitude and azimuth circle. The following
are the principal objections, as stated in Vol. I., of the ‘Memoirs of
the Astronomical Society of London.’ 1. The origin of the repeating
circle is due to _bad dividing_, which ought not to be tolerated in any
instrument in the present state of the art. 2. There are three sources
of fixed error which cannot be exterminated, as they depend more on the
materials than on the workmanship; first, the zero of the level changes
with variations of temperature; secondly, the resistance of the centre
work to the action of the tangent screws; and thirdly, the imperfection
of the screws in producing motion, and in securing permanent positions.
3. The instrument is applied with most advantage to slowly moving or
circumpolar stars; but in low altitudes these stars are seen near the
horizon, where refraction interferes. 4. Much time and labour are
expended, first in making the observations, and again in reducing them.
5. When any one step in a series of observations is bad, the whole time
and labour are absolutely lost. 6. When the instrument has a telescope
of small power, the observations are charged with errors of vision,
which the repeating circle will not cure. 7. This instrument cannot be
used as a transit instrument, nor for finding the exact meridian of a
place.

A great variety of directions is necessary in order to enable the
student of practical astronomy thoroughly to understand and to apply
this instrument to practice, which the limited nature of the present
work prevents us from detailing.--As this instrument consists of a
variety of complicated pieces of machinery, it is necessarily somewhat
expensive. A six inch brass astronomical circle for altitudes, zenith
or polar distances, azimuths, with achromatic telescope, &c., is marked
in Messrs. W. and S. Jones’ catalogue of astronomical instruments, at
£27 6s. A circle 12 inches diameter, from £36 15s. to £68 5s. An 18
inch ditto, of the best construction, £105. The larger astronomical
circles for public observatories, from 100 to a 1000 guineas and
upwards, according to their size, and the peculiarity of their
construction.


THE TRANSIT INSTRUMENT.

A Transit instrument is intended for observing celestial objects as
they pass across the meridian. It consists of a telescope fixed at
right angles to a horizontal axis--which axis must be so supported that
what is called _the line of collimation_, or the line of sight of the
telescope, may move in the plane of the meridian. This instrument was
first invented by Romer in the year 1689, but has since received great
improvements by Troughton, Jones and other modern artists. Transit
instruments may be divided into two classes, _Portable_, and _Fixed_.
The portable instrument, when placed truly in the meridian, and well
adjusted, may be advantageously used as a stationary instrument in an
observatory, if its dimensions be such as to admit of a telescope of
3-1/2 feet focal length; but when the main tube is only from 20 to
30 inches long, with a proportional aperture, it is more suited for
a travelling instrument to give the exact time; and, when carried on
board a ship in a voyage of discovery, may be taken on shore at any
convenient place, for determining the solar time of that place, and for
correcting the daily rate of the Chronometer giving the time at the
first meridian, so that the longitude of the place of observation may
be obtained from the difference of the observed and indicated times,
after the proper corrections have been made.

[Illustration: _figure 90._]

The following is a brief description of one of Mr. Troughton’s Portable
Transit Instruments. In fig. 90. PP is an achromatic telescope firmly
fixed, by the middle to a double conical and horizontal axis HH, the
pivots of which rest on angular bearings called Ys, at the top of
the standards B, B, rendered steady by oblique braces DD, fastened
to the central part of the circle, AA. In large fixed instruments,
the pivots and angular bearings are supported on two massive stone
pillars, sunk several feet into the ground, and are sometimes supported
by mason-work, to secure perfect stability. The axis HH has two
adjustments, one for making it exactly level, and the other for placing
the telescope in the meridian. A graduated circle L is fixed to the
extremity of the pivot which extends beyond one of the Ys, and the two
radii that carry the verniers _aa_, are fitted to the extremities of
the pivot in such a way as to turn round independent of the axis. The
double verniers have a small level attached to them, and a third arm
_b_, which is connected with the standard B by means of a screw _s_.
If the verniers are placed by means of the level, in a true horizontal
position, when the axis of the telescope is horizontal, and the arm _b_
screwed by the screw _s_ to the standard B, the verniers will always
read off the inclination of the telescope, and will enable the observer
to point it to any star, by means of its meridian altitude. The whole
instrument rests on three foot screws entered into the circle AA. In
the field of view of the telescope, there are several parallel vertical
wires, crossed at right angles with a horizontal one, and the telescope
is sometimes furnished with a diagonal eye-piece, for observing stars
near the zenith. A level likewise generally accompanies the instrument,
in order to place it horizontal, by being applied to the pivots of the
axis.

In order to fix the transit instrument exactly in the meridian, a good
clock regulated to sidereal time is necessary. This regulation may be
effected by taking equal altitudes of the sun or a star before and
after they pass the meridian, which may be done by small quadrants,
or by a good sextant. The axis H of the instrument is then to be
placed horizontal by a spirit level, which accompanies the transit,
and the greatest care must be taken that the axis of vision describes
in the heavens a great circle of the sphere. To ascertain whether the
telescope be in the plane of the meridian, observe by the clock when
a circumpolar star seen through the telescope transits both above and
below the pole; and if the times of describing the eastern and western
parts of its circuit be equal, the telescope is then in the plane of
the meridian; otherwise, certain adjustments must be made. When the
telescope is at length perfectly adjusted, a land-mark must be fixed
upon, at a considerable distance--the greater the better. This mark
must be in the horizontal direction of the intersection of the cross
wires, and in a place where it can be illuminated, if possible, in the
night time, by a lantern hanging near it; which mark being on a fixed
object, will serve at all times afterwards for examining the position
of the telescope.

Various observations and adjustments are requisite in order to fixing a
transit instrument exactly in the plane of the meridian. There is the
adjustment of the _level_--the horizontal adjustment of the axis of
the telescope--the placing of the parallel lines in the focus of the
eye-glass, so as to be truly vertical, and to determine the equatorial
value of their intervals--the collimation in _azimuth_, so that a
line passing from the middle vertical line to the optical centre of
the object-glass, is at right angles with the axis of the telescope’s
motion--the collimation in _altitude_, so that the horizontal line
should cross the parallel vertical lines, not only at right angles, but
also in the optical centre of the field of view--with various other
particulars; but of which our limited space will not permit us to enter
into details. Those who wish to enter into all the minute details in
reference to the construction and practical application of this and the
other instruments above described, as well as all the other instruments
used by the Practical Astronomer, will find ample satisfaction in
perusing the Rev. Dr. Pearson’s Introduction to Practical Astronomy,
4to., Vol. II.

A portable Transit instrument, with a cast-iron stand, the axis 12
inches in length, and the achromatic telescope about 20 inches, packed
in a case, sells at about 16 guineas: with a brass-framed stand and
other additions, at about 20 guineas. Transit instruments of larger
dimensions are higher in proportion to their size, &c.




CHAPTER III.


ON OBSERVATORIES.

In order to make observations, with convenience and effect, on the
heavenly bodies, it is expedient that an _observatory_, or place for
making the requisite observations, be erected in a proper situation.
The following are some of the leading features of a spot adapted for
making celestial observations: 1. It should command an extensive
visible horizon all around, particularly towards the south and the
north. 2. It should be a little elevated above surrounding objects.
3. It should be, if possible, at a considerable distance from
manufactories, and other objects which emit much smoke or vapour,
and even from chimney-tops where no sensible smoke is emitted, as
the heated air from the top of funnels causes undulations in the
atmosphere. 4. It should be at a distance from swampy ground or valleys
that are liable to be covered with fogs and exhalations. 5. It should
not, if possible, be too near public roads, particularly if paved with
stones, and frequented by heavy carriages, as in such situations,
undulations and tremulous motions may be produced, injurious to the
making of accurate observations with graduated instruments. 6. It is
expedient that the astronomical observer should have access to some
distant field within a mile of the observatory, on which a meridian
mark may be fixed, after his graduated instruments are properly
adjusted. The distance at which a meridian mark should be erected will
depend in part on the focal length of the telescope generally used
for making observations on the Right Ascensions and declinations of
the stars. It should be fixed at such a distance that the mark may
be distinctly seen without altering the focus of the telescope when
adjusted to the sun or stars, which, in most cases, will require to be
at least half a mile from the place of observation, and more if it can
be obtained.

Observatories may be distinguished into public and private. A _private_
observatory may be comprehended in a comparatively small building, or
in the wing of a building of ordinary dimensions for a family, provided
the situation is adapted to it. Most of our densely-peopled towns
and cities, which abound in narrow streets and lanes, are generally
unfit for good observatories, unless at an elevated position at their
extremities. Public observatories, where a great variety of instruments
is used, and where different observers are employed, require buildings
of larger dimensions, divided into a considerable number of apartments.
The observatory of _Greenwich_ is composed principally of two separate
buildings--one of which is the observatory properly so called, where
the assistant lives and makes all his observations; the other is the
dwelling-house in which the astronomer-royal resides. The former
consists of three rooms on the ground-floor, the middle of which is
the assistant’s sitting and calculating room, furnished with a small
library of such books only as are necessary for his computations, and
an accurate clock made by the celebrated Graham, which once served Dr.
Halley as a transit-clock. Immediately over this is the assistant’s
bed-room, with an alarum to awake him to make his observations at
the proper time. The room on the eastern side of this is called the
_transit_-room, in which is an 8 feet transit instrument, with an
axis of 3 feet, resting on 2 pieces of stone, made by Mr. Bird, but
successively improved by Messrs. Dollond, Troughton and others. Here
is also a chair to observe with, the back of which lets down to any
degree of elevation that convenience may require. On the western side
is the _quadrant room_, with a stone pier in the middle running north
and south, having on its eastern face a mural quadrant of 8 feet
radius, by which observations are made on the southern quarter of the
meridian, through an opening in the roof, of 3 feet wide, produced by
means of two sliding shutters. On the western face is another mural
quadrant of 8 feet radius, the frame of which is of iron, and the arch
of brass, which is occasionally applied to the north quarter of the
meridian. In the same room is the famous zenith sector, 12 feet long,
with which Dr. Bradley made the observations which led to the discovery
of the nutation of the earth’s axis and the aberration of the light
of the fixed stars. Here are also Dr. Hooke’s reflecting quadrant and
three time-keepers by Harrison. On the south side of this room a small
wooden building is erected for the purpose of observing the eclipses
of Jupiter’s satellites, occultations of stars by the moon, and other
phenomena which require merely the use of a telescope, and the true
or mean time. It is furnished with sliding shutters on the roof and
sides to view any part of the hemisphere from the Prime Vertical down
to the southern horizon. It contains a 40-inch achromatic, with a
triple object-glass; and also a 5 feet achromatic by Messrs. John and
Peter Dollond--a 2 feet reflecting telescope by Edwards, and a 6 feet
reflector by Herschel. Above the dwelling-house is a large octagonal
room, which is made the repository for certain old instruments, and for
those which are too large to be used in the other apartments. Among
many other instruments, it contains an excellent 10 feet achromatic by
Dollond, and a 6 feet reflector by Short. Upon a platform, in an open
space, is erected the great reflecting telescope constructed by Mr.
Ramage of Aberdeen, on the Herschelian principle, which has a speculum
of 15 inches diameter, and 25 feet focal length, remarkable for the
great accuracy and brilliancy with which it exhibits celestial objects.
Various other instruments of a large size, and of modern construction,
have of late years been introduced into this observatory, such as the
large and splendid transit instrument constructed by Troughton, in
1816--the two large _mural circles_ by Troughton and Jones--the transit
clock, by Mr. Hardy, and several other instruments and apparatus which
it would be too tedious to enumerate and describe.

Every observatory, whether public or private, should be furnished with
the following instruments. 1. A transit instrument for observing the
meridian passage of the sun, planets and stars. 2. A good clock whose
accuracy may be depended upon. 3. An achromatic telescope, at least
44 inches focal distance, with powers of from 45 to 180 for viewing
planetary and other phenomena--or, a good reflecting telescope at least
3 feet long, and the speculum 5 inches diameter. 4. An equatorial
instrument, for viewing the stars and planets in the day-time, and for
finding the Right Ascension and declination of a comet, or any other
celestial phenomenon. Where this instrument is possessed, and in cases
where no great degree of accuracy is required, the equatorial may be
made to serve the general purposes of a transit instrument.

A private observatory might be constructed in any house which has a
commanding view of the heavens, provided there is an apartment in it,
in which windows may be placed, or openings cut out fronting the north,
the south, the east and the west. The author of this work has a small
observatory erected on the top of his house, which commands a view of
20 miles towards the east, 30 miles towards the west, and north-west,
and about 20 miles towards the south, at an elevation of above 200 feet
above the level of the sea, and the banks of the Tay, which are about
half a mile distant. The apartment is 12-1/2 feet long by 8-1/2 wide,
and 8-1/2 feet between the floor and the roof. It has an opening on
the north by which observations can be made on the pole-star; a window
on the south by which the meridian-passages of the heavenly bodies may
be observed; another opening towards the east, and a fourth opening,
consisting of a door, towards the west. There is a pavement of lead
on the outside, all around the observatory-room, enclosed by a stone
parapet 3-1/2 feet high, the upper part of which is coped with broad
flat stones, in certain parts of which groves or indentations are made
for receiving the feet of the pedestal of an achromatic telescope,
which form a steady support for the telescope in the open air, when
the weather is calm and serene, and when observations are intended
to be made on any region of the heavens. By placing an instrument
on this parapet, it may be directed to any point of the celestial
canopy, except a small portion near the northern horizon, which is
partly intercepted by a small hill. In the following ground-plan, fig.
91. AAA, is the parapet surrounding the observatory-room; BBB, a walk
around it nearly 3 feet broad, covered with lead. O is the apartment
for the observatory, having an opening C to the north, another opening
D to the east, E is a window which fronts the south, and F is a door
fronting the west, by which an access is obtained to the open area on
the outside. GHI is an area on the outside towards the south, covered
with lead, 15 feet long from G to H, and 6-1/2 feet from E to I, from
which a commanding view of the southern, eastern and western portions
of the heavens may be obtained: _eeee_ are positions on the top of the
parapet where a telescope may be conveniently placed, when observations
are intended to be made in the open air. The top of this parapet is
elevated about 30 feet from the level of the ground. On the roof of
the observatory, about 12 feet above its floor, on the outside is a
platform of lead, surrounded by a railing, 6 feet by 5, with a seat,
on which observations either on celestial or terrestrial objects may
occasionally be made. K is a door or hatchway, which forms an entrance
into the observatory from the apartments below, which folds down, and
forms a portion of the floor.

[Illustration: _figure 91._]

In the perspective view of the building fronting the title-page, the
position and general aspect of the observatory-part of the building may
be more distinctly perceived.

In public observatories, where zenith or polar distances require to be
measured, it is necessary that there should be a dome, with an opening
across the roof and down the north and south walls. Should an altitude
or azimuth circle, or an equatorial instrument be used, they will
require a revolving roof with openings and doors on two opposite sides,
to enable an observer to follow a heavenly body across all the cardinal
points. The openings may be about 15 inches wide, and the roof needs
not be larger than what is requisite for giving room to the observer
and the instrument, lest its bulk and weight should impede its easy
motion. There have been various plans adopted for revolving domes.
Fig. 92 represents a section of the rotatory dome constructed at East
Sheen by the Rev. Dr. Pearson. This dome turns round on three detached
spheres of lignum vitæ, in a circular bed, formed partly by the dome,
and partly by the cylindrical frame-work, which surrounds the circular
room of 9 feet diameter. A section of this bed forms a square which the
sphere just fills, so as to have a small play to allow for shrinking;
and, when the dome is carried round, the spheres, having exactly
equal diameters of 4-1/4 inches each, when placed at equal distances
from one another, keep their relative places, and move together in a
beautifully smooth manner. These spheres act as friction rollers in
two directions at the four points of contact, in case any obstacle is
opposed to their progressive motion by the admission of dirt, or by
any change of figure of the wood that composes the rings of the dome,
and of the gang-way. No groove is here made, but what the weight of
the roof resting on the hard sphere occasions. The dome itself moves
twice round for the balls once, and has, in this way, its friction
diminished. The wood of this dome is covered by Wyatt’s patent copper,
one square foot of which weighs upwards of a pound; and the copper is
so turned over the nails that fix it at the parts of junction, that not
a single nail is seen in the whole dome. This covering is intended to
render the dome more permanent than if it had been made of wood alone.
At the observatory at Cambridge the dome is made chiefly of iron. In
the figure _a_, _a_ represents one of the two oblong doors that meet
at the apex of the cone, and a piece of sheet-copper bent over the
upper end of the door which shuts last, keeps the rain from entering at
the place of junction. The two halves of the dome are united by brass
rods passing through the door-cheeks of wainscot at _a_ and _a_ by
means of nuts that screw upon their ends, which union allows the dome
to be separated into two parts when there may be occasion to displace
it. The wooden plate _bb_, which appears in a straight line, is a
circular broad ring to which the covering wainscot boards are made fast
above the eaves, and _cc_ is a similar ring forming the wall-plate or
gang-way on which the dome rests and revolves.

[Illustration: _figure 92._]

[Illustration: _figure 92_*.]

Fig. 92* shows a small door that lies over the summit of the dome, and
may be separately opened for zenith observations; the rod of metal with
a ring at the lower end passing through it, serves to open and shut
this door, and at the same time carries upon its upper end a large ball
that falls back on the roof when the door is open, and keeps the door
in a situation to be acted upon by the hook of a handle that is used
for this purpose. The doors _aa_ being curved, are made to open in two
halves, the upper one being opened first, on account of its covering
the end of the other; and the observer may open one or two doors as may
best suit his purpose. The weight of this dome is such that a couple of
wedges, inserted by a gentle blow between the rings _bb_ and _cc_, will
keep it in its situation under the influence of the strongest wind.

It may not be improper to remark, that in all observatories, and in
every apartment where celestial observations are made, there should,
if possible, be a uniform temperature; and consequently a fire should
never be kept in such places, particularly when observations are
intended to be made, as it would cause currents of air through the
doors and other openings, which would be injurious to the accuracy of
observations. When a window is opened in an ordinary apartment where a
fire is kept, there is a current of heated air which rushes out at the
top, and a current of cold air which rushes in from below, producing
agitations and undulations, which prevent even a good telescope from
showing celestial objects distinct and well defined; and, I have no
doubt, that many young observers have been disappointed in their
views of celestial phenomena, from this circumstance, when viewing
the heavenly bodies from heated rooms in cold winter evenings; as the
aërial undulations before the telescope prevent distinct vision of such
objects as the belts of Jupiter, the spots of Mars, and the rings of
Saturn.




CHAPTER IV.


ON ORRERIES OR PLANETARIUMS.

An orrery is a machine for representing the order, the motions, the
phases, and other phenomena of the planets. Although orreries and
planetariums are not so much in use as they were half a century ago,
yet as they tend to assist the conceptions of the astronomical tyro in
regard to the motions, order, and positions of the bodies which compose
the solar system, it may not be inexpedient shortly to describe the
principles and construction of some of these machines.

The reason why the name _Orrery_ was at first given to such machines,
is said to have been owing to the following circumstance. Mr. Rowley,
a mathematical-instrument-maker, having got one from Mr. George
Graham, the original inventor, to be sent abroad with some of his own
instruments, he copied it and made the first for the Earl of Orrery.
Sir R. Steele, who knew nothing of Mr. Graham’s machine--thinking to
do justice to the first encourager, as well as to the inventor of such
a curious instrument, called it an _Orrery_, and gave Mr. Rowley the
praise due to Mr. Graham. The construction of such machines is not
a modern invention. The hollow sphere of Archimedes was a piece of
mechanism of this kind, having been intended to exhibit the motions
of the sun, the moon, and the five planets, according to the Ptolemaic
system. The next orrery of which we have any account was that of
Posidonius, who lived about 80 years before the Christian era, of which
Cicero says, ‘If any man should carry the sphere of Posidonius into
Scythia or Britain, in every revolution of which the motions of the
sun, moon and five planets, were the same as in the heavens, each day
and night, who in those barbarous countries could doubt of its being
finished--not to say actuated--by perfect reason?’ The next machine of
this kind, which history records, was constructed by the celebrated
Boethius, the Christian Philosopher, about the year of Christ 510--of
which it was said ‘that it was a machine pregnant with the universe--a
portable heaven--a compendium of all things.’ After this period, we
find no instances of such mechanism of any note till the 16th century,
when science began to revive, and the arts to flourish. About this time
the curious clock in Hampton Court Palace was constructed, which shows
not only the hours of the day, but the motions of the sun and moon
through all the signs of the zodiac, and other celestial phenomena.
Another piece of mechanism of a similar kind is the clock in the
cathedral of Strasburg, in which besides the clock part, is a celestial
globe or sphere with the motions of the sun, moon, planets and the
firmament of the fixed stars, which was finished in 1574.

Among the largest and most useful pieces of machinery of this kind, is
the great sphere erected by Dr. Long in Pembroke Hall in Cambridge.
This machine, which he called the _Uranium_, consists of a planetarium
which exhibits the motion of the earth and the primary planets, the
sun, and the motion of the moon round the earth, all enclosed within a
sphere. Upon the sphere, besides the principal circles of the celestial
globe, the Zodiac is placed, of a breadth sufficient to contain the
apparent path of the moon, with all the stars over which the moon
can pass, also the ecliptic, and the heliocentric orbits of all the
planets. The Earth in the planetarium has a moveable horizon, to which
a large moveable brass circle within the sphere may be set coincident,
representing the plane of the horizon continued to the starry heavens.
The horizons being turned round sink below the stars on the east side,
and make them appear to rise, and rise above the stars on the west
side, and make them appear to set. On the other hand, the earth and the
horizon being at rest, the sphere may be turned round to represent the
apparent diurnal motion of the heavens. In order to complete his idea
on a large scale, the Doctor erected a sphere of 18 feet diameter, in
which above 30 persons might sit conveniently, the entrance to which is
over the South Pole, by six steps. The frame of the sphere consists of
a number of iron meridians, the northern ends of which are screwed to
a large round plate of brass with a hole in the centre of it; through
this hole, from a beam in the ceiling, comes the north pole, a round
iron rod about three inches long, and which supports the upper part of
the sphere, to its proper elevation for the latitude of Cambridge, so
much of it as is invisible in England being cut off, and the lower or
southern ends of the meridians terminate on, and are screwed down to a
strong circle of oak 13 feet diameter, which, when the sphere is put in
motion, runs upon large rollers of lignum vitæ, in the manner that the
tops of some wind-mills turn round. Upon the iron meridians is fixed
a zodiac of tin painted blue, on which the ecliptic and heliocentric
orbits of the planets are drawn and the stars and constellations
traced. The whole is turned round with a small winch, with as little
labour as it takes to wind up a Jack, although the weight of the iron,
tin, and the wooden circle is above a thousand pounds. This machine,
though now somewhat neglected, may still be seen in Pembroke Hall,
Cambridge, where I had an opportunity of inspecting it in November,
1839. The essential parts of the machine still remain nearly in the
same state as when originally constructed in 1758.

The machine which I shall now describe is of a much smaller and less
complex description than that which has been noticed above, and may
be made for a comparatively small expense, while it exhibits, with
sufficient accuracy, the motions, phases, and positions of all the
primary planets, with the exception of the new planets, which cannot be
accurately represented on account of their orbits crossing each other.
In order to the construction of the Planetarium to which I allude, we
must compare the proportion which the annual revolutions of the primary
planets bear to that of the Earth. This proportion is expressed in the
following table, in which the first column is the time of the Earth’s
period in days; the second, that of the planets; and the third and
fourth are numbers very nearly in the same proportion to each other.

  365-1/4  :     88      ::    83  :   20 for Mercury.
  365-1/4  :    224-2/3  ::    52  :   32 for Venus.
  365-1/4  :    687      ::    40  :   75 for Mars.
  365-1/4  :   4332-1/2  ::     7  :   83 for Jupiter.
  365-1/4  :  10759-1/3  ::     5  :  148 for Saturn.
  365-1/4  :  30686      ::     3  :  253 for Uranus.

[Illustration: _figure 93._]

On account of the number of teeth required for the wheel which moves
Uranus, it is frequently omitted in Planetariums, or the planet is
placed upon the arbor which supports Saturn. If we now suppose a
spindle or arbor with six wheels _fixed_ upon it in an horizontal
position, having the number of teeth in each corresponding to the
numbers in the third column, namely the wheel AM (fig. 93.) of 83
teeth, BL of 52, CK of 50, for the earth, DI of 40, EH of 7, and FG
of 5; and another set of wheels moving freely about an arbor having
the number of teeth in the fourth column, namely AN of 20, BO of 32,
CP of 50--for the earth; DQ of 75, ER of 83, and FS of 148. Then, if
these two arbors of fixed and moveable wheels be made of the size,
and fixed at the distance here represented, the teeth of the former
will take hold of those of the latter, and turn them freely when the
machine is in motion. These arbors, with their wheels, are to be placed
in a box of a proper size, in a perpendicular position; the arbor of
fixed wheels to move in pivots at the top and bottom of the box, and
the arbor of the moveable wheels to go through the top of the box,
and having on the top a wire fixed, and bent at a proper distance
into a right angle upwards, bearing on the top a small round ball,
representing its proper planet. If then, on the lower part of the arbor
of fixed wheels, be placed a pinion of screw-teeth, a winch turning a
spindle with an endless screw, playing in the teeth of the arbor, will
turn it with all its wheels, and these wheels will turn the others
about with their planets, in their proper and respective periods of
time. For, while the fixed wheel CK moves its equal CP once round, the
wheel AM will move AN a little more than four times round, and will
consequently exhibit the motion of Mercury; the wheel EH will turn the
wheel ER about 1/12 round, representing the proportional motion of
Jupiter; and the wheel FG will turn the wheel FS, about 1/29.5 round,
and represent the motion of Saturn, and so of all the rest.

[Illustration: _figure 94._]

The following figure (fig. 94.) represents the appearance of the
instrument when completed. Upon the upper part of the circular box
is pasted a Zodiac circle divided into 12 signs, and each sign into
30 degrees, with the corresponding days of the month. The wheel-work
is understood to be within the box, which may either be supported by
a tripod, or with four feet, as here represented. The moon, and the
satellites of Jupiter, Saturn and Uranus, are moveable only by the
hand. When the winch W is turned, then all the primary planets are
made to move in their respective velocities. The ball in the centre
represents the Sun, which is either made of brass or of wood gilded
with gold.

By this Planetarium, simple as its construction may appear, a variety
of interesting exhibitions may be made and problems performed, which
may be conducive to the instruction of young students of astronomy. I
shall mention only a few of those as specimens.

1. When the planets are placed in their respective positions by means
of an Ephemeris or the Nautical Almanack, the relative positions of
those bodies in respect to each other, the quarters of the heavens
where they may be observed, and whether they are to be seen in the
morning before sun-rise or in the evening after sun-set, may be at
once determined. For example, on the 19th of December, 1844, the
_heliocentric_ places of the planets are as follows:--Uranus 2° Aries;
Saturn 8° 27´ of Aquarius; Jupiter 7° 4´ Aries; Mars 12° 45´ Libra;
the Earth 27° 46´ Gemini; Venus 29° 48´ Virgo; Mercury 7° 53´ Pisces.
When the planets are placed on the planetarium in these positions, and
the eye placed in a line with the balls representing the Earth and the
Sun, all those situated to the left of the sun are to the east of
him, and are to be seen in the evening, and those on the right, in the
morning. In the present case, Uranus, Saturn, Jupiter, and Mercury are
evening stars, and Mars and Venus can only be seen in the morning.
Jupiter is in an aspect nearly _quartile_, or 3 signs distant from the
sun, and Uranus is nearly in the same aspect. Saturn is much nearer the
sun, and Mercury is not far from the period of its greatest _eastern_
elongation. Mars is not far from being in a quartile aspect, _west_ of
the sun, and Venus is near the same point of the heavens, approaching
to the period of its greatest _western_ elongation, and consequently
will be seen before sun-rise as a beautiful morning star. Jupiter and
Uranus, to the east of the sun, appear nearly directly opposite to
Venus and Mars, which are to the west of the sun. The phase[47] of
Venus is nearly that of a half-moon, and Mercury is somewhat gibbous,
approaching to a half-moon phase. If, now, we turn the machine by the
winch till the Index of the earth point at the 8th of August, 1845, we
shall find the planets in the following positions:--Mars and Saturn are
nearly in opposition to the sun; Venus and Mercury are evening stars at
no great distance from each other, and Jupiter is a morning star. In
like manner if we turn the machine till the Index point to any future
months, or even succeeding years, the various aspects and positions of
the planets may be plainly perceived. When the planets are moved by the
winch, in this machine, we see them all _at once_ in motion around the
sun, with the same respective velocities and periods of revolution
which they have in the heavens. As the planets are represented in the
preceding positions, Mercury, Jupiter and Mars, are evening stars,
and Venus, Saturn, and Uranus, morning stars, if we suppose the earth
placed in a line with our eye and the sun.

2. By this instrument, the truth of the Copernican or Solar system is
clearly represented. When the planets are in motion, we perceive the
planets Venus and Mercury to pass both before and behind the sun, and
to have two conjunctions. We observe Mercury to be never more than
a certain angular distance from the sun, as viewed from the earth,
namely 27°; and Venus 47°. We perceive that the superior planets,
particularly Mars, will be sometimes much nearer to the earth than at
others, and therefore must appear larger at one time than at another,
as they actually appear in the heavens. We see that the planets cannot
appear from the earth to move with uniform velocity; for when nearest
they appear to move faster, and slower when most remote. We likewise
observe that the planets appear from the earth to move sometimes
_direct_, or from west to east, then become retrograde, or from east
to west, and between both to be _stationary_. All which particulars
exactly correspond with celestial observations. For illustrating these
particulars there is a simple apparatus represented by fig. 95, which
consists of a hollow wire with a slit at top which is placed over the
arm of Mercury or Venus at E. The arm DG represents a ray of light
coming from the planet at D to the earth at F. The planets being then
in motion, the planet D, as seen in the heavens from the earth at F,
will undergo the several changes of position, which we have described
above, sometimes appearing to go backwards and at other times forwards.
The wire prop, now supposed to be placed over Mercury at E, may
likewise be placed over any of the other planets, particularly Mars,
and similar phenomena will be exhibited.

[Illustration: _figure 95._]

This machine may likewise be used to exhibit the falsity of the
Ptolemaic system, which places the Earth in the centre, and supposes
the sun and all the planets to revolve around it. For this purpose, the
ball representing the Sun is removed, and placed on the wire or pillar
which supports the Earth, and the ball representing the Earth is placed
in the centre. It will then be observed, that the planets Mercury and
Venus, being both within the orbit of the sun, cannot at any time be
seen to go behind it, whereas, in the heavens we as often see them go
behind as before the sun. Again, it shows that as the planets move in
circular orbits about the central earth, they ought at all times to
appear of the same magnitude; while, on the contrary, we observe their
apparent magnitudes in the heavens to be very variable; Mars, for
example, appearing sometimes nearly as large as Jupiter, and at other
times only like a small fixed star. Again, it is here shown that the
planets may be seen at all distances from the sun; for example, when
the sun is setting, Mercury and Venus, according to this arrangement,
might be seen, not only in the south but even in the eastern quarter
of the heavens--a phenomenon which was never yet observed in any age;
Mercury never appearing beyond 27° of the Sun, nor Venus beyond 48°.
In short, according to the system thus represented, it is seen, that
the motions of the planets should all be regular, and uniformly the
same in every part of their orbits, and that they should all move the
same way, namely from west to east; whereas, in the heavens, they are
seen to move with variable velocities, sometimes appearing stationary,
and sometimes moving from east to west, and from west to east. All
which circumstances plainly prove that the Ptolemaic cannot be the true
system of the universe.

    A Planetarium, such as that now described, might be constructed
    with brass wheel-work, for about 5 guineas. The brass
    wheel-work of one which I long since constructed cost about 3
    guineas, and the other parts of the apparatus about 2 guineas
    more. The following are the prices of some instruments of this
    kind as made by Messrs. Jones, 30, Lower Holborn, London. ‘An
    Orrery, showing the motions of the Earth, Moon, and inferior
    planets, Mercury and Venus, by wheel-work, the board on which
    the instrument moves being 13 inches diameter, £4: 14s. 6d.’ ‘A
    Planetarium showing the motions of all the primary planets by
    wheel-work with 1-1/2 inch or 3 inch papered globes,--according
    to the wheel-work and the neatness of the stands, from £7:
    17s. 6d. to £10: 10s.’ ‘Ditto, with wheel-work to show the
    parallelism of the Earth’s axis, the motions of the Moon, her
    phases, &c., £18: 18s.’ ‘Ditto, with wheel-work, to show the
    earth’s diurnal motion, on a brass stand in mahogany case, £22:
    1s.’ ‘A small _Tellurian_, showing the motion of the Earth and
    Moon, &c., £1: 8s.’


HENDERSON’S PLANETARIUM.

The following is a description of the most complete and accurate
planetarium I have yet seen. The calculations occupied more than eight
months. For this article I am indebted to my learned and ingenious
friend Dr. Henderson, F.R.A.S., who is known to many of my readers by
his excellent astronomical writings.

[Illustration: _figure 96._]

Section of the wheel-work of a Planetarium for shewing with the utmost
degree of accuracy the mean tropical revolutions of the planets round
the sun, calculated by E. Henderson, LL.D. &c.

In the above section the dark horizontal lines represent the wheel-work
of the Planetarium, and the annexed numerals, the numbers of teeth in
the given wheel. The machine has three axes or arbors, indicated by
the letters A, B, C.--Axis ‘C,’ the ‘Yearly axis,’ is assumed to make
one revolution in 365.242,236 days, or, in 365 days 5^h 48^m 49.19^s
and is furnished with wheels 17, 44, 54, 36, 140, 96, 127, 86, which
wheels are all firmly riveted to said axis, and consequently they turn
round with it in the same time. Axle ‘B’ is a fixture; it consists
of a steel rod, on which a system of pairs of wheels revolve; thus
wheels 40 and 77 are made fast together by being riveted on the same
collet represented by the thick dark space between them, as also of
the rest: the several wheels on this axis may be written down thus;
40/77, 49/129, 20/94, 79/81, 30, 27/50, 41/65, 59/65, 96, 77/47, 67/42.
On axis A a system of wheels, furnished with tubes revolve, and these
tubes carry horizontal arms, supporting perpendicular stems with the
planets. The wheels on this axis are 173, 117/190, 111, 119, 122/130,
123/127, 83, 239, 96, 128, 72. From the following short description
the nature of their several actions will, it is presumed, be readily
understood--viz.,

[Sidenote: MERCURY’S PERIOD.]

On the axis ‘C’ at the bottom is wheel 86, which turns round in 365
days 5^h 48^m 49.19^s, this wheel impels a small wheel of 22 teeth, to
which is made fast to wheel 67, both revolving together at the foot
of axis B; wheel 67 drives a wheel of 72 once round in the period of
87 days, 23^h 14^m 36.1^s: this last mentioned wheel has a long tube,
which turns on the steel axis A, and carries a horizontal arm with the
planet Mercury round the sun in the time above noted.

[Sidenote: VENUS’S PERIOD.]

On axis ‘C’ is wheel 127, which drives wheel 47, to which is riveted a
wheel of 77 teeth, which impels a wheel of 128 teeth on axis A, and
causes it to make a revolution in 224 days, 16^h 41^m 31.1^s, and is
furnished with a tube, which revolves over that of Mercury and ascends
through the cover of the machine, and bears an arm on which is placed a
small ball representing this planet in the time stated.

[Sidenote: THE EARTH’S PERIOD.]

The motion of the earth round the sun is simply effected as
follows--the assumed value of axis ‘C;’ the ‘Yearly axis’ is 365 days
5^h 48^m 49.19^s; hence a system of wheels having the same numbers of
teeth, or at all events, the first mover, and last wheel impelled must
be equal in their numbers of teeth; in this machine three wheels are
employed, thus; a wheel having 96 teeth is made fast to the Yearly
axis C and of course moves round with it in a mean solar year, as
above noted, this wheel impels another wheel of 96 teeth, on axis B,
and this in its turns drives a third wheel of 96 teeth on axis A, and
is furnished with a long tube which revolves over that of Venus, and
ascends above the cover-plate of the machine, and bears a horizontal
arm which supports a small terrestrial globe, which revolves by virtue
of said wheels once round the sun in 365 days 5^h 48^m 49.19^s.

[Sidenote: MARS’ PERIOD.]

The revolution of this planet is effected as follows--a wheel of 140
teeth is made fast to the yearly axis C, and drives on axis B a wheel
of 65 teeth, to which is fixed a wheel of 59 teeth, which impels a
large wheel of 239 teeth on axis A once round the sun in 686 days
22^h 18^m 33.6^s, this last-mentioned wheel is also furnished with a
tube which revolves over that of the earth, and carries a horizontal
arm bearing the ball representing Mars, and causes it to complete a
revolution round the sun in the period named.

[Sidenote: _THE ASTEROIDS._ VESTA’S PERIOD.]

The period of Vesta is accomplished thus, viz. On the Yearly axis C,
is made fast a wheel of 36 teeth, which drives a wheel of 65 teeth on
axis B, to which is fixed a wheel of 41 teeth, which impels a wheel of
83 teeth on axis A, once round in 1336 days 0^h 21^m 19.8^s: The tube
of which last wheel ascends on that of Mars, and like the rest bears an
arm supporting a ball representing this planet.

[Sidenote: JUNO’S PERIOD.]

For the revolution of Juno, the yearly axis C is furnished with a wheel
of 54 teeth, which impels a wheel of 50 teeth on axis B, to which is
made fast a wheel of 27 teeth which turns a wheel of 127 teeth on axis
A, once round in 1590 days 17^h 35^m 2.7^s, and the tube of which
ascends on that of Vesta, and supports a horizontal arm which carries a
small ball representing this planet in the period named.

[Sidenote: CERES’ PERIOD.]

The revolution of Ceres is derived from the period of Juno, because
wheel-work taken from the unit of a solar year was not sufficiently
accurate for the purpose, therefore on Juno’s wheel of 127 teeth is
fixed a wheel of 123 teeth, which drives a thick little bevel sort
of wheel of 30 teeth on axis B: the reason of this small wheel being
bevelled is to allow its teeth to suit both wheels 123/130; wheel 30
drives wheel 130, on axis A once round in 1681 days, 6^h 17^m 22.4^s
and the tube of wheel 130 turns on the tube of Juno, and ascends in a
similar manner with the rest and carries an horizontal arm supporting a
small ball representing this planet, and is caused to revolve round the
Sun in the above mentioned period (the period of Ceres to that of Juno
is as 130 is to 123; hence the wheels used.)

[Sidenote: PALLAS’S PERIOD.]

The Period of Pallas could not be derived from the solar year with
sufficient accuracy, and recourse was had to an engrafted fraction on
the period of Ceres, thus. On wheel 130 of Ceres is made fast a wheel
of 122 teeth, which drives a wheel of 81 teeth on axis B, to which is
fixed a wheel 79 which impels a wheel of 119 teeth on axis A, and is
furnished with a tube which ascends, and turns on that of Ceres, and
supports a horizontal arm, which bears a small ball representing this
planet, which by virtue of the above train of wheels is caused to
complete a revolution round the Sun in 1681^d 10^h 28^m 25.1^s.

[Sidenote: JUPITER’S PERIOD.]

The motion of this planet is derived from the period of a solar year;
from the ‘yearly axis’ thus, on this axis is made fast a wheel of 44
teeth which turns a wheel of 94 teeth on axis B, to which is riveted
a small wheel of 20 teeth, which impels a wheel on axis A having 111
teeth, which is furnished with an ascending tube which revolves over
that of Pallas, and bears an horizontal arm which supports a ball
representing this planet, which by the said train of wheels is caused
to revolve round the Sun in 4330^d 14^h 39^m 35.7^s.

[Sidenote: SATURN’S PERIOD.]

The periodic revolution of Saturn is also taken from the solar
year--viz., a small wheel of 17 teeth is fixed to the ‘yearly axis’
near its top, and drives a wheel of 129 teeth on axis B, to which is
made fast a wheel of 49 teeth, which turns a wheel of 190 teeth on
axis A, whose tube ascends and revolves on that of Jupiter’s tube, and
supports an arm, having a ball representing Saturn and its rings, and
which by the train of wheels is caused to perform a revolution round
the sun in the period of 10746^d 19^h 16^m 50.9^s.

URANUS’S PERIOD.

The revolution of this planet could not be attained with sufficient
accuracy from the period of a solar year--the period is engrafted on
that of Saturn’s, thus, a wheel of 117 teeth is made fast to wheel 190
of Saturn, and consequently revolves in Saturn’s period. This wheel
of 117 teeth drives a wheel on axis B, having 77 teeth, to which is
fixed a wheel of 40 teeth, which turns on axis A, a large wheel of 173
teeth, whose tube ascends and revolves over that of Saturn, and carries
a horizontal arm which supports a ball representing this planet, which
is caused to complete its revolution by such a train of wheels in the
period of 30589^d 8^h 26^m 58.4^s. Such is a brief description of the
motions of this comprehensive and very accurate machine.

The axis A, on which the planetary tubular wheels revolve, performs
a rotation in 25 days 10 hours, by virtue of the following train of
wheels, 61/14 + 70/12 of 24 hours, that is, a pinion of 14 is assumed
to revolve in 24 hours, and to drive a wheel of 61 teeth, to which is
fixed a pinion of 12, which turns the wheel 70 in the period noted; to
this wheel-axis, it is made fast, and by revolving with it, exhibits
the Sun’s rotation.

[Sidenote: DIURNAL HAND.]

The machine is turned by a handle or winch, which is assumed to turn
round in 24 hours, and from this rotation of 24 hours a train of
wheel-work is required to cause the ‘yearly axis’ C, to turn once
round in 365^d 5^h 48^m 49.19^s, which is effected in the following
manner--viz, the train found by the process of the reduction of
continuous fractions is 61/14 + 144/18 + 211/23 that is, in the train
for turning the sun, the same pinion 14 turns the same wheel 61,
and turns a pinion of 18 leaves, to which is fixed a wheel of 144
teeth, having a pinion of 23 leaves, which impels a large wheel of
241 teeth once round in 365.242236^d or 365^d 5^h 48^m 49.19^s, this
last-mentioned wheel of 241 teeth is made fast to the under part of
the ‘yearly axis’ C at D, the handle having a pinion of 14 leaves
therefore, and transmitting its motion through the above train, causes
the yearly axis to revolve in the same period.

[Sidenote: REGISTRATING DATES.]

The planetarium is also furnished with a system of wheels for
registrating dates for either 10,000 years past or to come, the
arrangement is not shewn in the engraving (to prevent confusion) but
it might be shortly described thus:--Near the top of the yearly axis
is a hooked piece _e_, which causes the tooth of a wheel of 100 teeth
to start forward yearly, consequently 100 starts of said wheel will
cause it to revolve in 100 solar years, and it has a hand which points
on a dial on the cover of the machine the years; thus for the present
year this hand will be over the number 45. This last-named wheel of
100 teeth has a pin which causes a tooth of another wheel of 100 teeth
to start once in 100 years, hence this last wheel will complete one
revolution in 10,000 years, and it is for this purpose the former index
or hand moves over a number yearly. The second index will pass over a
number every 100 years--for the present year the second hand or index
will be over the number 18, and will continue over it until the first
index moves forward to 99, then both indexes will move at one time,
viz., the first index to 00 on the first concentric circle of the
dial, and the second index to 19, denoting the year 1900, and so of
the rest. By the ecliptic being divided in a series of four spirals,
the machine makes a distinction between common and leap years, and
indicates the common year as containing 365 days, and the leap-year 366
days, by taking in a day in February every fourth year; thus for any
given period for 10,000 years past or to come, the various situations
and aspects of the planets may be ascertained by operating with this
machine, and this for thousands of years without producing a sensible
error either in space or time. This planetarium wheel-work is enclosed
in an elegant mahogany box of twelve sides--is about 5 feet in diameter
by 10 inches in depth; at each of the twelve angles, or sides, small
brass pillars rise and support a large Ecliptic circle on which are
engraven the signs, degrees and minutes of the Ecliptic--the days of
the month, &c. This mahogany box with the wheel-work is supported by a
tripod stand three feet in height, and motion is communicated to the
several balls representing the planets by turning the handle as before
described. A Planetarium of this complicated sort, costs sixty guineas.

The following is a tabular view of the wheel-work, periods, &c.

  ----------+-----------------------------+---------------------+------------------
  Planets’  |        Wheel-work.          | Tropical periods    |True mean Tropical
  Names.    |                             | produced by the     | Periods of
            |                             | wheel-work.         | the Planets.
  ----------+-----------------------------+---------------------+------------------
            |                             |    da. ho.  m.  s.  |   da. ho. m.  s.
            |                             |                     |
  Mercury   |     22/85 + 67/72 of a Year |    87. 23. 14. 36.1 |   87. 23. 14. 36
            |                             |                     |
  Venus     |          47/127 + 128/77 "  |   224. 16. 41. 31.1 |  224. 16. 41. 36
            |                             |                     |
  The Earth |  Prime mover 96 + 96 + 96 " |   365.  5. 48. 49.19|  365.  5. 48. 49
            |                             |                     |
  Mars      |           65/140 + 239/59 " |   686. 22. 18. 33.6 |  686. 22. 18. 34
            |                             |                     |
  Vesta     |            65/36 + 83/41 "  |  1335.  0. 21. 19.8 | 1335.  0. 21. 20
            |                             |                     |
  Juno      |           50/54 + 127/27 "  |  1590. 17. 35.  2.7 | 1590. 17. 35.  1
            |                             |                     |
  Ceres     |       130/123 + 30 of Juno  |  1681.  6. 17. 22.4 | 1681.  6. 17. 29
            |                             |                     |
  Pallas    |    81/122 + 119/79 of Ceres |  1681. 10. 28. 25.1 | 1681. 10. 28. 42
            |                             |                     |
  Jupiter   |    94/44 + 111/20 of a Year |  4330. 14. 39. 35.7 | 4330. 14. 39. 32
            |                             |                     |
  Saturn    |   129/17 + 190/49 "         | 10746. 19. 16. 50.9 |10746. 19. 16. 52
            |                             |                     |
  Uranus    |   77/117 + 173/40 of Saturn | 30589.  8. 26. 58.4 |30589.  8. 26. 59
  ----------+-----------------------------+---------------------+------------------
  The Sun’s      61/14 + 70/12 of 24 ho.  |    25. 10.  0.  0   |   25. 10.  0.  1
  Rotation                                |                     |
  The tropical}  61/14 + 144/18 + 241/23 "|   365.  5. 48. 49.19|  365.  5. 48. 49
   period of  }                           |                     |
  the Earth   }                           |                     |
  roundthe    }                           |                     |
  Sun.        }                           |                     |
  ----------------------------------------+---------------------+------------------

In the month of October last year, Dr. Henderson made a series of
calculations for a new Planetarium for the use of schools. It shows
with considerable accuracy for 700 days, the mean tropical revolutions
of the Planets round the sun--the machine consists of a system of brass
wheels peculiarly arranged, and is enclosed in a circular case three
feet in diameter, the top of which has the signs and degrees of the
ecliptic laid down on it, as also the days of the months, &c. This
Planetarium costs only 45s. or on a tripod stand, table-high, 55s.; the
machine is put in motion by a handle on the outside. To the teachers
and others connected with education this Planetarium must be of great
importance, for without a proper elucidation of the principles of
astronomy, that of Geography must be but confusedly understood. This
Planetarium is at present made by Mr. Dollond, 9, White Conduit Grove,
Islington, London.

The _Tellurian_ is a small instrument which should be used in
connection with the Planetarium formerly described. This instrument is
intended to show the annual motion of the earth, and the revolution
of the moon around it. It also illustrates the moon’s phases, and the
motion of her nodes, the inclination of the Earth’s axis, the causes
of eclipses, the variety of seams, and other phenomena. It consists of
about eight wheels, pinions and circles. A small instrument of this
description may be purchased for about one pound eight shillings, as
stated in the note, page 527.


ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SATURN’S RING.

[Illustration: _figure 97._]

The striking and singular phenomenon connected with the planet
Saturn--though now ascertained beyond dispute to be a Ring, or Rings,
surrounding its body at a certain distance--was a subject of great
mystery, and gave rise to numerous conjectures and controversies, for
a considerable time after the invention of the telescope by which it
was discovered. Though it was first discovered in the year 1610, it
was nearly 50 years afterwards, before its true form and nature were
determined. Galileo was the first who discovered anything uncommon
connected with Saturn: through his telescope he thought he saw that
planet appear like two smaller globes on each side of a larger one; and
after viewing the planet in this form for two years, he was surprised
to see it becoming quite round, without its adjoining globes, and
some time afterwards to appear in the triple form. This appearance
is represented in fig. 1 of the above engraving. In the year 1614,
Scheiner, a German astronomer, published a representation of Saturn,
in which this planet is exhibited as a large central globe, with two
smaller bodies, one on each side, partly of a conical form, attached to
the planet and forming a part of it, as shown fig. 2. In the year 1640
and 1643, Ricciolus, an Italian mathematician and astronomer, imagined
he saw Saturn as represented in fig. 3. consisting of a central
globe, and two conical shaped bodies completely detached from it, and
published an account of it corresponding to this view. Hevelius, the
celebrated astronomer of Dantzig, author of the _Selenographia_ and
other works, made many observations on this planet about the years
1643, 1649 and 1650, in which he appears to have obtained different
views of the planet and its appendages, gradually approximating to the
truth, but still incorrect. These views are represented in figures
4, 5, 6, and 7. Fig. 4 nearly resembles two hemispheres, one on each
side of the globe of Saturn. The other figures very nearly resemble
the extreme parts of the ring as seen through a good telescope, but
he still seems to have considered them as detached from each other as
well as from Saturn. Figures 8 and 9 are views given by Ricciolus at a
period posterior to that in which he supposed Saturn and his appendages
in the form delineated in fig. 3. In these last delineations the
planet was supposed to be enclosed in an elliptical ring, but this ring
was supposed to be _fixed_ to its two opposite sides.

Fig. 10, is a representation by Eustachius Divini, a celebrated
Italian optician at Bologna. The shades represented on Saturn and the
elliptical curve are incorrect, as this planet presents no such shadowy
form. The general appearance here presented is not much unlike that
which the ring of Saturn exhibits, excepting that at the upper side the
ring should appear covering a portion of the orb of Saturn. But Divini
seems to have conceived that the curve on each side was attached to the
body of Saturn. For when Huygens published his discovery of the ring
of Saturn in 1659, Divini contested its truth, because he could not
perceive the ring through his own telescopes; and he wrote a treatise
on the subject in opposition to Huygens, in 1660, entitled ‘Brevis
Annotatio in Systema Saturninum.’ Huygens immediately replied to him,
and Divini wrote a rejoinder in 1661.--Fig. 11 is the representation
given by Francis Fontana, a Neapolitan astronomer. This figure
represents Saturn as having two crescents, one on each side, attached
to its body, with intervals between the planet and the crescents. Fig.
12 is a view delineated by Gassendus, a celebrated French philosopher.
It represents the planet as a large ellipsoid, having a large circular
opening near each end, and, if this representation were the true one,
each opening would be at least 30,000 miles in diameter. Fig. 13,
which is perhaps the most singular of the whole, is said to be one
of the view’s of this planet given by Ricciolus. It represents two
globes--each of which, in the proportion they here bear to Saturn, must
be more than thirty thousand miles in diameter. These globes, were
conceived as being attached to the body of Saturn by curves or bands,
each of which, in the proportion represented, must have been at least
7000 miles in breadth, and nearly 40,000 miles long. This would have
exhibited the planet Saturn as a still more singular body than what
we have found it to be; but no such construction of a planet has yet
been found in the universe, nor is it probable that such a form of a
planetary body exists.

It is remarkable that only two general opinions should have been
formed respecting the construction of Saturn--as appears from these
representations--either that this planet was composed of three distinct
parts, separate from each other,--or that the appendage on each side
was _fixed_ to the body of the planet. The idea of a ring surrounding
the body of the planet, at a certain distance from every part of it,
seems never to have been thought of till the celebrated Huygens, in
1655, 1656 and 1657, by numerous observations made on this planet,
completely demonstrated that it is surrounded by a solid and permanent
ring, which never changes its situation, and, without touching the
body of the planet, accompanies it in its revolution around the sun.
As the cause of all the erroneous opinions above stated was owing to
the imperfection of the telescopes which were then in use, and their
deficiency in magnifying power,--this ingenious astronomer set himself
to work in order to improve telescopes for celestial observations. He
improved the art of grinding and polishing object-glasses, which he
finished with his own hands, and produced lenses of a more correct
figure, and of a longer focal distance than what had previously been
accomplished. He first constructed a telescope 12 feet long, and
afterwards one 23 feet long, which magnified about 95 times; whereas
Galileo’s best telescope magnified only about 33 times. He afterwards
constructed one 123 feet long, which magnified about 220 times. It was
used without a tube, the object-glass being placed upon the top of a
pole and connected by a cord with the eye-piece. With such telescopes
this ingenious artist and mathematician discovered the fourth satellite
of Saturn, and demonstrated that the phenomenon, which had been so
egregiously misrepresented by preceding astronomers, consisted of an
immense ring surrounding the body, and completely detached from it. His
numerous observations and reasonings on this subject were published
in Latin, in 1659, in a quarto volume of nearly 100 pages, entitled
‘_Systema Saturnium, sive de causis mirandorum Saturni Phenomenôn, et
Comite ejus Planeta Nova_,’ from which work the figures and some of the
facts stated above have been extracted.


ON THE SUPPOSED DIVISIONS OF THE EXTERIOR RING OF SATURN.

From the period in which Huygens lived till the time when Herschel
applied his large telescopes to the heavens, few discoveries were made
in relation to Saturn. Cassini, in 1671, discovered the fifth satellite
of this planet; in 1672, the third; and the first and second in March,
1684. In 1675, Cassini saw the broad side of its ring bisected quite
round by a dark elliptical line, of which the inner part appeared
brighter than the outer. In 1722, Mr. Hadley, with his 5 feet Newtonian
Reflector observed the same phenomenon, and perceived that the dark
line was stronger next the body, and fainter towards the upper edge
of the ring. Within the ring he also discovered two belts across the
disk of Saturn. But it does not appear that they had any idea that this
dark line was empty space separating the ring into two parts. This
discovery was reserved for the late Sir W. Herschel, who made numerous
observations on this planet, and likewise ascertained that the ring
performs a revolution round the planet in ten hours and thirty minutes.

Of late years, some observers have supposed that the exterior ring
of Saturn is divided into several parts, or, in other words, that it
consists of two or more concentric rings. The following are some of
the observations on which this opinion is founded. They are chiefly
extracted from Captain Kater’s Paper on this subject, which was read
before the Astronomical Society of London.

The observations, we are told, were made in the years 1825 and 1826,
and remained unpublished, from a wish on the part of the observer
to witness the appearances again. The planet Saturn has been much
observed by Captain Kater, for the purpose of trying the light, &c.,
for which the ring and satellites are good tests. The instruments
which were employed in the present investigations were two Newtonian
Reflectors--one by Watson, of 40 inches focus and 6-1/4 aperture; and
another by Dollond, of 68 inches focus, and 6-3/4 aperture. The first,
under favourable circumstances, gave a most excellent image, the latter
is a very good instrument. The following are extracts from the author’s
journal.

_Nov. 25, 1825._--The double ring beautifully defined, perfectly
distinct all around, and the principal belts well seen. I tried many
concave glasses, and found that the image was much sharper than with
convex eye-glasses, and the light apparently much greater. Dollond,
259, the best power, 480, a single lens, very distinct. _Nov. 30_, the
night very favourable, but not equal to the 25th. The exterior ring
of Saturn is not so bright as the interior, and the interior is less
bright close to the edge next the planet. The inner edge appears more
yellow than the rest of the ring, and nearer in colour to the body
of the planet. _Dec. 17_.--The evening extremely fine. With Dollond,
I perceived the outer ring of Saturn to be darker than the inner,
and the division of the ring all around with perfect distinctness;
but with Watson I fancied that I saw _the outer ring separated by
numerous dark divisions extremely close, one stronger than the rest,
dividing the ring about equally_. This was seen with my most perfect
single eye-glass power. A careful examination of some hours confirmed
this opinion.--_Jan. 16_ and 17, 1826.--Captain Kater believed that
he saw the divisions with the Dollond, but was not positive. Concave
eye-glasses found to be superior to convex. _Feb. 26, 1826_.--The
division of the outer ring not seen with Dollond. On the 17th Dec.,
when the divisions were most distinctly seen, Captain Kater made a
drawing of the appearance of Saturn and his rings. The phenomena
were witnessed by two other persons on the same evening, one of whom
saw several divisions in the outer ring, while the other saw one
middle division only; but the latter person was short-sighted, and
unaccustomed to telescopic observations. It may be remarked, however,
that these divisions were not seen on other evenings, which yet were
considered very favourable for distinct vision.

It is said that the same appearances were seen by Mr. Short, but the
original record of his observations cannot be found. In Lalande’s
_Astronomy_ (3rd edition, article 3351,) it is said, ‘Cassini remarked
that the breadth of the ring was divided into two equal parts by a
dark line having the same curvature as the ring, and the _exterior_
portion was the less bright. Short _told_ me that he observed still
more singular phenomena with his large telescope of 12 feet. The
breadth of the ansæ, or extremities of the ring; was, according to him,
divided into two parts,--an inner portion without any break in the
illumination, and an outer divided by several lines concentric with the
circumference; which would lead to a belief, _that there are several
rings in the same plane_.’ De Lambre and Birt severally state that
Short saw the outer ring divided, probably on the authority of Lalande.
In Brewster’s _Ferguson’s Astronomy_, vol. ii, p. 125, 2nd edition,
there is the following note on this subject. ‘Mr. Short assures us,
that with an excellent telescope, he observed the surface of the ring
divided by several dark concentric lines, which seem to indicate a
number of rings proportional to the number of dark lines which he
perceived.’

In Dec. 1813, at Paris, Professor Quetelet saw the outer ring divided
with the achromatic telescope of 10 inches aperture, which was
exhibited at the exposition. He mentioned this the following day to M.
de la Place, who observed, that ‘those or even more divisions, were
conformable to the system of the world.’ On the other hand the division
of the outer ring was not seen by Sir W. Herschel in 1792, nor by Sir
J. Herschel in 1826, nor by Struve in the same year; and on several
occasions when the atmospheric conditions were most favourable, it
has not been seen by Captain Kater. It has been remarked by Sir W.
Herschel, Struve and others, that the exterior ring is much less
brilliant than the interior. And it is asked, may not this want of
light in the outer ring arise from its having a very dense atmosphere?
and may not this atmosphere in certain states admit of the divisions
of the exterior ring being seen, though, under other circumstances,
they remain invisible? The above observations are said to have been
confirmed by some recent observations by Decuppis at Rome, who
announced, some years ago, that Saturn’s outer ring is divided into two
or three concentric rings.

Some of the observations stated above, were they perfectly correct,
would lead to the conclusion that Saturn is encompassed with a number
of rings, concentric with and parallel to each other. But while such
phenomena as described above are so seldom seen, even by the most
powerful telescopes and the most accurate observers, a certain degree
of doubt must still hang over the subject; and we must suspend our
opinion on this point, till future observations shall either confirm
or render doubtful those to which we have referred. Should the Earl
of Rosse’s great telescope, when finished for observation, be found
to perform according to the expectations now entertained, and in
proportion to its size and quantity of light, we shall expect that our
doubts will be resolved in regard to the supposed divisions of the ring
of Saturn.




APPENDIX.


BRIEF DESCRIPTION OF THE EARL OF ROSSE’S TELESCOPE.

This telescope, the largest and most magnificent that ever was
attempted, reflects the greatest honour on the genius, the inventive
powers, and the scientific acquirements of its noble contriver, as
well as on the elevated station in which he is placed. With rank and
fortune, and every circumstance that usually unfit men for scientific
pursuit, he has set a bright example to his compeers of the dignity and
utility of philosophical studies and investigations, and of the aids
they might render to the progress of science, were their wealth and
pursuits directed in a proper channel.

Previously to his Lordship’s attempting the construction of his
largest--or ‘Monster Telescope,’ he had constructed one with a speculum
of 3 feet in diameter, which was considered one of the most accurate
and powerful instruments that had ever been made, not excepting even
Sir W. Herschel’s forty-feet Reflector. In the account of this
telescope, published in the Philosophical Transactions for 1840,
his Lordship speaks of the possibility of a speculum of six feet in
diameter being cast. At that time, it was considered by some as little
short of a chimera to attempt the construction of such a monstrous
instrument. But the idea no sooner occurred to this ingenious and
persevering nobleman than he determined to put it to the test, and the
result has been attended with complete success. The materials of which
this speculum is composed are _copper_ and _tin_, united very nearly
in their atomic proportions, namely, copper 126.4 parts, to tin 58.9
parts. This compound has a specific gravity of 8.8, and it is found to
preserve its lustre with more splendor, and to be more free from pores
than any other. A foundry was constructed expressly for the purpose of
casting the speculum. Its chimney built from the ground was 18 feet
high, and 16-1/2 square at the base, tapering to four at the top. At
each of its sides, communicating with it by flue, was sunk a furnace
8 feet deep, and 5-1/2 square, with a circular opening 4 feet in
diameter. About seven feet from the chimney was erected a large crane,
with the necessary tackle for elevating and carrying the crucibles from
the furnace to the mould, which was placed in a line with the chimney
and crane, and had three iron baskets supported on pivots hung round
it; and four feet farther on was the annealing oven. The crucibles
which contained the metal were each 2 feet in diameter, 2-1/2 deep,
and together weighed one ton and a half; they were of cast iron and
made to fit the baskets at the side of the mould. These baskets were
hung on wooden uprights or pivots, to one of these on each side was
attached a lever, by depressing which it might be turned over, and
the contents of the crucible poured into the mould. The bottom of the
mould was made by binding together tightly layers of hoop-iron, and
turning the required shape on them edgewise. This mould conducted the
heat away through the bottom, and cooled the metal towards the top in
infinitely small layers, while the interstices, though close enough to
prevent the metal from escaping, were sufficiently open to allow the
air to penetrate. This bottom was six feet in diameter and 5-1/2 inches
thick, and was made perfectly horizontal by means of spirit levels, and
was surrounded by a wooden frame; a wooden pattern, the exact size of
the speculum, being placed on the iron; sand was well packed between
it and the frame, and the pattern was removed. Each of the crucibles
containing the melted metal was then placed in its basket, and every
thing being ready for discharging their contents, they were at the same
instant turned over, and the mould being filled, the metal in a short
time safely set into the required figure. Whilst it was red hot, and
scarcely solid, the frame-work was removed, and an iron ring connected
with a bar which passed through the oven, being placed round it, it
was drawn in by means of a capstan at the other side, on a railroad,
when charcoal being lighted in the oven, and turf fires underneath
it, all the openings were built up, and it was left for sixteen weeks
to anneal. It was cast on the 13th of April, 1842, at 9 o’clock in
the evening. The crucibles were ten hours heating in the furnaces
before the metal was introduced, which in about ten hours more was
sufficiently fluid to be poured. When the oven was opened the speculum
was found as perfect as when it entered it. It was then removed to the
grinding machine, where it underwent that process, and afterwards was
polished, without any accident having occurred.

This speculum weighed _three tons_, and lost about one eighth of
an inch in grinding. Lord Rosse has since cast another speculum of
the same diameter four tons in weight. He can now, with perfect
confidence, undertake any casting, so great an improvement has the
form of mould which he has invented proved. The speculum was placed
on an equilibrium bed, composed of nine pieces resting on points at
their centres of gravity; the pieces were lined with pitch and felt,
before the speculum was placed on them. The speculum box is also lined
with felt and pitched; this prevents any sudden change of temperature
affecting the speculum by means of the bad conducting power of the
substances employed. A vessel of lime is kept in connection with the
speculum-box to absorb the moisture, which otherwise might injure the
mirror. The process of grinding was conducted under water, and the
moving power employed was a steam-engine of three-horse power. The
Polisher is connected with the machinery by means of a large ring of
iron, which loosely encircles it; and instead of either the speculum
or the polisher being stationary, both move with a regulated speed;
the ring of the polisher, and therefore the polisher itself, has a
transverse and a longitudinal motion; it makes 80 strokes in the
minute, and 24-1/2 strokes backward and forward for every revolution
of the mirror, and at the same time 1-72/100 strokes in the transverse
direction. The extent of the latter is 27/100 of the diameter of the
speculum. The substance made use of to wear down the surface was emery
and water, a constant supply of these was kept between the grinder
and the speculum. The Grinder is made of cast iron, with grooves
cut lengthways, across and circularly on its face. The polisher and
speculum have a mutual action upon each other; in a few hours, by the
help of the emery and water, they are both ground truly circular,
whatever may have been their previous defects. The grinding is
continued till the required form of surface is produced; and this is
ascertained in the following manner. There is a high tower over the
house in which the speculum is ground, on the top of which is fixed a
pole, to which is attached the dial of a watch; there are trap doors
which open, and by means of a temporary eye-piece, allow the figure
of the dial to be seen in the speculum brought to a slight polish. If
the dots on the dial are not sufficiently well-defined, the grinding
is continued; but if they appear satisfactorily, the polishing is
commenced. It required six weeks to grind it to a fair surface. The
polisher was cut into grooves, to prevent the abraded matter from
accumulating in some places more than in others--a thin layer of pitch
was spread over it, it was smeared over with rouge and water, and a
supply of it kept up till the machinery brought it to a fine black
polish. The length of time employed for polishing the 3 feet speculum
was six hours.[48]

This large telescope is now completed, or nearly so. The tube is 56
feet long, including the speculum box, and is made of deal, one inch
thick, hooped with iron. On the inside, at intervals of 8 feet, there
are rings of iron 3 inches in depth and 1 inch broad, for the purpose
of strengthening the sides. The diameter of the tube is 7 feet. It is
fixed to mason-work, in the ground, to a large universal hinge which
allows it to turn in all directions. At 12 feet distance, on each side,
a wall is built, 72 feet long, 48 high on the outer side, and 56 on
the inner--the walls being 24 feet distant from each other, and lying
exactly in the meridional line. When directed to the south, the tube
may be lowered till it become almost horizontal; but when pointed to
the north, it only falls till it is parallel with the earth’s axis,
pointing then to the pole of the heavens. Its lateral movements take
place only from wall to wall, and this commands a view for half an hour
on each side of the meridian--that is, the whole of its motion from
east to west is limited to 15 degrees. At present it is fitted up in a
temporary way to be used as a Transit instrument; but it is ultimately
intended to connect with the tube-end galleries, machinery which shall
give an automaton movement, so that the telescope shall be used as an
Equatorial Instrument. All the works connected with this instrument
are of the strongest and safest kind; all the iron-work was cast in
his Lordship’s laboratory by men instructed by himself, and every part
of the machinery was made under his own eye, by the artizans in his
own neighbourhood, and not a single accident worth mentioning happened
during the whole proceeding.

The expence incurred by his Lordship in the erection of this noble
instrument was not less than _twelve thousand pounds_! besides the
money expended in the construction of the telescope of three feet
diameter. Sufficient time has not yet been afforded for making
particular observations with this telescope; but from slight trials
which have been made, even under unfavourable circumstances, it
promises important results. Its great superiority over every telescope
previously constructed consists in the great quantity of light it
reflects, and the brilliancy with which it exhibits objects even when
high powers are applied. It has a reflecting surface of 4,071 square
inches, while that of Herschel’s 40-feet telescope had only 1811 square
inches on its polished surface, so that the quantity of light reflected
from the speculum is considerably more than double that of Herschel’s
largest reflector. This instrument has already exceeded his Lordship’s
expectations. Many appearances before invisible in the Moon, have been
perceived, and there is every reason to expect that new discoveries
will be made by it in the _Nebulæ_, double and triple stars, and other
celestial objects. The following is an extract of a communication
from Sir James South, on this subject, addressed to the Editor of the
‘_Times_.’ ‘The leviathan telescope on which the Earl of Rosse has
been toiling upwards of two years, although not absolutely finished,
was on Wednesday last directed to the Sidereal Heavens. The letter
which I have this morning received from its noble maker, in his usual
unassuming stile, merely states, that the metal only just polished,
was of a pretty good figure, and that with a power of 500, the nebula
known as No. 2., of Messier’s catalogue, was even more magnificent than
the nebula, No. 13 of Messier, when seen with his Lordship’s telescope
of 3 feet diameter, and 27 feet focus. Cloudy weather prevented him
from turning the leviathan on any other nebulous object. Thus, then,
we have all danger of the metal breaking before it could be polished,
overcome. Little more, however, will be done with it for some time, as
the Earl is on the eve of quitting Ireland for England to resign his
post at York as President of the British Association. I look forward
with intense anxiety to witness its first severe trial, when all its
various appointments shall be completed, in the confidence that those
who may then be present, will see with it what man has never seen
before. The diameter of the large metal is 6-feet, and its focus 54
feet; yet the immense mass is manageable by one man. Compared with
it, the working telescopes of Sir William Herschel, which in his
hands conferred on astronomy such inestimable service, and on himself
astronomical immortality, were but playthings.’

The following is a more recent account of observations made by this
telescope, chiefly extracted from Sir James South’s description of
this telescope, inserted in the _Times_ of April 16th, 1845, and the
‘_Illustrated London News_’ of April 19.

‘The night of the 5th of March, 1845, was the finest I ever saw in
Ireland. Many nebulæ were observed by Lord Rosse, Dr. Robinson and
myself. Most of them were for the first time since their creation,
seen by us as groups or clusters of stars; while some, at least to
my eyes, showed no such resolution. Never, however, in my life did
I see such glorious sidereal pictures as this instrument afforded
us. Most of the nebulæ we saw I certainly have observed with my own
large achromatic; but although that instrument, as far as relates to
magnifying power, is probably inferior to no one in existence, yet to
compare these nebulæ, as seen with it and the 6-feet telescope, is like
comparing, as seen with the naked eye, the dinginess of the planet
Saturn to the brilliancy of Venus. The most popularly-known nebulæ
observed this night were the ring nebulæ in the _Canes Venatici_, or
the 51st of Messier’s catalogue, which was resolved into stars with a
magnifying power of 548, and the 94th of Messier, which is in the same
constellation, and which was resolved into a large globular cluster of
stars, not much unlike the well-known cluster in Hercules, called also
13th Messier.’ Perfection of figure, however, of a telescope, must be
tested, not by nebulæ, but by its performance on a star of the first
magnitude. If it will, under high power, show the star round and free
from optical appendages, we may safely take it for granted it will not
only show nebulæ well, but any other celestial object as it ought.
To determine this point, the telescope was directed to _Regulus_,
with the entire aperture, and a power of 800, and ‘I saw’ says Sir
James, ‘with inexpressible delight, the star free from wings, tails or
optical appendages; not indeed like a planetary disk, as in my large
achromatic, but as a round image resembling voltaic light between
charcoal points; and so little aberration had this brilliant image,
that I could have measured its distance from, and position with any of
the stars in the field with a spider’s line micrometer, and a power of
1,000, without the slightest difficulty; for, not only was the large
star round, but the telescope, although in the open air, and the wind
blowing rather fresh, was as steady as a rock.’

‘On subsequent nights, observations of other nebulæ, amounting to some
30 or more, removed most of them from the list of nebulæ, where they
had long figured, to that of clusters; while some of these latter, more
especially 5 Messier, exhibited a sidereal picture in the telescope
such as man before had never seen, and which for its magnificence
baffles all description. Several double stars were seen with various
apertures of the telescope, and with powers between 360 and 800; and
as the Earl had told us before we should,--before the speculum was
inserted in the tube, in consequence of his having been obliged to quit
the superintendence of the polishing at the most critical part of the
process,--we found that a ring of about 6 inches broad, reckoning from
the circumference of the speculum, was not perfectly polished, and to
_that_ the little irradiation seen about Regulus was unquestionably
referable. The only double stars of the 1st class which the weather
permitted us to examine with it were Xi Ursæ Majoris, and Gamma
Virginis, which I could have measured with the greatest confidence.
D’Arrest’s comet we observed on the 12th of March, with a power of
400, but nothing worthy of notice was detected. Of the Moon, a few
words must suffice. Its appearance in my large achromatic of 12 inches
aperture is known to hundreds of readers; let them then imagine that
with it they look _at_ the moon, whilst with Lord Rosse’s 6 feet they
look _into it_, and they will not form a very erroneous opinion of
the performance of the Leviathan. On the 15th of March, when the moon
was 7 days old, I never saw her unilluminated disk so beautifully,
nor her mountains so temptingly measurable. On my first looking into
the telescope, a star of about the 7th magnitude was some minutes of
a degree from the moon’s dark limb, and its occultation by the moon
appeared inevitable. The star, however, instead of disappearing the
moment the moon’s edge came in contact with it, apparently glided on
the moon’s dark face, as if it had been seen through a transparent
moon, or as if the star were between me and the moon. It remained on
the moon’s disk nearly two seconds of time, and then disappeared. I
have seen this apparent projection of a star on the moon’s face several
times, but from the great brilliancy of the star, this was the most
beautiful I ever saw. The cause of this phenomenon is involved in
impenetrable mystery.’

The following is a representation of the Great Rosse Telescope, along
with part of the buildings with which it is connected. In the interior
face of the eastern wall a very strong iron arc of about 43 feet radius
is firmly fixed, provided with adjustments, whereby its surface facing
the telescope may be set very accurately in the plane of the meridian.
On this bar, lines are drawn, the interval between any adjoining two of
which, corresponds to one minute of time on the Equator. The tube and
speculum, including the bed on which the speculum rests, weigh about
15 tons. The telescope rests on an universal joint placed on masonry
about 6 feet below the ground, and is elevated or depressed by a chain
and windlass; and although it weighs about 15 tons, the instrument is
raised by two men with great facility. Of course, it is counterpoised
in every direction. The observer when at work, stands in one of four
galleries, the three highest of which are drawn out from the western
wall, while the fourth or lowest has for its base an elevating
platform, along the horizontal surface of which a gallery slides from
wall to wall by a machinery within the observer’s reach, but which
a child may work. When the telescope is about half an hour east of
the meridian, the galleries, hanging over the gap between the walls,
present to a spectator below an appearance somewhat dangerous; yet the
observer, with common prudence, is as safe as on the ground, and each
of the galleries can be drawn from the wall to the telescope’s side so
readily, that the observer needs no one else to move it for him.

[Illustration: _figure 98._]

The above figure represents only the upper part of the tube of the
telescope, at which the observer stands when making his observations.
The telescope is at present of the Newtonian construction, and
consequently, the observer looks into the side of the tube at the upper
end of the telescope, but it is proposed to throw aside the plane
speculum, and to adapt it to the _Front view_, on the plan already
described (see pp. 306, 313, &c.) so that the observer will sit or
stand with his back towards the object, and his face looking down upon
the speculum; and, in this position, he will sometimes be elevated
between 50 and 60 feet above the ground. As yet, the telescope has no
equatorial motion, but it very shortly will; and at no very distant
day, clock-work will be connected with it, when the observer will,
while observing, be almost as comfortable, as if he were reading at a
desk by his fire-side.

[Illustration: _figure 99._]

The following figure shews a section of the machinery connected with
this telescope. It exhibits a view of the inside of the eastern wall,
with all the machinery as seen in section. A is the mason-work on the
ground, B the universal joint, which allows the tube to turn in all
directions; C the speculum in its tube; D the box; E the eye-piece; F
the moveable pulley; G the fixed one; H the chain from the side of the
tube; I the chain from the beam; K the counterpoise; L the lever; M
the chain connecting it with the tube; Z the chain which passes from
the tube to the windlass over a pulley on a truss-beam which runs from
W to the same situation on the opposite wall--the pulley is not seen.
X is a railroad on which the speculum is drawn either to or from its
box; part is cut away to show the counterpoise. The dotted line _a_
represents the course of the weight R as the tube rises or falls; it
is a segment of a circle of which the chain I is the radius. The tube
is moved from wall to wall by the ratchet and wheel at R; the wheel is
turned by the handle O, and the ratchet is fixed to the circle on the
wall. The ladders in front, as shown in the preceding sketch, enable
the observer to follow the tube in its ascent to where the galleries on
the side wall commence; these side galleries are three in number, and
each can be moved from wall to wall by the observer, after the tube,
the motion of which he also accomplishes by means of the handle O.

I shall conclude the description of this wonderful instrument in the
words of Sir James South.

‘What will be the power of this telescope when it has its Le Mairean
form’ [that is, when it is fitted up with the front view] ‘it is not
easy to divine;--what nebulæ will it resolve into stars; in what nebulæ
will it not find stars;--how many satellites of Saturn will it show
us;--how many will it indicate as appertaining to Uranus;--how many
nebulæ never yet seen by mortal eye, will it present to us;--what spots
will it show us on the various planets; will it tell us what causes
the variable brightness of many of the fixed stars;--will it give us
any information as to the constitution of the planetary nebulæ;--will
it exhibit to us any satellites encircling them; will it tell us
why the satellites of Jupiter, which generally pass over Jupiter’s
face as disks nearly of white light, sometimes traverse it as black
patches;--will it add to our knowledge of the physical construction of
nebulous stars;--of that mysterious class of bodies which surround
some stars, called, for want of a better name, ‘photospheres;’--will
it show the annular nebulæ of Lyra, merely as a brilliant luminous
ring, or will it exhibit it as thousands of stars arranged in all the
symmetry of an ellipse; will it enable us to comprehend the hitherto
incomprehensible nature and origin of the light of the great nebulæ
of Orion;--will it give us, in easily appreciable quantity, the
parallax of some of the fixed stars, or will it make sensible to us
the parallax of the nebulae themselves;--finally, having presented to
us original portraits of the moon and of the sidereal heavens, such as
man has never dared even to anticipate--will it, by Daguerreotype aid,
administer to us copies founded upon truth, and enable astronomers of
future ages to compare the moon and heavens as they then may be, with
the moon and heavens as they were? Some of these questions will be
answered affirmatively, others negatively, and that, too, very shortly;
for the noble maker of the noblest instrument ever formed by man, “has
cast his bread upon the waters, and will, with God’s blessing, find it
before many days.”’


HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF
TELESCOPES.

As there are many among the lower ranks of the community who have a
desire to be possessed of a telescope, which will show them some of the
prominent features of celestial scenery, but who are unable to purchase
a finished instrument at the prices usually charged by Opticians, the
following hints may perhaps be acceptable to those who are possessed of
a mechanical genius.

The lenses of an Achromatic telescope may be purchased separately
from glass-grinders or Opticians, and tubes of a cheap material may
be prepared by the individual himself for receiving the glasses.
The following are the prices at which achromatic object-glasses for
astronomical telescopes are generally sold. Focal length 30 inches,
diameter 2-1/4 inches, from 2 to 3-1/2 guineas. Focal length 42 inches,
diameter 2-3/4 inches, from 5 to 8 guineas. Focal length 42 inches,
diameter 3-1/4 inches, from 12 to 20 guineas. Focal length 42 inches,
diameter 3-3/4 inches, from 25 to 30 guineas. Eye-pieces, from 10s. 6d.
to 18 shillings. The smallest of these lenses, namely that of 2-1/4
inches diameter, if truly achromatic, may be made to bear a power of
from 80 to 100 times, in clear weather, for celestial objects, which
will show Jupiter’s moons and belts, Saturn’s ring and other celestial
phenomena. The tubes may be made either of tin plates, _papier maché_,
or wood. Wood, however, is rather a clumsy article, and it is sometimes
liable to warp, yet excellent tubes have sometimes been made of it.
Perhaps the cheapest and most convenient of all tubes when properly
made, are those formed of paper. In forming these a wooden roller of
the proper diameter should be procured, and paper of a proper size,
along with book-binder’s paste. About three or four layers only of the
paper should be pasted at one time, and, when sufficiently dry, it
should be smoothed by rubbing it with a smooth stick or ruler; after
which another series of layers should be pasted on, and allowed to dry
as before, and so on till the tube has acquired a sufficient degree
of strength and firmness. In this way, I have, by means of a few old
Newspapers, and similar materials, formed tubes as strong as if they
had been made of wood. If several tubes be intended to slide into each
other, the smallest tube should be made first, and it will serve as a
roller for forming the tube into which it is to slide.

An achromatic object glass of a shorter focal distance, and a smaller
diameter than any of those stated above, may be fitted up as a useful
astronomical telescope, when a better instrument cannot be procured. In
the Pawn-broker’s shops in London, and other places, an old achromatic
telescope, with an object-glass 20 inches focal distance and about
1-1/2 inch diameter, may be purchased at a price varying from 15 to 20
shillings. By applying an astronomical eye-piece to such a lens, if
a good one, it may bear a power, for celestial objects, of 50 or 60
times. If two plano-convex glasses, 3/4 inch focal distance, be placed
with their convex sides near to each other, they will form an eye-piece
which will produce a power on such an object-glass, of above 50 times,
which will show Jupiter’s belts and satellites, Saturn’s ring, the
solar spots, and the mountains and cavities of the moon. I have an
object-glass of this description which belonged to an old telescope,
which cost me only 12 shillings, and with which I formerly made some
useful astronomical observations. It was afterwards used as the
telescope of a small Equatorial instrument, and, with it, I was enabled
to perceive stars of the first and second magnitude, and the planets
Venus, Jupiter, and Mars, in _the day-time_.

But, should such a glass be still beyond the reach of the astronomical
amateur, let him not altogether despair. He may purchase a single lens,
3 feet focal distance, for about a couple of shillings, and by applying
an eye-glass of 1 inch focus, which may be procured for a shilling, he
will obtain a power of 36 times, which is a higher power than Galileo
was able to apply to his best telescope; and consequently, with such an
instrument, he will be enabled to perceive all the celestial objects
which that celebrated astronomer first described, and which excited so
much wonder, at that period, in the learned world. But, whatever kind
of telescope may be used, it is essentially requisite that it be placed
on a firm stand in all celestial observations: and any common mechanic
can easily form such a stand at a trifling expence.

There is a certain optical illusion to which most persons are subject,
in the first use of telescopes, especially when applied to the
celestial bodies, on which it may not be improper to make a remark.
The illusion to which I allude is this--that they are apt to imagine,
the telescope does not magnify nearly so much as it really does. They
are apt to complain of the small appearance which Jupiter and Saturn,
for example, present when magnified 160 or 200 times. With such powers
they are apt to imagine, that these bodies do not appear so large as
the moon to the naked eye. Yet it can be proved that Jupiter, when
nearest the earth, viewed with such a power, appears about 5 times
the diameter of the full moon, and 25 times larger in surface. This
appears from the following calculation. Jupiter, when in opposition,
or nearest the Earth, presents a diameter of 47´´: the mean apparent
diameter of the moon is about 31´. Multiply the diameter of Jupiter by
the magnifying power, 200, the product is 9400´´ or 156´ or 2° 36´,
which, divided by 31´, the moon’s diameter, produces a quotient of 5,
showing that this planet with such a power appears five times larger in
diameter than the full moon to the naked eye, and consequently 25 times
larger in surface. Were a power of only 50 times applied to Jupiter,
when nearest the earth, that planet would appear somewhat larger than
the full moon. For 47´´ multiplied by 50 gives 2350´´ or 39´, which
is 8´ more than the diameter of the moon. Yet with such a power most
persons would imagine that the planet does not appear one third of the
size of the full moon.

The principal mode by which a person may be experimentally convinced
of the fallacy to which I allude is the following:--At a time when
Jupiter happens to be within a few degrees of the moon, let the planet
be viewed through the telescope with the one eye, and the magnified
image of the planet be brought into contact with the moon as seen with
the other eye--the one eye looking at the moon, and the other viewing
the magnified image of Jupiter through the telescope when brought into
apparent contact with the moon--then it will be perceived, that with
a magnifying power of 50 the image of Jupiter will completely cover
the moon as seen by the naked eye;--and with a power of 200--when the
moon is made to appear in the centre of the magnified image of the
planet--it will be seen that Jupiter forms a large and broad circle
around the moon, appearing at least 5 times greater than the diameter
of the moon. This experiment may be varied as follows: Suppose a person
to view the moon through a small telescope or opera-glass, magnifying
three times, he will be apt to imagine, at first sight, that she is not
in the least magnified, but rather somewhat diminished. But let him
bring the image as seen in the telescope in contact with the moon as
seen with the naked eye, and he will plainly perceive the magnifying
power, by the size of the image. It may be difficult in the first
instance to look, at the same time, at the magnified image and the real
object, but a few trials will render it easy.


THE END.


L. SEELEY PRINTER, THAMES DITTON.




ERRATA.


  Page 72 line 4 for EI, read FI.
   -- 103 -- 30 -- depend, read depends.
   -- 135 -- 10 -- refacting, read refracting.
   -- 136 -- 10 -- colour, read colours.
   -- 146 -- 27 -- G, read C.
   -- 146 -- 32 -- prisms, read prism.
   -- 153 -- 35 -- 28_{o} 3´, read 28° 10´
   -- 165 -- 32 -- some, read since.
   -- 165 -- 33 dele that.
   -- 166 --  5 for these, read their.
   -- 166 -- 21 -- those, read their.
   -- 178 -- 32 -- variety, read vanity.
   -- 187 --  7 -- in, read an.
   -- 187 -- 11 -- (p. 103.), read (p. 72.)
   -- 189 -- 30 -- lens, read lenses.
   -- 199 -- 31 -- punice, read pumice.
   -- 216 -- 10 -- nine, read ten.
   -- 236 -- 12, 13 -- “more distant from,” read “nearer to.”
   -- 337 -- 27 -- 1, read 1-1/2.




FOOTNOTES:

[1] Those unfortunate individuals who have been confined in the darkest
dungeons have declared, that though on their first entrance, no object
could be perceived, perhaps for a day or two, yet, in the course
of time, as the pupils of their eyes expanded, they could readily
perceive mice, rats, and other animals that infested their cells, and
likewise the walls of their apartments; which shows that, even in
such situations, light is present, and produces a certain degree of
influence.

[2] Letters to a German Princess, vol. l. pp. 68, 69, &c.

[3] The manner in which the motion of light was discovered is explained
in the author’s work, entitled ‘Celestial Scenery,’ pp. 369-371, and
the circumstances which led to the discovery of the aberration of light
are stated and illustrated in his volume on the ‘Sidereal Heavens,’ pp.
71-74, and pp. 284-292.

[4] Nicolson’s Introduction to Natural Philosophy, vol. 1.

[5] Light of a phosphoric nature, is frequently emitted from various
putrescent animal substances which, in the ages of superstition,
served to astonish and affright the timorous. We learn from Fabricius,
an Italian, that three young men, residing at Padua, having bought a
lamb, and eaten part of it on Easter Day, 1592, several pieces of the
remainder which they kept till the following day, shone like so many
candles when they were casually viewed in the dark. The astonishment of
the whole city was excited by this phenomenon, and a part of the flesh
was sent to Fabricius, who was Professor of anatomy, to be examined by
him. He observed, that those parts which were soft to the touch and
transparent in candle-light, were the most resplendent: and also that
some pieces of kid’s flesh which had happened to have lain in contact
with them were luminous, as well as the fingers and other parts of the
bodies of those persons who touched them. Bartholin gives an account
of a similar phenomenon, which happened at Montpelier in 1641. A poor
woman had bought a piece of flesh in the market, intending to make use
of it the following day, but happening not to be able to sleep well
that night, and her bed and pantry being in the same room, she observed
so much light come from the flesh as to illuminate all the place where
it hung. We may judge of the terror and astonishment of the woman
herself, when we find that a part of this luminous flesh was carried as
a very extraordinary curiosity to Henry, Duke of Conde, the Governor of
the place, who viewed it several hours with the greatest astonishment.
The light was as if gems had been scattered over the surface, and
continued till the flesh began to putrify, when it vanished, which
it was believed to do in the form of a cross. Hence the propriety of
instructing the mass of the community in the knowledge of the facts
connected with the material system, and the physical causes of the
various phenomena of nature.

[6] Memoires de la Soc. d’Aroncil, vol. ii.

[7] By a _medium_, in optics, is meant the space in which a ray of
light moves, whether pure space, air, water, glass, diamond, or any
other transparent substance through which the rays of light can pass in
straight lines.

[8] Edinburgh Philosophical Journal for October 1819, p. 411.

[9] This mode of finding the focus of a concave lens may be varied as
follows:--let the lens be covered with paper, having two small circular
holes; and on the paper for receiving the light, describe also two
small circles, but with their centres at twice the distance from each
other of the centres of the circles. Then move the paper to and from,
till the middle of the sun’s light, coming through the holes, falls
exactly on the middle of the circles; that distance of the paper from
the lens will be the focal length required.

[10] Small glass mirrors for performing some of the experiments, and
illustrating some of the principles above alluded to,--may be made of
the flattest kind of common watch glasses, by foliating or covering
with tin leaf and quicksilver the convex surfaces of such glasses.
Their focal distances will generally be from one to two inches. Such
mirrors afford a very large and beautiful view of the eye, when held
within their focal distance of that organ. Such mirrors will also serve
the purpose of reflecting light on the objects viewed by microscopes.
Larger mirrors, of from four to eight inches diameter, may be had of
the optician at different prices varying from five to ten or fifteen
shillings.

[11] Nicholson’s Journal of Natural Philosophy, &c. 4to. series, p. 225.

[12] There can be little doubt that some of the facts ascribed, in the
western highlands of Scotland, to _second sight_, have been owing to
the unusual refraction of the atmosphere; as one of the peculiarities
attributed to those who possessed this faculty was, that they were
enabled to descry boats and snips, before they appeared in the horizon.

[13] Fraunhofer was in the highest sense of the word, an _Optician_,
an original discoverer in the most abstruse and delicate departments
of this science--a competent mathematician, an admirable mechanist,
and a man of a truly philosophical turn of mind. By his extraordinary
talents, he was soon raised from the lowest station in a manufacturing
establishment to the direction of the _optical_ department of the
business, in which he originally laboured as an ordinary workman.
He then applied the whole power of his mind to the perfection of
the achromatic telescope, the defects of which in reference to the
optical properties of the materials used--he attempted to remedy; and
by a series of admirable experiments, succeeded in giving to optical
determinations, the precision of astronomical observations, surpassing,
in this respect all who had gone before him, except perhaps, the
illustrious Newton. It was in the course of these researches, that he
was led to the important discovery of the dark lines which occur in the
solar spectrum. His achromatic telescopes are scattered over Europe,
and are the largest and best that have hitherto been constructed. He
died at Munich, at a premature age, in 1826; his death, it is said
being accelerated by the unwholesome nature of the processes employed
in his glass-house; leaving behind him a reputation rarely attained
by one so young. His Memoir “On the refractive and dispersive power
of different species of glass, in reference to the improvement of
Achromatic telescopes, and an account of the lines on the spectrum,”
will be found in the “Edinburgh Philosophical Journal,” Vol. ix. pp.
288-299, and Vol. x. pp. 26-40, for 1823-4.

[14] Philosophical Transactions. Vol. 50. p. 294.

[15] Ecclesiasticus xliii. 11, 12.

[16] It is a question which has been frequently started--Whether there
was any rainbow before the flood? Some have conceived that the rainbow
was something of a _miraculous_ production, and that it was never
seen before the flood. The equivocal sense of the word ‘set’ in our
translation, has occasioned a mistaken impression of this kind. The
Hebrew word thus translated, signifies more properly ‘I do give,’ or
‘I _appoint_.’ The whole passage in reference to this circumstance,
literally translated, runs thus;--“I appoint my bow which is in the
cloud, that it may be for a sign or token of a covenant between me and
the earth; and it shall come to pass when I bring a cloud over the
earth, and the bow shall be seen in the cloud, that I will remember my
covenant that is between me and you,” &c. As the rainbow is produced
by the immutable laws of refraction and reflection, as applied to the
rays of the sun striking on drops of falling rain, the phenomenon must
have been occasionally exhibited from the beginning of the world:
unless we suppose that there was no rain before the flood, and that the
constitution of things in the physical system was very different from
what it is now. The passage affirms no more than that the rainbow was
_then appointed_ to be a _symbol_ of the covenant between God and man,
and although it may have been frequently seen before, it would serve
the purpose of a sign equally well, as if it had been miraculously
formed for this purpose, and even better, as its frequent appearance,
according to natural laws, is a perpetual memorial to man of the divine
faithfulness and mercy.

[17] Though Borellus mentions this circumstance, yet there is some
reason to doubt the accuracy of this statement, as young Jansen appears
to have been at that period, not more than six years old; so that it
is more probable that Galileo was the first discoverer of Jupiter’s
satellites.

[18] The reader may see an engraving of this instrument in the author’s
work entitled ‘the _Improvement of Society_.’--p. 209.

[19] It is one of the properties of concave lenses to render convergent
rays less convergent, and when placed as here supposed, to render them
parallel; and it is parallel rays that produce distinct vision.

[20] The word _aperture_ as applied to object-glasses, signifies the
opening to let in the light, or that part of the object-glass which is
left uncovered. An object-glass may be 3 inches in diameter, but if one
inch of this diameter be covered, its aperture is said to be only 2
inches.

[21] An achromatic telescope is said to be in possession of Mr. Cooper,
M.P. for Sligo, which is 26 feet long, and the diameter of the object
glass 14 inches.

[22] This telescope, which was made by Dollond, with a power of 240
times, gives a beautiful view of the belts of Jupiter and the double
ring of Saturn, and with a power of 50, the stars in the milky way and
some of the nebulæ appear very numerous and brilliant. Its owner is a
gentleman who unites science with Christianity.

[23] For a more particular account of Dr. Blair’s instruments and
experiments, the reader is referred to his Dissertation on this subject
in Vol. II. of the ‘Transactions of the Royal Society of Edinburgh,’
which occupies 76 pages--or to Nicholson’s ‘Journal of Natural
Philosophy,’ &c. Quarto Series, Vol. I., April, September, 1797.

[24] A more detailed account of the processes connected with the
construction of this telescope, will be found in a paper presented
to the Royal Society, in 1827, and published in the Philosophical
Transactions of that Society, for 1828, and likewise another paper,
published in the Transactions for 1829. From these documents, chiefly,
the preceding account has been abridged. See also the ‘Edinburgh
New Philosophical Journal’ for Jan.,--April, 1828, and Brewster’s
‘Edinburgh Journal of Science,’ for October, 1829.

[25] A particular description of this telescope, with the machinery
for moving it, illustrated with an engraving, may be seen in Reid and
Gray’s ‘Abridgement of the Philosophical Transactions.’--Vol. vi. Part
I. for 1723, pp. 147-152.

[26] Miss Short, who has erected and who superintends an observatory
on the Calton hill, Edinburgh, is the descendant of a brother of Mr.
Short. She is in possession of a large Gregorian reflector, about 12
feet long, made by Mr. Short, and mounted on an Equatorial axis. It was
originally placed in a small observatory erected on the Calton hill,
about the year 1776, but for many years past it has been little used.

[27] A particular account of the Earl of Rosse’s fifty-feet Reflector,
which is now finished, is given in the _Appendix_.

[28] Philosophical Transactions for 1800, Vol. XC. p. 80, &c.

[29] In using telescopes within doors, care should generally be taken,
that there be no fires in the apartment where they are placed for
observation, and that the air within be nearly of the same temperature
as the air of the surrounding atmosphere; for if the room be filled
with heated air, when the windows are opened, there will be a current
of cold air rushing in, and of heated air rushing out, which will
produce such an undulation and tremulous motion, as will prevent any
celestial object from being distinctly seen.

[30] The above directions and remarks are abridged with some
alterations from Dr. Pearson’s “Introduction to Practical
Astronomy.”--Vol. II.

[31] Pearson’s “Practical Astronomy.”--Vol. II.

[32] The mother-of-pearl dynameter may be purchased for about twelve
shillings. See fig. 57, _a_, _b_, _c_, p. 260.

[33] Reid’s Enquiry into the Human Mind, chap. iv.

[34] The distance of Saturn from the sun is 906,000,000 of miles; it
is sometimes nearer to and at other times farther from the earth,
according as it is near the point of its opposition to, or conjunction
with the sun. If this number be divided by 200, the supposed magnifying
power of the telescope, the quotient is 4,530,000, which expresses
the distance in miles at which it enables us to contemplate this
planet. If this number be subtracted from 906,000,000, the remainder
is 901,470,000, which expresses the number of miles from the earth at
which we are supposed to view Saturn with such an instrument.

[35] _Irish Transactions_, Vol. X. and Nicholson’s Philosophical
Journal, Vol. XVI.

[36] Brewster’s Appendix to ‘Ferguson’s Lectures.’

[37] A particular description of the micrometers here enumerated,
and several others, will be found in Dr. Pearson’s ‘Introduction to
Practical Astronomy,’ Vol. II.

[38] Adams’ Introduction to Practical Astronomy.

[39] Or find the sun’s right ascension for the given day; substract
this from the star or planet’s right ascension, and the remainder
is the approximate time of the star’s coming to the meridian. The
difference between this time and the time of observation, will then
determine the point to which the telescope is to be directed.

[40] The right ascensions, declinations, longitudes, &c., stated in
these memoranda--which were noted at the time of observation--are only
approximations to the truth; perfect accuracy in these respects being
of no importance in such observations. They are, however, in general,
within a minute or two of the truth. The _times_ of the observations,
too, are noted in reference--not to the _astronomical_, but to the
_civil_ day. The astronomical day commences at 12 noon, and the hours
are reckoned, without interruption, to the following noon. The civil
day commences at 12 midnight.

[41] This observation is inserted in the ‘Edinburgh Philosophical
Journal’ for January, 1844.

[42] The late Mr. Benjamin Martin, when describing the nature of the
solar telescope, in his ‘_Philosophia Britannica_,’ Vol. iii. p. 85,
gives the following relation:--‘I cannot here omit to mention a very
_unusual phenomenon_ that I observed about ten years ago in my darkened
room. The window looked towards the west, and the spire of Chichester
Cathedral was before it at the distance of 50 or 60 yards. I used very
often to divert myself by observing the pleasant manner in which the
sun passed behind the spire, and was eclipsed by it for sometime; for
the image of the sun and of the spire were very large, being made by a
lens of 12 feet focal distance. And once as I observed the occultation
of the sun behind the spire, just as the disk disappeared, I saw
several small, bright, round bodies or balls running toward the sun
from the dark part of the room, even to the distance of 20 inches.
I observed their motion was a little irregular, but rectilinear,
and seemed accelerated as they approached the sun. These luminous
globules appeared also on the other side of the spire, and preceded
the sun, running out into the dark room, sometimes more, sometimes
less, together in the same manner as they followed the sun at its
occultation. They appeared to be in general one-twentieth of an inch
in diameter, and therefore, must be very large luminous globes in some
part of the heavens, whose light was extinguished by that of the sun,
so that they appeared not in open day light; but whether of the meteor
kind, or what sort of bodies they might be, I could not conjecture.’
Professor Hansteen mentions, that when employed in measuring the zenith
distances of the pole star, he observed a somewhat similar phenomenon,
which he described as ‘a luminous body which passed over the field of
the universal telescope--that its motion was neither perfectly equal
nor rectilinear, but resembled very much the unequal and somewhat
serpentine motion of an ascending rocket;’ and he concluded that it
must have been ‘a meteor’ or ‘shooting star’ descending from the higher
regions of the atmosphere.[43]

In my frequent observations on Venus, to determine the nearest
positions to the sun in which that planet could be seen, I had several
times an opportunity of witnessing similar phenomena. I was not a
little surprised, when searching for the planet, frequently to perceive
a body pass across the field of the telescope, apparently of the same
size as Venus, though sometimes larger and sometimes smaller, so that
I frequently mistook that body for the planet, till its rapid motion
undeceived me. In several instances _four_ or _five_ of these bodies
appeared to cross the field of view, sometimes in a perpendicular, and,
at other times in a horizontal direction. They appeared to be luminous
bodies, somewhat resembling the appearance of a planet when viewed in
the day-time with a moderate magnifying power. Their motion was nearly
rectilinear, but sometimes inclined to a waving or serpentine form, and
they appeared to move with considerable rapidity--the telescope being
furnished with a power of about 70 times. I was for a considerable
time at a loss what opinion to form of the nature of these bodies;
but having occasion to continue these observations almost every clear
day for nearly a twelvemonth, I had frequent opportunities of viewing
this phenomenon in different aspects; and was at length enabled to
form an opinion as to the cause of at least _some_ of the appearances
which presented themselves. In several instances, the bodies alluded
to appeared much larger than usual, and to move with a more rapid
velocity; in which case I could plainly perceive that they were nothing
else than _birds_ of different sizes, and apparently at different
distances, the convex surfaces of whose bodies, in certain positions,
strongly reflected the solar rays. In other instances, when they
appeared smaller, their true shape was undistinguishable by reason of
their motion and their distance.

Having inserted a few remarks on this subject, in No. XXV. of the
Edinburgh Philosophical Journal for July, 1825, particularly in
reference to Professor Hansteen’s opinion, that article came under the
review of M. Serres, Sub-Prefect of Embrun, in a paper inserted in the
_Annales de Chemie_, for October, 1825, entitled, ‘Notices regarding
fiery meteors seen during the day.’[44] In the discussion of this
subject, M. Serres admits that the light reflected very obliquely from
the feathers of a bird is capable of producing an effect similar to
that which I have now described; but that ‘the explanation ought not
to be _generalized_.’ He remarks, that, while observing the sun at the
repeating circle, he frequently perceived, even through the 
glass adapted to the eye-piece, large luminous points which traversed
the field of the telescope, and which appeared too well defined not to
admit them to be distant, and subtended too large angles to imagine
them birds. In illustration of this subject he states the following
facts. On the 7th September, 1820, after having observed for some time
the eclipse of the sun which happened on that day, he intended to take
a walk in the fields, and on crossing the town, he saw a numerous group
of individuals of every age and sex, who had their eyes fixed in the
direction of the sun. Further on, he perceived another group having
their eyes in like manner turned towards the sun. He questioned an
intelligent artist who was among them to learn the object that fixed
his attention. He replied, ‘We are looking at the stars which are
detaching themselves from the sun.’ ‘You may look yourself; that will
be the shortest way to learn the fact.’ He looked, and saw, in fact,
not stars, but balls of fire of a diameter equal to the largest stars,
which were projected in various directions from the upper hemisphere
of the sun, with an incalculable velocity, and although this velocity
of projection appeared the same in all, yet they did not all attain
the same distance. These globes were projected at unequal and pretty
short intervals. Several were often projected at once, but always
diverging from one another. Some of them described a right line, and
were extinguished in the distance; some described a parabolic line, and
were in like manner extinguished; others again, after having removed
to a certain distance in a right line, retrograded upon the same line,
and seemed to enter, still luminous, into the sun’s disk. The ground of
this magnificent picture was a sky blue, somewhat tinged with brown.
Such was his astonishment at the sight of so majestic a spectacle, that
it was impossible for him to keep his eyes off it till it ceased, which
happened gradually as the eclipse wore off and the solar rays resumed
their ordinary lustre. It was remarked by one of the crowd that ‘the
sun projected most stars at the time when it was palest;’ and that the
circumstance which first excited attention to this phenomenon was that
of a woman who cried out ‘Come here!--come and see the flames that are
issuing from the sun!’

I have stated the above facts because they may afterwards tend to throw
light upon certain objects or phenomena with which we are at present
unacquainted. The phenomenon of ‘falling stars’ has of late years
excited considerable attention, and it seems now to be admitted, that,
at least, certain species of these bodies descend from regions far
beyond the limits of our atmosphere. This may be pronounced as certain
with regard to the ‘November Meteors.’ May not some of the phenomena
described above, be connected with the fall of meteoric stones--the
showers of falling stars seen on the 12th and 13th of November, or
other meteoric phenomena whose causes we have hitherto been unable to
explain? Or, may we conceive that certain celestial bodies, with whose
nature and destination we are as yet unacquainted, may be revolving in
different courses in the regions around us--some of them opaque and
others luminous, and whose light is undistinguishable by reason of the
solar effulgence?

[43] See Edinburgh Philosophical Journal, for April, 1825. No. XXIV.

[44] See Edinburgh Philosophical Journal, for July, 1826, p. 114.

[45] For an explanation of the manner of viewing Venus at her superior
conjunction, see ‘Celestial Scenery,’ 5th thousand, p. 102.

[46] See Long’s Astronomy, vol. 2, p. 487,--and Encyclopedia
Britannica, vol. ii. p. 436, 3rd edition.

[47] The balls which represent the different planets, on this machine,
have their hemispheres painted black, with the white side turned
directly to the sun, so that if the eye be placed in a line with the
earth, and the planet, particularly Mercury and Venus, its phase in the
heavens, at that time, as viewed with a telescope, may be distinctly
perceived.

[48] The above description has been selected and abridged from a small
volume entitled ‘The Monster Telescope, erected by the Earl of Rosse,
Parsontown,’--and also from the ‘Illustrated London News’ of September
9th, 1843. In the volume alluded to a more particular description will
be found, accompanied with engravings.


[Transcriber’s Note:

The corrections listed in the Errata list have been made.

The high resolution image for the image on page 196 does not have a
caption. I have captioned this image as "figure 40" and the one on page
206 as "figure 40*" to comply with the "List of Engravings".

Inconsistent double quotes and capitalization are as in the original.

Inconsistent spelling and hyphenation are as in the original.]





End of the Project Gutenberg EBook of The Practical Astronomer, by Thomas Dick

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