Mechanisms of Seeing 36-1 The sensation of color In discussing the sense of sight, we have to realize that(outside of a gallery 36-1 The sensation of color of modern art! )one does not see random spots of color or spots of light. When 36-2 The physiology of the eye we look at an object we see a man or a thing; in other words, the brain interprets what we see. How it does that, no one knows, and it does it, of course, at a very 36-3 The rod cells high level. Although we evidently do learn to recognize what a man looks like after much experience, there are a number of features of vision which are more 36-4 The compound (insect)ey elementary but which also involve combining information from different parts 36-5 Other eyes what we see. To help us understand how we make an interpretation of an entire image, it is worth while to study the earliest stages of the putting together of in 36-6 Neurology of vision formation from the different retinal cells. In the present chapter we shall concen trate mainly on that aspect of vision, although we shall also mention a number of side issues as we go along An example of the fact that we have an accumulation, at a very elementary level, of information from several parts of the eye at the same time, beyond our voluntary control or ability to learn, was that blue shadow which was produced by white light when both white and red were shining on the same screen. This effect at least involves the knowledge that the background of the screen is pink, even though, when we are looking at the blue shadow, it is only"white"light coming into a particular spot in the eye; somewhere, pieces of information have been put together. The more complete and familiar the context is, the more the eye will make corrections for peculiarities. In fact, Land has shown that if we mix that apparent blue and the red in various proportions, by using two photographic transparencies with absorption in front of the red and the white in different pro- portions, it can be made to represent a real scene, with real objects, rather faithfully In this case we get a lot of intermediate apparent colors too, analogous to what we would get by mixing red and blue-green; it seems to be an almost complete set of olors, but if we look very hard at them, they are not so very good. Even so, it is surprising how much can be obtained from just red and white. The more the scene looks like a real situation, the more one is able to compensate for the fact that all the light is actually nothing but pink! Another example is the appearance of"colors"in a black-and-white rotating isc, whose black and white areas are as shown in Fig. 36-1. when the disc is rotated, the variations of light and dark at any one radius are exactly the same; it g.36-1 Sc only the background that is different for the two kinds of"stripes. "Yet one of above is spun, colors appear in the"rings"appears colored with one color and the other with another. No one of the two darker"rings. "If yet understands the reason for those colors, but it is clear that information is being direction is reversed, the colors m卿m put together at a very elementary level, in the eye itself, most likely in the other ring Almost all present-day theories of color vision agree that the color-mixing data indicate that there are only three pigments in the cones of the eye, and that it is the spectral absorption in those three pigments that fundamentally produces he color sense. But the total sensation that is associated with the absorption characteristics of the three pigments acting together is not necessarily the sum of the individual sensations. We all agree that yellow simply does not seem to be reddish green; in fact it might be a tremendous surprise to most people to discover that light is, in fact, a mixture of colors, because presumably the sensation The colors depend on speed of rotation, on the brightness of illumination, and to some extent on who looks at them and how intently he stares at them