Chapter 9 - Color:
Simultaneous Contrast

In order to simplify the presentation of the general subject, discussions of color have been omitted in so far as possible from the preceding chapters. There are almost numberless phenomena involving color, many of which are optical illusions, or seemingly so. It will be obvious that many are errors of sense; some are errors of judgment; others are errors due to defects of the optical system of the eye; and many may be ascribed to certain characteristics of the visual process. It is not the intention to cover the entire field in detail; indeed, this could not be done within the confines of a large volume. However, substantial glimpses of the more important phases of color as related to optical illusions are presented in this chapter. In the early chapters pertaining to the eye and to vision some of the following points were necessarily touched upon, but the repetition in the paragraphs which follow is avoided as much as possible.

Simultaneous Contrast. - That the life of color is due to contrast is demonstrable in many ways. If a room is illuminated by deep red light, at first this color is very vivid in consciousness; however, gradually it becomes less saturated. After a half hour the color is apparently a much faded red but upon emerging from the room into one normally lighted, the latter appears very markedly greenish in tint. The reason that the pure red light does not appear as strongly colored as it really is, is due to the lack of contrast. In a similar manner at night we see white objects as white even under the yellowish artificial light. The latter appears very yellow in color when it is first turned on as daylight wanes but as darkness falls and time elapses it gradually assumes a colorless appearance.

An apparatus constructed after the plan of Fig. 62 is very effective for demonstrating the remarkable effects of color-contrast but some additions will add considerably to its convenience. If the lamps F are divided into three circuits, each emitting, respectively, red, green, and blue primary colors, it is possible by means of controlling rheostats to illuminate E, the environment, with light of any hue (including purple), of any saturation, and of a wide range of intensities or resulting brightnesses. Thus we have a very simple apparatus for quickly providing almost numberless environments for H. The same scheme can be applied to lamps R, with the result that a vast array of colors may be seen through the hole H. If the hole is the shape of the star in Fig. 66 it will be found very effective. The observer will actually see a star of any desired color amid an environment of any desired color. Care should be taken to have the star cut in very thin material in order to eliminate conspicuous boundary lines. It is quite satisfactory to use a series of colored papers on a slide at C and ordinary clear lamps at R. By means of this apparatus both contrasts - hue and brightness - may be demonstrated. Of course, for black and white only brightness-contrast is present; but in general where there is color-contrast there is also brightness-contrast. The latter may be reduced or even eliminated if the brightness of the star and of its surroundings are made equal, but it is difficult to make a satisfactory balance in this respect. Assuming, however, that brightness-contrast is eliminated, we have left only hue and saturation contrast, or what will be termed (rather loosely, it is admitted) color-contrast.

Fig. 66. - An excellent pattern for demonstrating color-contrast.

If, for example, an orange star is seen by itself in the midst of dark surroundings it will not appear very colorful. However, if the surroundings are now made bright with white light, the star appears quite saturated. With blue or green light the orange star appears even more intensely orange, but when the color-contrast is reduced, as in the case of yellow or red surroundings, the vividness of the orange star again decreases. This may be summarized by stating that two widely different colors viewed in this manner will mutually affect each other so that they appear still more different in hue. If their hues are close together spectrally this effect is not as apparent. For example, if orange and green are contrasted, the orange will appear reddish in hue and the green will appear bluish.

Let us now assume the star to be white, and that the surroundings are of any color of approximately the same brightness. The star which is really white will now appear decidedly tinted and of a hue approximately complementary to that of the surroundings. When the latter are of a green color the white star will assume a purplish tinge; when red the white star will appear of a blue-green tint; when yellow the white star will appear bluish. This is an optical illusion in any sense of the term.

The strength of this optical illusion caused by simultaneous contrast is very remarkable. For example, if a grayish purple star is viewed amid intense green surroundings it will appear richly purple, but when the surroundings are changed to a rich purple the grayish purple star will even appear greenish. The apparent change of a color to its complementary by merely altering its environment is really a remarkable optical illusion.

The importance of simultaneous contrast is easily demonstrated upon a painting by isolating any colored object from its surroundings by means of a hole in a gray card. For example, an orange flower-pot amid the green foliage of its surroundings will appear decidedly different in color and brightness than when viewed through a hole in a white, black, or gray cardboard. By means of colored papers the same color may be placed in many different environments and the various contrasts may be viewed simultaneously. The extent of the optical illusion is very evident when revealed in this simple manner. However, too much emphasis cannot be given to Figs. 62 and 66 as a powerful means for realizing the greatest effects.

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