About 4 per cent of all males are more or less color blind. While cases have been reported in women, it is rare and not typical in them and the conclusion has been that it is a sex-linked character of males.
We know of many sex-linked hereditary qualities. In the case of color blind individuals all of the germ cells with the X chromosome are defective in the gene for color discrimination.
The typical color blind individual has difficulty only with red and green. They both look alike and they both appear grayish. The only colors he distinguishes are yellows and blues.
Varieties of moderate degree are common. A very few persons are violet blind. Most of the subjects, however, exhibit the classical red-green blindness.
The physiological explanation of color blindness is not very clear. Discrimination of color in normal eyes occurs only in the center of the visual field. At the periphery of the circle of your vision when you are looking straight ahead, you see only white and black and formno colors. The anatomical elements in the retina in color blind individuals are probably all like the elements which pick up vision at the periphery.
Tests for color blindness are simple, accurate, and require little time. They are universally applied to railway employes. There is no reason why, with the number of automobile drivers who have to exercise color discrimination, they should not be part of the examination for a driver’s license.
Certainly with the appalling death rate from automobile accidents, and many of these occurring at night when the color blind individual has nothing to go by, a definite method of reducing them might lie in color-vision tests.
Remember that the color blind are usually entirely unaware of their defect even into adult life. It is totally impossible for such a person to distinguish a red signal from a green at a street crossing or to see the red signal at the back of a car in front.
The tests consist of requiring the applicant to match strands of different colored worsteds. A pale green skein (1), which must not incline toward yellow green, a medium purple (2) and a vivid red (3) are given him and he is asked to select from the other skeins on the table the ones which match these. The red-green color blind will select grays and yellow grays and bluish skeins to match all three test skeins. Because they see all three test skeins as gray with a little yellow or blue shading.
It is curious to recall that such testing of railway employes is only of recent date. In 1877 the Massachusetts Board of Railroad Commissioners wrote: “The subject of color-blindness is one which has never come to the attention of this board.” Railway employes even resented, at first, being required to pass such tests. Doubtless motorists will now.