PROPERTIES OF THE RETINA. 351 



Color Blindness. — The fact that some eyes do not possess 

 normal color vision does not seem to have attracted the attention 

 of scientific observers until it was studied with some care by Dalton, 

 the distinguished EngHsh chemist, at the end of the eighteenth 

 century. Dalton himself suffered from color blindness, and the 

 particular variety exhibited by him was for some time described 

 as Daltonism, but is now usually designated as red blindness. The 

 subject was given practical importance by later observers, espe- 

 cially by the Swedish physiologist Holmgren,* who emphasized its 

 relations to possible accidents by rail or at sea in connection with 

 colored signals. It is now the practice in all civiUzed 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. Among those persons who possess a tri- 

 chromatic color sense differences may be observed in regard 

 to the proportions of different colors which must be combined 

 to make a match with a given standard. This condition has 

 been shown to exist particularly for the combinations of red and 

 green required to match a homogeneous yellow. Individuals 

 who differ sensibly from the normal in the amounts of red and 

 green selected to make such a match have been described as 

 having an " anomalous trichromatic vision." f Those who are 

 completely color blind as regards 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 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 



* Holmgren, " Color Blindness in its Relations to Accidents by Rail and 

 Sea," "Smithsonian Institution Reports," Washington, 1878. See also Jef- 

 fries, " Color Blindness, its Danger and its Detection," Boston. 



t Consult von Kries in Nagel's Handbuch der Physiologie, vol. 3, p. 124. 



