696 THE SENSES 



Complemental Colors, and After-images of Color. Certain colors, 

 when allowed to stimulate the retina at the same time, tend to neutralize 

 each other. That is, they produce sensations approaching white, usually 

 some shade of gray, which will have a tinge of one or the other primary colors 

 according to the proportion of stimulation. These pairs of colors are called 

 complemental colors. Each spectral color has its complemental color, a fact 

 that is represented in figure 473. The complemental colors of greatest physi- 

 cal significance are red and green (greenish-blue), yellow and deep blue 

 (indigo blue), green (greenish-yellow), and violet. 



FIG. 473. Geometrical Color Table for Determining the Complemental Colors. 



Positive after-images of color exist for a brief moment, but the greatest 

 significance attaches to the negative after-images. The negative after-images 

 of color following the stimulus of colored light upon the retina are not the 

 sensation of color produced by the color of an object, but are the opposite 

 or complemental color. The after-image of red is, therefore, green, and 

 that of green, red; that of violet, yellow, and of yellow, violet, etc. The 

 same relation holds with the other colors. A condition for the development 

 of a strong after-image is that the primary image shall have continued to a 

 certain degree of fatigue. The colors which reciprocally excite each other 

 in the retina are those placed at opposite points in the color table, figure 473. 

 The after-images of color are most intense in the axis of the visual field and 

 are not always present in the periphery of the retina, as can readily be seen 

 by examining the chart, figure 471. 



Color sensations may also be produced by contrast. Thus, a very small 

 dull gray strip of paper, lying upon an extensive surface of any bright color, 

 does not appear gray, but has a faint tint of the color which is the comple- 

 ment of that of the surrounding surface. A strip of gray paper upon a green 

 field, for example, appears to have a tint of red, and when lying upon a red 

 surface, a greenish tint; it has an orange-colored tint upon a bright blue 

 surface, and a bluish tint upon an orange-colored surface; a yellowish color 



