Let us imagine that you arrive late at an afternoon movie. As you move suddenly from the bright illumination outside to the dim illumination inside, you are temporarily blind. After a few minutes you can see relatively well and the invisible legs you stumbled over on your way to your seat are now clearly visible. On getting into your car after the movie, you notice that, in the dim illumination of the evening, your red car is relatively dimmer and the blue car beside it is relatively brighter. You drive to a quiet spot to look at the stars and notice that it is easier to see a dim star by looking at it out of the corner of your eye than by looking at it directly.

It's not as easy to observe the physiological facts. However, if you were able to look very closely into the eyes of your date, you would notice that there are short, fat cells concentrated at the centre (cones) and long thin cells concentrated at the periphery (rods). The rods are composed of a chemical called iodopsin which is maximally sensitive to light with a wavelength of 560 millimicrons; the cones are composed of a chemical called rhodopsin which is maximally sensitive to a wavelength of 510 millimicrons.

Different structures imply different functions. We do indeed have two visual systems - one involving the cones for bright illumination and the other involving the rods for dim illumination. This is the duplicity theory. Diurnal organisms require only the bright illumination system and indeed chickens have only cones; nocturnal organisms require only the dim illumination system and indeed owls have only rods. Since our species roams around day and night, we require both systems. Your temporary blindness in the movie theatre is due to the fact that it takes time to switch from the bright to the dim illumination system. This shift is called dark adaptation.^₇ The relative dullness of the red car in dim illumination is due to the fact that the rods are more sensitive to the lower end of the spectrum than the cones. The apparently incongruous fact that dim stars are easier to see out of the corner of your eye is due to the fact that light is thus focused on the periphery of your retina where the rods are.

If the world were perceived as a black and white movie, then the duplicity theory would be adequate. However, since the world is seen in Technicolor, it is necessary to go beyond the duplicity theory to a supplementary theory. This theory must explain not only the brightness dimension from black to white but the hue dimension of the visual spectrum and the saturation dimension from the most vivid to the most insipid color. The relationship between those three dimensions is summarized in the color cone, with brightness as the axis, hue as the circumference, and saturation as the radius (Figure 7-5a). All the myriad colors in our world fall somewhere within this cone. The location of each color can be pinpointed by finding its position on each of those three dimensions.

The best candidate to supplement duplicity theory is the Young-Helmholtz theory that the cones are further divided into three types maximally sensitive to blue, green, and red respectively (see Figure 7-5b). By varying light along the wavelength dimension, we generate the corresponding psychological dimension of hue. By varying from light which stimulates only one receptor to light which stimulates all three receptors equally, we generate the corresponding psychological dimension of saturation. By varying from light which provides maximum stimulation to all three receptors to light which provides minimum stimulation of all three receptors, we generate the corresponding psychological dimension of brightness.