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when two groups, George Wald and Paul Brown at Harvard and William Marks, William Dobelle, and Edward MacNichol at Johns Hopkins, examined microscopically the abilities of single cones to absorb light of different wavelengths and found three, and only three, cone types. Meanwhile, scientists had had to do the best they could by less direct means, and they had, in fact, in the course of several centuries arrived at substantially the same result, proving Young's theory that just three types of cones were necessary and estimating their spectral sensitivities. The methods were mainly psychophy steal: scientists learned what colors are produced with various mixtures of monochromatic lights, they studied the effects on color vision of selective bleaching with monochromatic lights, and they studied color blindness.
Studies of color mixing are fascinating, partly because the results are so surprising and counterintuitive. No one without prior knowledge would ever guess the various results shown in the illustration on page 42—for example, that two spots, one vivid blue and the other bright yellow, when overlapped would mix to produce a white indistinguishable to the eye from the color of chalk or that spectral green and red would combine to give a yellow almost indistinguishable from monochromatic yellow.
Before discussing other theories of color, I should perhaps say more about the variety of colors that theories must account for. What colors are there besides the colors in the rainbow? I can think of three. One kind is the purples, which we don't find in rainbows, but which result from stimulating the red and blue cones, that is, from adding long- and short-wavelength light, or, loosely, red and blue light. If to a mixture of spectral red and blue lights—to purple—we add the right amount of the appropriate green, we get white, and so we say that the green and purple are complementary. You can, if you like, imagine a circular dial that gives all the spectral colors from red through yellow and green to blue and violet, then purples, first bluish-purple and then reddish-purple, and finally back to red. You can even arrange these hues so that complements arc opposite each other. The concept of primary colors does not even enter this scheme: if we think of primaries in terms of the three receptor types, we have greenish-yellow, green, and violet, shades hardly consistent with the idea of three pure, basic colors. But if by primary we mean three colors from which any other hues can be generated, these three will do, as will any other three that are far enough apart. Thus nothing I have said so far gives any justification for the idea of three unique primary colors.
A second kind of color results from adding white to any spectral color or to purple; we say that the white "washes out" the color, or makes it paler—the technical term is that it desaturates it. To match any two colors, we have to make their hues and saturations the same (for example, by selecting the appropriate position on the circle of colors and then adding the right amount of white), and then we need to equate the intensities. Thus we can specify a color by giving the wavelength of the color (or in the case of purple, its complement), the relative content of white, and a single number specifying intensity.
A mathematically equivalent option for specifying color is to give three numbers representing the relative effects of the light on the three cone types. Either way, it takes three numbers.
A third kind of color these explanations do not cover is typified by brown. I will come to it later.
Young's theory was adopted and championed by Hermann von Helmholtz and came to be known as the Young-Helmholtz theory. It was Helmholtz, incidentally, who finally explained the phenomenon mentioned at the beginning of this chapter, that mixing yellow and blue paints gives green. You can easily see how this differs from adding yellow and blue light by doing the following experiment, for which you need only two slide projectors and some yellow and blue cellophane. First, put the yellow cellophane over the lens of one projector and the blue over the other and then overlap the projected images. If you adjust the relative intensities, you will get a pure white in the area of overlap. This is the kind of color mixing we have been talking about, and we have said that the white arises because the combined yellow and blue light manages to activate all three of our cones with the same relative effectiveness that broad-band, or white, light does. Now turn off one projector and put both filters in front of the other one, and you will get green. To understand what is happening we need to know that the blue cellophane absorbs longwavelength light, the yellows and reds, from the white and lets through the rest, which looks blue, and that the yellow filter absorbs mainly blue and lets through the rest, which looks yellow. The diagram on the facing page shows the spectral composition of the light each filter passes. Note that in both cases the light that gets through is far from monochromatic, the yellow light is not narrow-band spectral yellow but a mixture of spectral yellow and shorter wavelengths, greens, and longer wavelengths, oranges and reds. Similarly, the blue is spectral blue plus greens and violet. Why don't we see more than just yellow or just blue? Yellow is the result of equal stimulation of the red and the green cones, with no stimulation of the blue cone; this stimulation can be accomplished with spectral yellow (monochromatic light at 580 nanometers)
or with a broader smear of wavelengths, such as we typically get with pigments, as long as the breadth is not so great as to include short wavelengths and thereby stimulate the blue cone. Similarly, as far as our three cones are concerned, spectral blue light has about the same impact as blue plus green plus violet. Now, when we use the two filters, one in front of the other, what we get is what both filters let through, namely, just the greens. This is where the graphs shown on this page, for broad-band blue and yellow, overlap. The same thing happens with paints: yellow and blue paints together absorb everything in the light except greens, which are reflected. Note that if we used monochromatic yellow and blue filters in our experiment, putting one in front of the other would result in nothing getting through. The mixing works only because the colors produced by pigments have a broad spectral content.
Why discuss this phenomenon here? I do so partly because it is gratifying to explain the dramatic and startling result of mixing yellow and blue paint to get green, and the even more startling result—because it is so unfamiliar to most people—of mixing yellow and blue light to get white. (In a chapter on color theory in a book on weaving, I found the statement that if you mix yellow and blue threads, as in warp and weft, you get green. What you do get is gray—
for biological reasons.) The artificial results of mixing paints is doubtless what has led to the idea of "primary colors," such as red, yellow, and blue. If any special set of colors deserves to be called primary, it is the set of red, blue, yellow, and green. As we will see in the section on Hering's color theory, what justification all four have as candidates for primaries has little to do with the three cones and much to do with the subsequent wiring in the retina and brain.



                   THE GENETICS OF VISUAL PIGMENTS
In the early 1980s Jeremy Nathans, while still an MD-Ph D student at Stanford, managed to clone the genes for the protein portions of human rhodopsin and all three cone pigments. He found that all four pigments show strong homologies in their amino acid sequences: the genes for the red and green pigments, which lie on the X, or sex, chromosome, are virtually identical—the amino acid sequences of the proteins show 96 percent identity—
whereas the genes that code for the blue pigment, on chromosome 7, and for

   
 




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The blue filter passes a fairly broad band of wavelengths centered about 480 nanometers. The yellow filter passes a fairly broad band of wavelengths centered about 580 nanometers. Both together pass only wavelengths common to the two—light at a fairly broad band of wavelengths centered about 530, which gives a green.




With three slide projectors and three filters, three overlapping spots (red, green, and blue) are projected onto a screen so that they overlap. Red and green give yellow, blue and green give turquoise, red and blue give purple, and all three—red, blue, and green—give white.

 
 
 
 
 


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