Figure 35-7 shows a different kind of color blindness, that of the protanope which has a focus near the red end of the boundary curve. Y ustoa gets approxi- mately the same position in this case. Using the three different kinds of color blindness, the three pigment response curves have finally been determined, and are shown in Fig. 35-8. Finally Perhaps. There is a question as to whether the three-pigment idea is right, whether color blindness results from lack of one pigment, and even whether the color-mix data on color blindness are right. Differ ent workers get different results. This field is still very much under development. 35-6 Physiochemistry of color vision Now, what about checking these curves against actual pigments in the The pigments that can be obtained from a retina consist mainly of a pigment called eye? curves of a normal trichromat's receptor visual purple. The most remarkable features of this are, first, that it is in the eye of almost every vertebrate animal, and second, that its response curve fits beauti- fully with the sensitivity of the eye, as seen in Fig. 35-9, in which are plotted on the same scale the absorption of visual purple and the sensitivity of the dark-adapted eye. This pigment is evidently the pigment that we see with in the dark: visual purple is the pigment for the rods, and it has nothing to do with color vision This fact was discovered in 1877. Even today it can be said that the color pigments % A of the cones have never been obtained in a test tube. In 1958 it could be said s that the color pigments had never been seen at all. But since that time two of them have been detected by rushton by a very simple and beautiful techI ight compared with light of low intensity, it needs a lot of visual purple to see with the pigment in the eye, and measure it anyway. What he does is this. There is an Fig. 35-9. The sensitivity curve of instrument called an opthalmoscope for sending light into the eye through the lens the dark-adapted eye, compared with and then focusing the light that comes back out. With it one can measure how the absorption curve of visual purple much is reflected. So one measures the reflection coefficient of light which has gone twice through the pigment (reflected by a back layer in the eyeball, and coming out through the pigment of the cone again). Nature is not always so beautifully designed. The cones are interestingly designed so that the light that comes into the cone bounces around and works its way down into the little sensitive points at the apex. The light goes right down into the sensitive point, bounces at the bottom and omes back out again, having traversed a considerable amount of the color-visior pigment;also, by looking at the fovea, where there are no rods, one is not confused by visual purple. But the color of the retina has been seen a long time ago: it is a sort of orangey pink; then there are all the blood vessels, and the color of the material at the back, and so on. How do we know when we are looking at the pigment? Answer: First we take a color-blind person, who has fewer pigments and for whom it is therefore easier to make the analysis. Second, the various pigments ke visual purple, have an intensity change when they are bleached by light; when we shine light on them they change their concentration. So, while looking at the bsorption spectrum of the eye, Rushton put another beam in the whole eye, which changes the concentration of the pigment, and he measured the change in the spectrum, and the difference, of course, has nothing to do with the amount of blood or the color of the reflecting layers, and so on, but only the pigment, and in this manner Rushton obtained a curve for the pigment of the protanope eye, which is given in Fig. 35-10. The second curve in Fig. 35-10 is a curve obtained with a normal was obtained by taking a normal eye and, having already determined what gment was, bleaching the other one in the red where the first one is insensitive Red light has no effect on the protanope eye, but does in the normal eye,and thus ne can obtain the curve for the missing pigment. The shape of one curve fits beautifully with Yustova's green curve, but the red curve is a little bit displaced So perhaps we are getting on the right track. Or perhaps not-the latest work with deuteranopes does not show any definite pigment missing