PHYSIOLOGY OF COLOR VISION 457
these, nor are there any tliat have more direct application in the use of colors in picture painting; thus, a gray wall viewed against a sun-lit background of green is no gray, but like the piece of paper in our exper- iment it becomes tinted of a purplish hue. Similarly, a shadow cast on yellow sand is blue and one thrown on the skin when this is other- wise in strong light often acquires a striking quality of green.
The phenomenon of successive contrast is elicited by steadily regard- ing a patch of a certain color for some time and then either closing the eyes, or better still, directing the gaze to a neutral surface, such as a gray untinted wall. A vivid color impression of the same shape as that of the colored patch previously looked at will be seen in both cases, but exhibiting a hue which is complementary to that of the patch.
In the experiments above described the complementary color is demonstrated by the use of a gray surface. It is evident, however, that, if we cause it to be projected against a background which itself possesses a certain hue, the two hues (the complementary and that of the re- garded surface) will become blended and will have the same effect as if they had been spun on a Maxwell's disc. For example, suppose we regard for some time a blue surface and then direct the gaze to one of red, the impression will be that of orange, because the complementary of blue, being yellow, fuses with red and produces orange.
Having determined the complementaries by means of these contrast methods we may confirm our results by color synthesis; thus supposing we have determined by the contrast methods that the complementary for a certain yellow is a certain blue, we may proceed to ascertain whether this is strictly the case by preparing discs composed of these two hues and rotating them on Maxwell's machine. If the hues are com- plementary the greatest possible degree of whiteness will be produced.
Successive contrast finds only a limited application in art, although it is of course conceivable that the intensive fixation of one colored area in a painting, or a design, might, by successive contrast, greatly modify the colored impression created by shifting the eyes to another part. It is improbable, however, that any artist, either intentionally or uninten- tionally, has laid on his pigments with this object in view. Nevertheless, successive contrast may assist us greatly in the actual determination of the complementary hue. Thus, to take again our example of the gray wall against the green background, we may exaggerate the effect of the green on the gray by regarding the green for some time and then shift- ing the gaze to the wall, when its purplish hue will be found to be much intensified. On the other hand, simultaneous contrast is of paramount importance in art ; indeed it is as important in the final impression pro- duced by a painting or a design as any other quality which this may possess. This importance depends on the fact that when two colored surfaces are placed in apposition each becomes changed as if it were mixed to a certain extent with the complementary hue of the other; or