When a surface is illuminated with colored light, as in stage lighting effects, the hue actually seen depends on the local color of the surface, on the light reflected before absorption takes place and on the remainder reflected after ab sorption. Ogden Rood found that non-mono chromatic yellow light looked bright green on Prussian blue and white on ultramarine. The Prussian blue had evidently absorbed all but the green component that must evidently have been in the yellow, while, as will be seen later, the ultramarine had absorbed all but the mono chromatic yellow component. In the first case green was the dominant hue reflected, in the second case it was yellow plus blue. This shows that the parts played by absorption and reflec tion are difficult to predict.
Additive Combinations.— Combinations of colors are always combinations of spectral lights whether they are made additively by di rect superposition of the components or tractively by absorption as in a mixture of pow ders or solutions. When red and green spec trum rays are thrown upon a white surface both colors are simultaneously reflected to the eye and give the sensation of yellow; on the other hand a mixture of red and green paints will in certain proportions give a gray which may be reddish or greenish, but never yellow, because together they subtract from the white light (daylight, etc.) which illuminates them all colors except those whose combination im presses the eye as gray. Such apparently con tradictory results have been the source of many absurd controversial °theories of color.° There are facts about color and theories of the psycho physiological processes of color perception but no theories of color other than chemical the ories of molecular grouping.
Colors may be added in several interesting ways. Fine lines alternately red and green or
blue and yellow, etc., ruled close together on white paper and examined from a distance through the large end of opera glasses so that the individual lines are not distinguishable give the eye two simultaneous sensations whose ef fect is quite different from that of the corre sponding pigment mixtures. This method was used by Miles as early as 1839. It may be per formed also by painting one end of a pack of papers with one color and the opposite end with another and then reversing alternate pa pers; or otherwise by stippling points of differ ent hues on a white surface, or by using the color-wheel described later. Artists often em ploy the stippling process with beautiful results because the surface so treated fluctuates in hue according as one or another of the components momentarily predominates by reason of the varying degrees of fatigue of the color nerves in the retina. It has been found experiment ally that all hues can be produced by suitable combinations of three physiological primaries: monochromatic red, green, blue (or violet). Thus red plus green in different proportions produce additively all the hues from red through orange, orange — yellow, — yellow, yellow — green, green; red plus blue give violet, red —violet, blue—violet and the purples ; blue plus green produce blue—green and green—blue; red plus green blue in certain proportions give white, in others they reproduce colors obtained from pairs of the primaries. This is the first great group of phenomena that can he accounted for only by studying the subjective effect of color. It is strikingly illustrated by the Lumiere, Joly and Ives-Cros processes of color photography (see