PROPERTIES OF THE RETINA. 355 



colors fall into two groups: the dichromatic, whose color vision 

 may be represented by two fundamental colors and their com- 

 binations with white or black, and the achromatic, 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-Helmholtz 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 tritan- 

 opia. From this standpoint genuine color blindness may be 

 regarded as a reduction form of normal trichromatic vision of 

 such a character that all the color sensations may be conceived as 

 depending upon the existence of only two fundamental color 

 processes. 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 real- 

 ity 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- 

 scribe some of their after-images as red, the after-image of indigo- 

 blue, for example, but that they describe none as green. The 

 after-image of purple, for instance, which to the normal eye is 

 bright green, is described 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 individual differences. The green-blind are 

 also, according to recent descriptions, red-green blind; they also 



