456 TEE POPULAR SCIENCE MONTHLY 



on the retina just as when they are superimposed by means of Max- 

 well'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 paint- 

 ing. It must be viewed at a distance just short of that which is neces- 

 sary 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 strik- 

 ingly in the ease 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 con- 

 nection 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 comple- 

 mentary hue. There are two simple methods for performing this experi- 

 ment, 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 com- 

 plementary 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 com- 

 plementary hue. ]STo experiments in color vision are more striking than 



