Very instructive experiments may be made by putting together a colored disc with a black, a white or a gray one. The brightness or luminosity of a color may be diminished by ro tating black with it; this produces a shade such as would be seen in lessened illumination but does not reduce the hue or the saturation. The saturation may be lowered by using a white disc; the resulting colors arc tints. But if a neutral gray and a colored disc of equal lumi nosities are rotated simultaneously the satura tion will be lowered without change of hue or brightness. In short, black absorbs all colors and therefore changes only the amount of light reflected f rom the composite disc; white re flects all and, in effect, adds the physiological primaries to the disc; gray may be regarded as a mixture of black and white (or also, as will be seen presently, of other pairs) and is thus more luminous than black and more saturated than white. Ogden Rood succeeded in match ing almost 500 colors with combinations formed of seven discs painted with the following water color pigments, the numbers in parentheses being the wave lengths, in microns, of the cor responding spectral hues: matt white (zinc oxide), matt black (lampblack), vermilion (0.644), mineral orange (0.614), light chrome yellow (0.585), emerald green (0.521), artificial ultramarine (0.425). His formulas are given in the 'Standard Dictionary' under spectrum.
The wave lengths of the Bradley papers are: red, 0.657; orange, 0.608; yellow, 0.579; green, 0.516; blue, 0.469; violet, 0.421. Consult also Bradley, Milton, 'Elementary Color) (1895).
Some pairs of discs produce gray which must be regarded as a shade of white, or so to say, as a dark white, due to the comparatively low luminosity of the pigments; the correspond ing spectral hues, being monochromatic and of greater brightness, produce real white. Two colors whose white are called physiological complementaries; their number is unlimited. Physiological complements depend of course upon the source of illumination, for if the subdued white which they produce in daylight be examined in some other illuminant it will partake of its color. Additive combina tions in general tend toward white; if actual lights are superposed the luminosity of the re sultant equals the sum of the luminosities of the components, but on the color wheel the result ant has a luminosity equal to the average (weighted mean) brightness of the sectors. When the eye is stimulated to fatigue by any hue, say red, its complement is subjectively called up because the eye then sees all the constituents of white except red. Now since red and green make white, and all the spectral hues together also make white, white minus i red equals green; that is, green is complement ary to red.