468 THE SPECIAL SENSES 



rays which enter the eye just enough to overcome this defect. 

 In farsightedness {hypermUropia) the image is projected back of 

 the retina, and convergent glasses are necessary to remedy this 

 difSculty. In both of these cases the amount of convergence or 



Fig. 222 — Position of retina in near and far siglit; £, in natural sight; G, in far 

 sight ; C, in near sight. 



divergence exerted by the spectacle lenses should be only such as 

 to counterbalance the defect of the lens in this particular. 

 The selection of glasses, therefore, should always be the result of 

 prescription by an oculist who has determined by careful exam- 

 ination just how much correction is needed. Many eyes are ruined 

 by careless selection of glasses. 



Astigmatism. — A very common defect in the eyes of children 

 and adults is an irregular curvature in the surfaces of the cornea 

 or lens, or in both. The result of this is that rays which pass 

 through the affected parts are brought to a focus at a different 

 point from those which pass through the unaffected parts. This 

 produces a blurred image on the retina, and indistinct vision. 

 The eye muscles become strained in the constant attempts they 

 make to change the focus of the lens and secure a clear image, and 

 this strain often results in chronic headache. This defect receives 

 the name of astigmatism. It can be corrected only by the use of 

 glasses whose surfaces are so formed as to correct these irregu- 

 larities in surface, and the selection of these glasses should be based 

 upon careful examination by an oculist. 



Cataract. — Degeneration of the lenses at points renders them 

 opaque. This opacity, whether it occurs in spots or over the 

 whole surface of the lens or its inclosing capsule, is known as 

 cataract. The extent determines whether it produce partial or 



