To produce the large number of possible color tones it is not necessary to have recourse to a corresponding number of wave-lengths; for, given a small- number of tones, properly chosen, it is possible to produce all the others by the process of color-mixture. One of the most common methods of mixing colors is by the use of pigments; another is by rapid rotation of the color-wheel, which carries two or more sectors of colored paper or cardboard. At a certain rate of revolution of the heel the ob server ceases to see the separate sectors and sees instead a single homogenous color which is dif ferent from either of the sectors used. The tone of the ((mixed') color depends upon the re lation of the colors mixed. If tv%o colors stand near each other in the spectrum, the "mixed') color is intermediate: for example, red and low mixed give an orange; violet and red, a purple. There is, however, for each color, a °complementary" color which when mixed vdth it in a certain proportion gives — not a color tone at all, tut —a pure gray or white. It should be noted that, in every case, it is stinuili that are mixed and not sensations; that the mixture is of physical or physiological processes not mental processes. A 'mixed" color is as simple, psychologically, w, any other.
Visual sensation depends not only upon the light stimulus to which the eye is exposed and upon the mixture of stimuli; it depends also upon (a) the part of the retina stimulated, (b) the stimulation of adjacent areas and (c) cer tain more or less permanent effects left by the stimulus upon the visual organ.
(a) It is only in the central part of the retina— the part lying about the fovea or spot of clearest vision — that all the colors are seen; further out toward the periphery- of the retina lies a zone in which reds and greens are not sensed (only blues and yellows), and, still fur ther out, an outermost zone whose stimulation gives rise to brightness sensations only. That is to say, the normal eye is totally color-blind over a part of its sensitive area and partially color-blind over a second part. Abnormal color blindness, then, which is relatively common in the race, marbe regarded— at least in its com moner forms — as an extension to the fovea of the normal color-blindness of the normal eye.
The eye is a chemical sense; that is, chemical processes are interpolated between the reception by the eye of light-waves and the neural proc esses that are ultimately set into function by them. Two special results follow from this fact. First, stimulation is not strictly confined to the part of the retina directly affected by light; and, secondly, nervous excitation continues after the external stimulus has ceased to operate. (b) The first of these results, retinal irradiation of chemical or photo-chemical processes, is strik ingly demonstrated by the phenomena of con trast A patch of white looks whiter if it is placed on a black surface; black looks blacker in the neighborhood of white. In a similar
manner, the saturation of a color (for example, red) is enhanced if it is brought near a com plementary color (blue-green) • and, finally, a gray in the immediate neighbo;hood of a color (for example, green) is tinged, under certain conditions, with the complementary of that color (purplish red). The last case, induction of a contrast color upon a gray, niay be observed in the shadows of snow, which often look blue under yellow sunlight. The general effects, both of brighrness and of color contrast, are, more over,. frequently to be seen in clothing and in interior decorations. The fundamental law of contrast is that the contrast effect is always in the direction of the opposite brightness or of the complementary color. (c) The result of continued stimulation -of the eye is to change the condition of excitability of that organ. If large colored glasses be worn before the eyes for several minutes the unnatural hue of ob jects, which is at first very noticeable, gradu ally disappears. With blue glasses, objects at first look bluish, but finally return to their proper tones. The eyes are said to have be come blue-adapted. A similar effect is produced if only a small part — not the whole of the retina— is exposed for some moments to colored light. A piece of red paper, for ex ample, hung on a gray wall and fixated steadily gradually loses its saturation and approaches a gray. But not only is the excitability of the eye altered under continued stimulation; it re mains altered, as noted above, even after the exciting cause has ceased to operate. When the blue glasses are removed the landscape looks yellowish; the observer is yellow-sighted. Sim ilarly, when the red paper is removed, a bluish green patch appears upon the gray wall. The first result is due to general adaptation, the second to local adaptation. In both instances, adaptation forms a predisposition for seeing colors which are complementary to the original, stimulating colors. The result of local adapta tiOn (as the bluish-green patch induced by the reddish paper) is a negative after-image. Adaptation and after-images follow the same course with brightnesses as with colors. It is, for example, a matter of common observa tion that confinement in a darkened room en hances the brightness of objects seen subse quently in full light ; that exposure to a strong light (sunlight on snow) tends to reduce the apparent bnghtness of objects seen afterward in moderate illumination, and, finally, that fixa tion of a dark or a light area, induces a nega tive after-image of the opposite brightness. All these phenomena illustrate the effects of adapta tion.