SENSE OF SIGHT. 593 



colors red, green, and violet, but that blue must also be included 

 as a primary color. 



Color-blindness or Daltonism. It is estimated that 4 per cent, 

 of all males and 0.4 per cent, of all females are color-blind i. e., 

 unable to recognize all colors ; and this defect is congenital i. e., 

 its possessor is born with it. It is said that it may be inherited 

 also. Most of those who are color-blind are unable to distinguish 

 between red and green, while some cannot tell green from blue, 

 and others confound various colors. The name Daltonism, given 

 to color-blindness, is derived from the name of the great chemist, 

 Dalton, who when twenty-six years of age discovered that he was 

 color-blind. By some writers the term "Daltonism" is used as 

 a synonym for color-blindness in general, while others restrict its 

 use to red-blindness. Dalton himself, in matching silk, would 

 match red, pink, orange and brown with different shades of green ; 

 blue he would match with pink and violet, and lilac with gray. 

 It was not Dalton, however, but a shoemaker, Harris by name, 

 who discovered color-blindness, he himself being red-blind. 



As already stated, most color-blind persons are either red- 

 blind or green-blind. If a red-blind person looks at the spectral 

 colors, those from the red to the green look green, while the 

 extreme red end of the spectrum is invisible. The violet appears 

 blue, and at the end of the green near the blue of the spectrum 

 there appears to be a band of white or gray. To a green-blind 

 person the spectral colors from red to yellow appear to be all 

 yellow, but of different intensity : the green appears as a pale 

 yellow, having in its middle a gray or a white band, while the 

 violet appears blue. A report on color-blindness, made by the 

 Royal Society, states that " to the green-blind, red and yellow are 

 the same color; but the yellow being the brighter, he looks on 

 red as degraded or darkened yellow. On the other hand, to the 

 red-blind, green is brighter than yellow or orange, and these 

 appear as degraded green." 



The explanation of color-blindness varies according to the 

 theory of color-vision which is accepted. According to the 

 Young-Helmholtz theory, a red-blind person is deficient in the 

 red, and a green-blind person in the green, visual substance of 

 the retina, or else, if these substances are present, all the rays, 

 irrespective of their wave-lengths, stimulate them. In accordance 

 with the Hering theory, those who are red-blind or green-blind 

 lack the red-green visual substance and possess yellow-blue. On 

 this theory there is no difference between red-blindness and green- 

 blindness, but, as a matter of fact, this is not the case. If the 

 Franklin theory is accepted, then the explanation in the case of 

 the red- and green-blind would be that the white-black, or gray, 

 visual substance had developed into the yellow-blue, but had been 

 arrested in its development before the stage of red-green had 



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