524 THE HUMAN BODY. 



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 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 be- 

 cause 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 stimulated 

 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 

 will seem dark gray, with a white patch on it; an effect 

 due to the greater excitability of the retinal parts previously 

 rested by the black, when compared with the sensation 

 aroused elsewhere by light from the white wall acting on 

 the previously stimulated parts of the visual surface. 

 All persons will recall many instances of such phenomena, 

 which are especially noticeable soon after rising in the 

 morning. Similar things maybe noticed with colors; after 

 looking at a red patch the eye turned 011 a white wall sees 

 a blue-green patch ; the elements causing red sensations 

 having been fatigued, the white mixed light from the wall 

 now excites on that region of the retina only the other 

 primary color sensations. The blending of colors so as to 



