is due to some other process than a simple mixture like a chord in music, where the three notes are there at the same time and if we listen hard we can hear them individually. We cannot look hard and see the red and the green The earliest theories of vision said that there are three pigments and three kinds of cones, each kind containing one pigment; that a nerve runs from each cone to the brain, so that the three pieces of information are carried to the brain and then in the brain, anything can happen. This, of course, is an incomplete idea it does no good to discover that the information is carried along the optic nerve to the brain, because we have not even started to solve the problem. We must ask more basic questions: Does it make any difference where the information is put together? Is it important that it be carried right up into the brain in the optic nerve, or could the retina do some analysis first? We have seen a picture of the retina as an extremely complicated thing with lots of interconnections(Fig. 35-2) and it might make some analyses As a matter of fact, people who study anatomy and the development of the eye have shown that the retina is, in fact, the brain: in the development of the em- bryo, a piece of the brain comes out in front, and long fibers grow back, con- necting the eyes to the brain. The retina is organized in just the way the brain is organized and, as someone has beautifully put it, "The brain has developed a way to look out upon the world. The eye is a piece of brain that is touching light, so to speak, on the outside. So it is not at all unlikely that some analysis of the color has already been made in the retina This gives us a very interesting opportunity. None of the other senses involves uch a large amount of calculation, so to speak, before the signal gets into a nerve that one can make measurements on. The calculations for all the rest of the senses usually happen in the brain itself, where it is very difficult to get at specific places to make Its, because there are so many interconnections. Here, with the light, three layers of cells ma the results of the calculations being transmitted through the optic nerve. So we have the first chance to observe physiologically how, perhaps, the first layers Photochemical Absorptions of the brain work in their first steps. It is thus of double interest, not simply interesting for vision, but interesting to the whole problem of physiology yk1《r-2 The fact that there are three pigments does not mean that there must be three kinds of sensations. One of the other theories of color vision has it that there are really opposing color schemes( Fig. 36-2). That is, one of the nerve fibers carries a lot of impulses if there is yellow being seen, and less than usual for blue. Another Fig. 36-2. Neural connections ac- nerve fiber carries green and red information in the way, and another, white cording to an opponent "theory of and black. In other words, in this theory someone has already started to make a color vision guess as to the system of wiring, the method of calculation The problems we are trying to solve by guessing at these first calculations are questions about the apparent colors that are seen on a pink background, what hap- pens when the eye is adapted to different colors, and also the so-called psychological phenomena. The psychological phenomena are of the nature for instance, that hite does not"feel"like red and yellow and blue, and this theory was advanced because the psychologists say that there are four apparent pure colors: There are four stimuli which have a remarkable capacity to evoke psychologically simple blue, yellow, green, and red hues respectively. Unlike sienna, magenta, purple, or most of the discriminable colors, these simple hues are unmixed in the sense that none partakes of the nature of the other; specifically, blue is not yellowish, reddish or greenish, and so on; these are psychologically primary hues. That is a psycho- logical fact, so-called. To find out from what evidence this psychological fact was deduced, we must search very hard indeed through all the literature: In the modern literature all we find on the subject are repeats of the same statement, or of one by a german psychologist, who uses as f his authorities Leonardo da vinci who of course, we all know was a great artist. He says, "Leonardo thought there were five colors. "Then, looking still further, we find, in a still older book, the evidence for the subject. The book says something like this: "Purple is reddish-blue, orange is reddish-yellow, but can red be seen as purplish- orange? Are not red and yellow more unitary than purple or orange? The average person, asked to state which