VISION. 



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pound colors in which red enters the complementary color alone is 

 visible, white appearing as bluish green. Another theory of color- 

 vision is that of Hering. The six sensations of color readily fall 

 into three pairs, the members of each pair having similar relation- 

 ship. White and black naturally go together, the one being antagon- 

 istic of the other. According to Hering, the retina is undergoing 

 metabolic changes, and he supposes there are three distinct visual 

 substances which are undergoing anabolism and catabolism. When 

 breaking down, or catabolism, is in excess of the building up, or 

 anabolism, we have a sensation of white ; when upbuilding predomi- 

 nates, we have black. 



Anabolism of the visual substances by the rays of light produces 

 green, blue, and black; catabolism of these visual substances pro- 

 duces white, red, and yellow. 



White is catabolic 

 1. 1 and 



Black is anabolic. 



Red is catabolic 



and 

 Green is anabolic. 



2. 



Yellow is catabolic 



and 

 Blue is anabolic. 



In applying this theory to color-blindness it must be assumed 

 that those who are red-blind want the red-green visual substance; 

 they have only the black-white and yellow-blue visual substance in 

 the retina. 



According to the Young-Helmholtz theory, there is a defect cor- 

 responding to the three color-perceiving fibers. According to this 

 theory color-blindness is of four kinds: red, green, and violet, and 

 complete blindness to colors. In the Hering theory the kinds are: 

 (1) complete, (2) blue-yellow, (3) red-green, and (4) incomplete color- 

 blindness. 



Color-blindness is also called Daltonism, after Dalton, a Quaker, 

 who first described it. The percentage of color-blindness among per- 

 sons is about 4 per cent, in males, and 1 per cent, in females; and 

 among Quakers 4 1 / 2 , because for generations they have worn drabs. 

 The disease is hereditary. The cones in color-vision are, according 

 to Van Kries, for the perception of color, whilst the rods are for the 

 perception of light and darkness. 



COMPLEMENTARY COLORS. Those colors are complementary 

 which when mixed together produce white. The following table 

 gives the complementary colors of the spectrum: 



