ABSTRACT
The most difficult part of applying mathematics to the description of a physical phenomenon involves identifying the relevant aspects of the phenomenon. This is especially true of vision, as in any natural scene, it is not obvious what constitutes the stimulus to the visual system. However, consideration of a basic visual stimulus consisting of an isolated light source, which can vary in both intensity and wavelength, makes it clear that any visual stimulus will have a number of qualitatively different aspects. The appropriate mathematical structures for describing phenomena which vary both quantitatively and qualitatively are introduced in this chapter.
