THE EYE 161 



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 

 himself with a suitable green necktie and he hastened 

 to choose one. When he displayed it there was a 

 strenuous protest.; it was the Harvard crimson. 



The process in. the rods and cones must be a photo- 

 graphic one. Some chemicals are present there which 

 are changed in a definite way by the action of light. 

 The changes which occur must furnish the immediate 

 source of the stimuli which start the nerve-impulses 

 on their way to the brain. One difference between the 

 retina and a common photographic plate lies in the 

 fact that in the eye the taking of each picture is fol- 

 lowed by a marvellously rapid recovery of a condition 

 which permits a new set of images to be registered. In 

 the plate the outlines photographed are permanent 

 even though they may be overlaid by those of a second 

 n 



