Color vision 35-1 The human eye The phenomenon of colors depends partly on the physical world. We discuss 5-1 The human eye the colors of soap films and so on as being produced by interference. But also of course, it depends on the eye, or what happens behind the eye, in the brain 35-2 Color depends on intensity Physics characterizes the light that enters the eye, but after that, our sensations are 35-3 Measuring the color sensation the result of photochemical-neural processes and psychological responses 35-4 The chromaticity diagram There are many interesting phenomena associated with vision which involve a mixture of physical phenomena and physiological processes, and the full appreci- 35-5 The mechanism of color vision ation of natural phenomena, as we see them, must go beyond physics in the usual sense. We make no apologies for making these excursions into other fields, because 35-6 Physiochemistry of color vision the separation of fields, as we have emphasized, is merely a human convenience, and an unnatural thing. Nature is not interested in our separations, and many of the interesting phenomena bridge the gaps between fields In Chapter 3 we have already discussed the relation of physics to the other ornea sciences in general terms, but now we are going to look in some detail at a specific ris field in which physics and other sciences are very, very closely interrelated. That area is vision. In particular, we shall discuss color vision. In ent chapter we shall discuss mainly the observable phenomena of human and in the next chapter we shall consider the physiological aspects of vision, both in man and gament It all begins with the eye; so, in order to understand what phenomena we see, Vitreousiihumor some knowledge of the eye is required. In the next chapter we shall discuss in Choroid Reting ome detail how the various parts of the eye work, and how they are interconnected with the nervous system. For the present, we shall describe only briefly how the eye functions(Fig. 35-1) t enters the eye through the cornea; we have already discussed how it is Macula lutea bent and is imaged on a layer called the retina in the back of the eye, so that different optic nerve parts of the retina receive light from different parts of the visual field outside. The retina is not absolutely uniform: there is a place, a spot, in the center of our field 35-1.The of view which we use when we are trying to see things very carefully, and at which we have the greatest acuity of vision; it is called the fovea or macula. The side parts of the eye, as we can immediately appreciate from our experience in looking at things, are not as effective for seeing detail as is the center of the eye. There is also a spot in the retina where the nerves carrying all the information run out; that is a blind spot. There is no sensitive part of the retina here, and it is possible to demonstrate that if we close, say, the left eye and look straight at something, and then move a ger or another small object slowly out of the field of view it suddenly disappears omewhere. The only practical use of this fact that we know of is that some physiol ogist became quite a favorite in the court of a king of france by pointing this out to him; in the boring sessions that he had with his courtiers, the king could amuse himself by"cutting off their heads" by looking at one and watching another's head disappear Figure 35-2 shows a magnified view of the inside of the retina in somewhat schematic form. In different parts of the retina there are different kinds of struc tures. The objects that occur more densely near the periphery of the retina are called rods. Closer to the fovea, we find, besides these rod cells, also cone cells We shall describe the structure of these cells later. As we get close to the fovea, the number of comes increases, and in the fovea itself there are in fact nothing but cone Fig. 35-2. the structure of the retin cells, packed very tightly, so tightly that the cone cells are much finer, or narrower ( Light enters from below 35