546 VISION 



a mixture of two definite rays., a long- and a short-waved one, can be found 

 to match every homogeneous color. The color system of such eyes is dichro- 

 matic. This abnormality is as a rule inborn and is spoken of as color Mind- 

 ness. If the color system of the normal eye consists of three components, 

 that of the dichromatic eye might be derived from it by the absence of some 

 one constituent. According to the Young- Helmholtz theory there would thus 

 be three possible kinds of color blindness : red blindness, green blindness and 

 violet blindness. From facts which will not be' given here it appears that the 

 color systems of the red blind and the green blind do correspond fairly with 

 what would be expected from the theory. Little is known with regard to 

 the third form. 



How the color blind actually experience colors can of course only be an- 

 swered by persons in whom only one eye has been color blind from birth. 

 Hippel and Holmgren have investigated two such cases. It must suffice here 

 to remark that in one of them mixed white light appeared the same i. e., 

 colorless to the color-blind as to the normal eye. 



In indirect or averted vision the ability to distinguish colors decreases 

 gradually from the center toward the periphery of the field of vision. The 

 peripheral limits for the different colors, even for a perfectly normal eye, 

 depend upon the intensity of the light, the saturation of the color and the 

 size of the object. Thus Hess found that for a definite shade of red on a 

 gray background, the limit was 20 from the axis of vision, provided the size 

 of the object was 7 mm. in diameter ; with a diameter of 30 mm. the same red 

 could be recognized 32 from the axis. According to Landolt, if the intensity 

 of light could be made great enough and the object could be made extensive 

 enough, we would be able to see all colors at the very periphery of the retina. 



At all events the capacity for color is much less in the peripheral portions 

 of the retina than in the central portions, and with colored objects of moderate 

 size and moderate intensity of light one may say that a green (of 495 /*/*) 

 and a red with a moderate admixture of blue disappear entirely at a relatively 

 short angular distance from the line of vision. Yellow and blue can be recog- 

 nized for some distance farther toward the periphery, in fact all rays of 

 greater wave length than 495 /*//, are seen as yellow and all of less wave length 

 as blue. Still farther toward the periphery the sensations of yellow and blue 

 disappear and a zone which is tolerably color blind is reached (Hess). 



2. The Theory of Antagonistic Colors. Like the three-color theory, this 

 also proceeds on the assumption that all our visual sensations are conditioned 

 upon the cooperation of a few components (visual substances) in the organ 

 of vision. According to the former theory these are present in the retina itself ; 

 but the theory of antagonistic colors leaves it undecided whether these substances 

 occur in the retina, optic nerve or some portion of the brain concerned in vision. 



The three-color theory, as we have seen, explains the sensation of white 

 as the result of an equal excitation of the three components and regards white 

 and black as only quantitatively different sensations. According to Bering's 

 theory, black and white are qualitatively different sensations, accompanied by 

 opposite chemical processes in a special black-white-perceiving substance. The 

 sensation of white arises while a process of katabolism is going on in this 

 substance, that of black during a process of anabolism. The brightness or 



