124 REFRACTION 



crystalline lens is bulged so much that even when the muscles 

 are doing their best to pull it out from the center and flatten it, 

 the lens is still so bulged that it deviates the rays too much 

 and hence focuses them before they reach the retina (Fig. 81 ). 



FlG. 81. The nearsighted eye. The defect is remedied by concave glasses. 



Something must be done to scatter the rays so that they will 

 not come to a focus before they reach the retina. Hence 

 nearsighted persons wear concave lenses which are just thin 

 enough at the center to balance the too great thickness of the 

 crystalline lens. 



118. Headache and Eyes. Ordinarily the muscles of ac- 

 commodation adjust themselves easily and quickly; if, how- 

 ever, they do not, frequent and severe headaches occur as a 

 result of too great muscular effort toward accommodation. 

 Among young people headaches are frequently caused by 

 over-exertion of the crystalline muscles. Glasses relieve the 

 muscles of the extra adjustment, and hence are effective in 

 eliminating this cause of headache. 



An exact balance is required between glasses, crystalline 

 lens, and muscular activity, and only those who have studied 

 the subject carefully are competent to treat so sensitive and 

 necessary a part of the body as the eye. The least mistake 

 in the curvature of the glasses, the least flaw in the type of 

 glass (for example, the kind of glass used), means an im- 

 proper focus, increased duty for the muscles, and gradual 

 weakening of the entire eye, followed by headache and 

 general physical discomfort. 



