334 THE SPECIAL SENSES. 



colored signals. It is now the practice in all civilized countries to 

 require tests for color blindness in the case of those who in railways 

 or upon vessels may be responsible for the interpretation of signals. 

 The numerous statistics that have been gathered show that the 

 defect is fairly prevalent, especially among men. It is said that 

 on the average from 2 to 4 per cent, are color blind among males, 

 while among women the proportion is much smaller, 0.01 to 1 per 

 cent. Among the poorly educated classes the defect is said to be 

 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 Imholtz theory of color vision, the 

 red-blind, the green-blind, and the violet-blind. As the terms 

 red-blind and green-blind imply a more specific condition of 

 vision than is found to be the case on careful examination, von 

 Kries has suggested as a substitute the names protanopia and 

 deuteranopia, as indicating a defect in a first or second constit- 

 uent necessary for color vision. According to the same nomen- 

 clature so-called violet-blindness would be designated as tritanopia. 

 The most common by far of these groups is that of so-called red- 

 blindness (protanopia); 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, how- 

 ever, 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 de- 



