Add some notes about the new sequential mode to the docs.

README.rst

@@ -6,11 +6,12 @@ Raypier is a non-sequential ray-tracing framework for modelling optical systems.
 
 #. It's pretty fast. The tracing algorithms are written in Cython (i.e. C) and use efficient data-structures for handling many thousands of rays.
 #. Correctly handles polarisation
+#. Sequential and non-sequential tracing.
 #. Support for dispersion including a comprehensive library of glass-types (taken from refractive-index.info)
    and diffraction-gratings
 #. Point Spread Function (PSF) and E-Field evaluation by summation of of Gaussian modes.
 #. Paraxial Gaussian mode evaluation covers generally astigmatic modes.
-#. Tracing support for conics section and general aspherics (conic + polnomial)
+#. Tracing support for conics section, general aspherics (conic + polnomial) and 2D polynomial surfaces. 
 #. Zurnike polynomial surface distortions.
 #. An interactive user-interface with 3D model/ray rendering through VTK. Tracing is performed "live".
 #. A modest selection of optic-types including singlet and achromatic doublet lenses (with AR coating), plane-mirrors, 

doc/source/introduction.rst

@@ -1,11 +1,12 @@
 Introduction to Raypier
 =======================
 
-Raypier is a non-sequential optical ray-tracing program. It is intended as a 
+Raypier is a optical ray-tracing program. It is intended as a 
 design tools for modelling optical systems (cameras, imaging systems, telescopes etc.).
 
 The main features of ray-trace are:
  - Non-sequential tracing (no need to specify the order of optical components)
+ - Optional sequential tracing where the surface-sequence is discovered using a non-sequential initial trace. 
  - Physical optics propagation with "Gausslet" tracing and beam decomposition
  - Nice visualisation of the traced result
  - Live update to the traced result as the user adjusts the model

doc/source/ray_sources.rst

@@ -5,6 +5,10 @@ Ray sources generate the input rays for the model. The Ray-source objects also h
 :py:attr:`traced_rays` attribute, as a list of :py:class:`RayCollection` objects. Each item in this list represents one "generation"
 of rays. 
 
+Ray sources have a :py:attr:`sequential` attribute. When set True, the source will be traced sequentially using a pre-defined list of surfaces.
+If no list of surfaces exists, the trace will default to the usual non-sequential mode and the surface-sequence obtained by this trace
+will be stored for use in subsequent sequential traces. The stored sequence can be cleared by assigning to the :py:attr:`clear_sequence` event-trait.
+
 Ray source classes are subclasses of :py:class:`raypier.sources.BaseRaySource`. Besides generating the :py:attr:`input_rays` (a :py:class:`RayCollection`
 instance) attribute as the input to the model, and holding the trace results in the :py:attr:`traced_rays` member, the source
 objects also control the visualisation of the rays. The following visualisation-related attributes are available.