326 THE SPECIAL SENSES. 



more common than among educated persons. Color blindness 

 may exist in different degrees of completeness, from a total loss to 

 a simple imperfection or feebleness of the color sense, and it is 

 usually congenital. Those who are completely color blind as re- 

 gards some or all of the fundamental colors fall into two groups: 

 the dichromatic, whose color vision may be represented by two 

 fundamental colors and their combinations with white or black, 

 and the monochromatic or totally color blind, who see only the 

 white-gray-black series. 



Dichromatic Vision. The color-blind who belong to this class 

 fall into two or three groups, which have been designated, under 

 the influence of the Young-He lmholtz theory of color vision, the 

 red-blind, the green-blind, and the violet-blind. The most com- 

 mon by far of these groups is that of so-called red blindness; it 

 constitutes the usual form of color blindness. As a matter of fact, 

 persons so affected are in reality red-green blind. In what may be 

 called the most typical cases they distinguish in the spectrum only 

 yellows and blues. The red, orange, yellow, and green appear as 

 yellow of different shades, the green- blue as gray, and the blue- 

 violet and purple as blue. The red end of the spectrum is distinctly 

 shortened, especially if the illumination is poor, and the maximum 

 luminosity, instead of being in the yellow as in normal eyes, is in the 

 green. When the spectrum is examined by such persons a neutral 

 gray band is seen at the junction of the blue and green. In some 

 cases, however, this neutral band is not seen, the yellow passing with 

 but little change into the blue. As a matter of fact, in red blindness 

 the most characteristic defect is a failure to see or to appreciate the 

 green. This color is confused with the grays. and with dull shades 

 of red. When such persons are examined for their negative after- 

 images for different colors, it will be noted that they describe some 

 of their after-images as red, the after-image of indigo-blue, for ex- 

 ample, but that they describe none as green. The after-image of 

 purple, for instance, which to the normal eye is bright green, is de- 

 scribed by them as gray blue or pale blue. From the descriptions 

 given it is probable that the color vision of the so-called red blind 

 is not by any means the same in all cases, but exhibits many individ- 

 ual differences. The green-blind are also, according to recent des- 

 criptions, red-green blind; they also confuse reds and greens and 

 in the spectrum are conscious of only two color qualities, namely, 

 yellow and blue. They differ from the red blind in that the red end 

 of the spectrum is not shortened, and the maximum luminosity, as 

 with the normal eye, is placed in the yellow. In the matching and 

 combination of colors they show distinct differences from the red 

 blind, so that though resembling the latter in general features, they 

 differ obviously in some details. Violet blindness, so called, seems 



