Modern accounts of vision draw on the past in innumerable and often unstated ways. The divisions we draw between the physics of light, physiological responses to it, and the psychology of perception represent hard-won battles. The theories applied to these remain matters of debate. We are in a better position to appreciate the massive advances that have been made in understanding the nature of light, image formation, visual anatomy and physiology, and visual phenomena themselves when we can place them in the context of the past. The historical perspective is often overlooked or neglected in books on perception, which is a pity because it implies that we now have a privileged viewpoint, superior to those of the past. In fact, the same theoretical issues often recur, disguised by the new jargon to appear different. Seeing through the shroud of the present can facilitate our understanding of such issues, and remaining ignorant of past attempts to grapple with them can inhibit progress.
All sciences are based on observation. Naturally occurring or experimentally manipulated events are observed and if the observations prove to be consistent they become phenomena. That is, if different observers describe the events in much the same way then they are considered to be aspects of the world rather than aspects of the observer. The phenomena are accumulated and classified in order to interpret them and determine lawful relations between them. The history of vision is somewhat unusual because it is the history of observation itself. For most of its history, the phenomena of vision were those based on observation of natural events like rainbows and reflections, together with the consequences of diseases like cataract and failing sight in old age. Thus, investigations of both the physics of light and the physiology of sight added to the range of visual phenomena, although these two areas were not initially distinguished. The psychological dimension was gradually integrated with these. Vision became an experimental discipline rather late. For most of its long history it has drawn upon observations of naturally occurring events, with relatively little in the way of manipulating the conditions under which they occurred. Colour led the way into the laboratory; when sunlight could be separated into its spectral components and recombined in a variety of ways the nature of light itself could be