74-8 THE SENSES 



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- 



Green 



(OrceniiMlw) 

 Cyan-blue 



Ontnpe 



Red 



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



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; yellowish color 

 upon a bright violet, and a violet tint upon a bright yellow surface. The 

 color excited thus must arise as an opposite or antagonistic condition of the 

 retina, and the opposite conditions of which it thus becomes the subject, 

 would seem to balance each other by their reciprocal reaction. A necessary 

 condition for the production of the contrast colors is that the part of the 

 retina in which the new color is to be excited shall be in a state of compara- 

 tive repose; hence the small object itself must be gray. A second condition 

 is that the color of the surrounding surface shall be very bright. 



Color-blindness. Many persons are unable to distinguish one or 

 mo-e of the fundamental colors, and therefore have different perceptions 

 of the color combinations from that of the normal individual. It is said that 



