In general however it may be taken as a working hypothesis that good combinations are always more than 80°-90° apart on the circle, that is, they should be separated from one another by about one quarter of the circumference. Even complementaries may form displeasing combinations (i. e., certain reds and greens), in which case, as Rood has pointed out, the hues are usually far removed from the line which separates those that are cold and warm. When we are compelled to appose hues having a hurtful influence on one another, the unpleasing impression which they create may be lessened by certain tricks, such as by assigning one of the hues to a much smaller field, or by decreasing the saturation of one of them, or by adding a third hue whose position on the chromatic circle is as far as possible removed from the others: thus the disagreeable effect of a yellowish-green and yellow is much improved by the addition of some violet, etc.
So far, for simplicity sake, we have regarded but one quality of a color, its hue, although in doing this it has been impossible entirely to neglect the closely related qualities of brightness and saturation. These we shall now proceed to consider.
Brightness is most marked, under ordinary conditions of illumination, around the yellow portions of the spectrum. It is a property which is exhibited in marked degree by different grays. Indeed it is measured by finding a gray which appears of equal brightness to that of a given color. Such measurements may be made with considerable accuracy by finding a gray background against which the color becomes indistinguishable when viewed by the very outermost portions of the retina which are color blind, that is, which see no hue in a color but only a grayness, the degree of which is proportional to the brightness of the color.[1] To make such comparisons, the person must regard a dot in the center of a plain black surface and must then gradually move a small piece of colored or of gray paper, mounted on a suitable handle, from the periphery towards the center of the surface. At a certain position the colored paper will be seen as gray because the rays of light from it are striking the color-blind areas of the retina. Various grays are used until one is found which matches exactly with that created by the colored paper. A still simpler method consists in rotating the color on a Maxwell disc along with a synthetic gray. In this case judgment of equality may however be somewhat confused, on account of the gray assuming the complementary hue.
Brightness plays a most important part in the phenomenon of con-
- ↑ The power to judge hue depends on the presence in the retina of peculiar nerve endings called cones. These are absent from the peripheral portions and only gradually make their appearance towards the center. There is, therefore, a region between the periphery and the center of the retina which is partly color blind, blue and yellow being perceptible, but red and green still appearing as gray.
