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ANIMAL BIOLOGY 



In many people the eye is not entirely spherical, or 

 the lens has not the proper convexity, so that the eye 

 does not have the usual range of vision. Some people 

 are near sighted, that is, they see clearly only close at 

 hand, while others are far sighted and can see clearly only 



PIG. 220. Diagram to illustrate the adjustment of the crystalline 

 lens, A, for distant objects, B for near ones. (After Huxley.) 



things that are distant. In the first case the clearest 

 image is formed in front of the retina and in the other case 

 the image is behind it. These defects are corrected by 

 proper eye glasses. 



A very common eye defect, called astigmatism, is caused 

 by the unequal curvature of the eye ball or lens. This is 



FIG. 221. Diagram of the position of the retina. F, in far sight; N, 

 in near sight; and C, in natural sight. 



corrected by lenses which have a cylindrical instead of a 

 spherical curvature. Much eye strain is endured by many 

 people without their being aware of the fact; and many 

 disorders, such as headache, nervousness, indigestion and 

 other ailments are directly traceable to this cause. Read- 

 ing in dim light or with the light in front so that we get 



