Page:Popular Science Monthly Volume 83.djvu/460

456 on the retina just as when they are superimposed by means of Maxwell's machine. In the same way vibrations of the eyelids by moving the eyelashes across the palpebral cleft assist the synthesis, this being made evident by half closing the eyes, a method often used in studying pictures.

The success with which the desired impression can be created in a pointilistic picture often depends upon the purity of the colored dots, its vibrating quality being at the same time much enhanced by leaving a narrow margin of white around each dot. When this is successfully done there comes into play another physiological process known as flicker, which can be experimentally produced by rotating discs with black and white sectors at a speed which is just insufficient to cause a uniform gray. The resulting flicker possesses a glittering quality which makes it appear of distinctly greater brightness than the gray which results from complete synthesis. The same thing may be seen by observing the spokes of a wheel revolving at different velocities. Instead of black and white the sectors may be composed of different hues.

In the flicker experiments the gray remains of the same degree of saturation at whatever rate the disc is revolving, provided it is revolving more quickly than is necessary to produce complete fusion, and so in pointilistic painting, when the picture is viewed beyond the distance at which fusion occurs the impression is practically that of the older painting. It must be viewed at a distance just short of that which is necessary to produce complete synthesis. The post-impressionists such as Cezanne, Matisse, etc., realizing this limitation in pointilism, have been searching after a method by which the color scheme maintains its effect on us at whatever distance the picture is viewed. The physiological principle upon which this depends is that known as contrast, and this we will now proceed to study. Being a property exhibited most strikingly in the case of complementary hues, it becomes necessary for us to have, besides the color triangle, some simple experimental methods by which the complementary hues may be determined. Such methods include the experiments of simultaneous and successive contrast, in connection with which many facts of fundamental importance in the use of pigments are brought to light.

Simultaneous contrast is illustrated by regarding a strip of gray against a colored field when the gray becomes tinted with the complementary hue. There are two simple methods for performing this experiment, one is to spin a colored disc, midway between the center and circumference of which is a circle, composed partly of black and partly of white; this synthesizes to a gray which becomes tinted with the complementary hue of the colored field. The other way is to lay a narrow strip of gray paper (cut as a zigzag) on a colored sheet and then to cover the whole with thin tissue paper; the gray will assume the complementary hue. No experiments in color vision are more striking than