colors are unitary, names red, yellow, and blue, these three and some observers add a fourth, green. Psychologists are accustomed to accept the four as salient hues. So that is the situation in the psychological analysis of this matter: if every- body says there are three, and somebody says there are four, and they want it to be four, it will be four. That shows the difficulty with psychological researches It is clear that we have such feelings, but it is very difficult to obtain much informa tion about them So the other direction to go is the physiological direction, to find out experi mentally what actually happens in the brain, the eye the retina, or wherever, and perhaps to discover that some combinations of impulses from various cells move along certain nerve fibers. Incidentally, primary pigments do not have to be in separate cells; one could have cells in which are mixtures of the various pigments cells with the red and the green pigments, cells with all three(the information of all three is then white information), and so on. There are many ways of hooking the system up, and we have to find out which way nature has used. It would be hoped, ultimately, that when we understand the physiological connections we will have a little bit of understanding of some of those aspects of the psychology, so we look in that direction 36-2 The physiology of the eye We begin by talking not only about color vision, but about vision in general just to remind ourselves about the interconnections in the retina, shown in Fi 35-2. The retina is really like the surface of the brain. Although the actual picture through a microscope is a little more complicated looking than this somewhat schematized drawing, by careful analysis one can see all these interconnections There is no question that one part of the surface of the retina is connected to other parts, and that the information that comes out on the long axons, which produce the optic nerve, are combinations of information from many cells. There are three layers of cells in the succession of function there are retinal cells which are the ones that the light affects, an intermediate cell which takes information from a single or a few retinal cells and gives it out again to several cells in a third layer of cells and carries it to the brain. There are all kinds of cross connections between cells in the layers We now turn to some aspects of the structure and performance of the eye (see Fig 35-1). The focusing of the light is accomplished mainly by the cornea by the fact that it has a curved surface which"bends"the light. This is why we cannot see clearly under water, because we then do not have enough difference between the index of the cornea, which is 1.37, and that of the water. which is 1.33 Behind the cornea is water, practically, with an index of 1.33, and behind that is a lens which has a very interesting structure: it is a series of layers, like an onion, except that it is all transparent, and it has an index of 1. 40 in the middle and 1.38 at the outside. (It would be nice if we could make optical glass in which we could adjust the index throughout, for then we would not have to curve it as much as we do when we have a uniform index. ) Furthermore, the shape of the cornea is not that of a sphere. A spherical lens has a certain amount of spherical aberration The cornea is"flatter"at the outside than is a sphere, in just such a manner that he spherical aberration is less for the cornea than it would be if we put a spherical lens in there! The light is focused by the cornea-lens system onto the retina. As we look at things that are closer and farther away, the lens tightens and loosens and changes the focus to adjust for the different distances. To adjust for the total ￠ amount of light there is the iris, which is what we call the color of the eye, brown or blue, depending on who it is; as the amount of light increases and decreases, the iris moves in and out Fig. 36-3. The neural interconnect- of the lens, the motion of the eye, the muscles which turn the eye in the socket, and the eyes *he mechanical operation of Let us now look at the neural machinery for controlling the accommodation tions for the iris, shown schematically in Fig. 36-3. Of all the information that comes out of the optic nerve A, the great majority is divided into one of two bundles(which we will talk about later)and thence to the brain. but there are a few fibers, of 36-3