244 THE HUMAN BODY 



Color Blindness. This is a deficiency in color vision whereby 

 certain colors fail to produce the characteristic color sensations 

 that they do in normal eyes. The commonest sort of color blind- 

 ness is so-called red-green blindness. In- it neither red nor green 

 has the same value as in normal eyes. Both colors seem to give 

 the sensation of " neutral " tints, grays and browns. Two varieties 

 of red-green blindness are recognized; the difference between 

 them is, however, apparently one of perception of luminosity 

 rather than of color. To the red-blind person a red object looks 

 dim as well as of neutral tint; to the green-blind person a red ob- 

 ject appears to be bright, although in color of neutral tint likewise. 



Red-green blindness is the common form. It is usually con- 

 genital and occurs more frequently in males than in females. 

 One male in twenty-five, on the average, is color blind, and less 

 than one female in a hundred. It has been suggested that this 

 difference is at bottom one of training; women have from time 

 immemorial used brighter colors and more colors in their clothing 

 than have men, and have therefore become more accustomed to 

 making nice color discriminations. 



A form of violet blindness has been described as occurring in 

 rare pathological conditions. It can be brought on temporarily, 

 it is said, by taking the drug santonin. This form of color blind- 

 ness has not been thoroughly studied. Monochromatic blindness, 

 in which the only sensation is of degrees of grayness, shading at 

 one end into white, at the other into black, is also described. This 

 is accompanied in most cases by blindness of the fovea, and is 

 probably therefore the result of complete loss of cone function. 



A full explanation of red-green blindness cannot be had, of 

 course, until the mechanism of color vision is understood. From 

 what was said about the distribution of color perception in the 

 retina it is clear, however, that in all eyes there is an area of red- 

 green blindness between the area of complete color perception 

 and the area of white-black vision. If we suppose the cones in 

 the central area to be undifferentiated from those of this im- 

 mediately surrounding zone we have a condition of red-green 

 blindness involving the whole eye and corresponding to that of the 

 color-blind person. 



The detection of color blindness is often a matter of considerable 

 importance, especially in sailors and railroad operators since the 



