158 HUMAN PHYSIOLOGY 



The fovea and a zone extending some distance out- 

 side it have the capacity to differentiate all the colors. 

 Beyond the area with complete color vision there is a 

 tract not stimulated characteristically by reds and 

 greens though distinguishing yellows and blues. Still 

 beyond, all color comparison is lost and only light and 

 shade can be recognized. We say that the outlying 

 region of the retina is totally color-blind. The same 

 part is without cones, so it has been natural to infer that 

 the cones are adapted to discriminate color while the 

 rods are affected in the same way by all varieties of 

 light. If this is so their reactions can indicate nothing 

 more than the degree of illumination. 



Color-blindness. What is ordinarily meant by this 

 expression is not a complete inability to compare colors 

 but a confusion of reds and greens. It will be noted 

 that in any retina there is a zone in which this con- 

 fusion exists. A person is said to be color-blind when 

 the same difficulty extends even to the fovea. Since 

 red and green are colors used for railroad signals and 

 for the port and starboard lights of vessels it has been 

 found necessary to subject employes to careful tests. 

 In the trials the candidates are not asked to name any 

 colors, as that would be testing their education rather 

 than their natural endowment, but they are asked 

 to place in piles numerous skeins of yarn, putting those 

 together which have a general similarity. 



It is reported that this defect is to be found in one 

 man out of about thirty. It is rare in women. Color- 

 blindness is hereditary in families but, according to an 

 odd principle, a man who is color-blind will not have 

 color-blind sons or daughters. His son's sons will also be 

 free from the defect and all his granddaughters, but 

 it may be looked for in the sons of his daughters. Many 

 quaint stories are told of the mistakes made by color- 

 blind persons. A Dartmouth student with this handicap 

 came to Boston to attend the football game with Harvard. 

 At the last moment his friends suggested that he provide 



