Vision is utterly different. Its information-handling capacity resides largely in the image's being captured by an array of millions receptors, at every instant. We take in the complex scene in a flash. If we wanted in addition to handle wavelength the way the ear does, the retina would need not only to have an array of receptors covering its surface, but to have, say, one thousand receptors for each point on the retina, each one with maximum sensitivity to a different wavelength. But to squeeze in a thousand receptors at each point is physically not possible. Instead, the retina compromises. At each of a very large number of points it has three different receptor types, with three different wavelength sensitivities. Thus with just a small sacrifice in resolution we end up with some rudimentary wavelength-handling ability over most of our retina. We see seven colors, not eighty-eight (both figures should be much higher!), but in a single scene each point of the many thousands will have a color assigned to it. The retina cannot have both the spatial capabilities that it has and also have the wavelength-handling capacity of the auditory system.
The next thing is to get some feel for what it means for our color vision to have three visual receptors. First, you might ask, if a given cone works better at some wavelengths than at others, why not simply measure that cone's output and deduce what the color is? Why not have one cone type, instead of three? It is easy to see why. With one cone, say the red, you wouldn't be able to tell the difference between light at the most effective wavelength, about 560 nanometers, from a brighter light at a less effective wavelength. You need to be able to distinguish variations in brightness from variations in wavelength.
But suppose you have two kinds of cones, with overlapping spectral sensif tivities—say, the red cone and the green cone. Now you can determine wavelength simply by comparing the outputs of the cones. For short wavelengths, the green cone will fire better; at longer and longer wavelengths, the outputs ' will become closer and closer to equal; at about 580 nanometers the red surpasses the green, and does progressively better relative to it as wavelengths get still longer. If we subtract the sensitivity curves of the two cones (they are logarithmic curves, so we are really taking quotients), we get a curve that is independent of intensity. So the two cones together now constitute a device that measures wavelength.
Then why are not two receptors all we need to account for the color vision that we have? Two would indeed be enough if all we were concerned with was monochromatic light—if we were willing to give up such things as our ability to discriminate colored light from white light. Our vision is such that no monochromatic light, at any wavelength, looks white. That could not be true if we had only two cone types. In the case of red and green cones, by progressing from short to long wavelengths, we go continuously from stimulating just the green cone to stimulating just the red, through all possible green-to-red response ratios. White light, consisting as it does of a mixture of all wavelengths, has to stimulate the two cones in some ratio. Whatever monochromatic wavelength happens to give that same ratio will thus be indistinguishable from white. This is exactly the situation in a common kind of color blindness in which the person has only two kinds of cones: regardless of which one of the three pigments is missing there is always some wavelength of light that the person cannot distinguish from white. (Such subjects are color defective, but certainly not color-blind.)
To have color vision like ours, we need three and only three cone types. The conclusion that we indeed have just three cone types was first realized by examining the peculiarities of human color vision and then making a set of deductions that are a credit to the human intellect.
We are now in a better position to understand why the rods do not mediate color. At intermediate levels of light intensity, rods and cones can both be functioning, but except in rare and artificial circumstances the nervous system seems not to subtract rod influences from cone influences. The cones are compared with one another; the rods work alone. To satisfy yourself that rods do not mediate color, get up on a dark moonlit night and look around. Although you can see shapes fairly well, colors are completely absent. Given the simplicity of this experiment it is remarkable how few people realize that they do without color vision in dim light.
Whether we see an object as white or colored depends primarily (not entirely) on which of the three cone types are activated. Color is the consequence of unequal stimulation of the three types of cones. Light with a broad spectral curve, as from the sun or a candle, will obviously stimulate all three kinds of cones, perhaps about equally, and the resulting sensation turns out to be lack of color, or "white". If we could stimulate one kind of cone by itself (something that we cannot easily do with light because of the overlap of the absorption curves), the result, as already mentioned, would be vivid color—violet, green, or red, depending on the cone stimulated. That the peak sensitivity of what we call the "red cone" is at a wavelength (560 nanometers) that appears to us greenish-yellow is probably because light at 560 nanometers excites both the green-sensitive cone and the red-sensitive cone, owing to the overlap in the green- and red-cone curves. By using longer wavelength light we can stimulate the red cone, relative to the green one, more effectively.
The graphs on the facing page sum up the color sensations that result when various combinations of cones are activated by light of various wavelength compositions.
The first example and the last two should make it clear that the sensation "white"—the result of approximately equal stimulation of the three cones—
can be brought about in many different ways: by using broad-band light or by using a mixture of narrow-band lights, such as yellow and blue or red and blue-green. Two beams of light are called complementary if their wavelength content and intensities are selected so that when mixed they produce the sensation "white". In the last two examples, blue and yellow are complementary, as are red at 640 nanometers and blue-green.
THEORIES OF COLOR VISION
The statements I have made about the relationship between what cones are stimulated and what we see depend on research that began with Newton in 1704 and continues up to the present. The ingenuity of Newton's experiments is hard to exaggerate: in his work on color, he split up white light with a prism; he recombined the light with a second prism, obtaining white again; he made a top consisting of colored segments, which when spun gave white. These discoveries led to the recognition that ordinary light is made up of a continuous mixture of light of different wavelengths.
Gradually, over the eighteenth century, it came to be realized that any color could be obtained by mixtures of light of three wavelengths in the right proportions, provided the wavelengths were far enough apart. The idea that any color could be produced by manipulating three controls (in this case, controls of the intensity of the three lights) was termed trichromacy. In 1802 Thomas Young put forward a clear and simple theory to explain trichromacy: he proposed that at each point in the retina there must exist at least three "particles"—
tiny light-sensitive structures—sensitive to three colors, red, green, and violet. The long time span between Newton and Young is hard to explain, but various roadblocks, such as yellow and blue paints mixing to produce green, must surely have impeded clear thinking. The definitive experiments that finally proved Young's idea that color must depend on a retinal mosaic of three kinds of detectors was finally confirmed directly and conclusively in 1959,
The top graph, "cone sensitivities", repeats the graph on page the previous. The rest of the figure suggests which cones will be activated by various mixtures of colored light and what the resulting sensations will be.