Fig. 35-3. The spectral sensitivity of the eye. Dashed curve, rods; solid curve cones Another effect of the fact that rods take over in the dark, and that there are lo rods in the fovea, is that when we look straight at something in the dark,our vision is not quite as acute as when we look to one side , a faint star or nebula can sometimes be seen better by looking a little to one side than directly at it, because we do not have sensitive rods in the middle of the fovea Another interesting effect of the fact that the number of cones decreases as we go farther to the side of the field of view is that even in a bright light color disappears as the object goes far to one side. The way to test that is to look in some particular fixed direction, let a friend walk in from one side with colored cards, and try to decide what color they are before they are right in front of you One finds that he can see that the cards are there long before he can determine the olor. When doing this, it is advisable to come in from the side opposite the blind spot, because it is otherwise rather confusing to almost see the color, then not see anything, then to see the color again Another interesting phenomenon is that the periphery of the retina is very sensitive to motion. Although we cannot see very well from the corner of our eye if a little bug moves and we do not expect anything to be moving over there e are immediately sensitive to it. We are all "wired up"to look for something jiggling to the side of the field 35-3 Measuring the color sensation Now we go to the cone vision, to the brighter vision, and we come to the question which is rhost characteristic of cone vision and that is color. As we know, white light can be split by a prism into a whole spectrum of wavelengths which appear to us to have different colors that is what colors are, of course appearances. Any source of light can be analyzed by a grating or a prism,and one can determine the spectral distribution, i.e, the"amount"of each wavelength A certain light may have a lot of blue, considerable red, very little yellow, and so on. That is all very precise in the sense of physics, but the question is, what color will it appear to be? It is evident that the different colors depend somehow the spectral distribution of the light, but the problem is to find what characteristics of the spectral distribution produce the various sensations. For example, what do we have to do to get a green color? We all know that we can simply take a piece of the spectrum which is green. But is that the only way to get green, or orange, or any other color? Is there more than one spectral distribution which produces the same apparent visual effect? The answer is, definitely yes. There is a very limited number of visual effects, in fact just a three-dimensional manifold of them, as we shall shortly see, but there is an infinite number of different curves that we can draw for the light that comes from different sources. Now the question we have to discuss is, under what conditions do different distributions of light appear as exactly the same color 35-3