546 THE HUMAN BODY. 



observation of color-blind persons thus helps in deciding 

 which these are. The most common form is red color blind- 

 ness; persons afflicted with it confuse reds and greens. .Red 

 to the normal eye is red because it excites red sensation 

 much, green some, and violet less; and a white page white, 

 because it excites red, green, and violet sensations about 

 equally. In a person without red sensation a red object 

 would arouse only some green and violet sensation and so would 

 be indistinguishable from a bluish green; in practice it is 

 found that many persons confound these colors. Cases of 

 green and violet color blindness are also met with, but they 

 are much rarer than the red color blindness or " Daltonism." 



The detection of color blindness is often a matter 

 of considerable importance, especially in sailors and railroad 

 officials, since the two colors most commonly confounded, red 

 and green, are those used in maritime and railroad signals. 

 Persons attach such different names to colors that a decision 

 as to color blindness cannot be safely arrived at by simply 

 showing a color and asking its name. The best plan is to^ 

 take a heap of worsted of all tints, select one, say a red, and 

 tell the man to put alongside it all those of the same color, 

 whether of a lighter or a darker shade; if red blind he will 

 select not only the reds but the greens, especially the paler 

 tints. About one man in eight is more or less red blind. 

 The defect is much rarer in women. 



Fatigue of the Retina. The nervous visual apparatus is 

 easily fatigued. Usually we do not observe this because its 

 restoration is also rapid, and in ordinary life our eyes, when 

 open, are never at rest; we move them to and fro, so that 

 parts of the retina receive light alternately from brighter and 

 darker objects and are alternately excited and rested. How 

 constant and habitual the movement of the eyes is can be 

 readily observed by trying to fix for a short time a small spot 

 without deviating the glance; to do so for even a few seconds 

 is impossible without practice. If any small object is steadily 

 " fixed " for twenty or thirty seconds it will be found that the 

 whole field of vision becomes grayish and obscure, because 

 the parts of the retina receiving most light get fatigued, and 

 arouse no more sensation than those less fatigued and stimu- 

 lated by light from less illuminated objects. Or look steadily 

 at a black object, say a blot on a white page, for twenty 

 seconds, and then turn the eye on a white wall; the latter 



