THE EYE AND THE PHYSIOLOGY OF VISION. 557 



Applying this to the eye we find that it not infrequently 

 occurs that a cornea possesses such irregularities of curva- 

 ture, and astigmatism is the result. The presence of as- 

 tigmatism can be readily detected by looking at a disk on 

 which are placed the numerals arranged like those on the 

 face of a clock. Astigmatic persons will see some of these 

 numerals in focus, while others are indistinct and blurred. 

 When the focus is changed to the blurred ones and these 

 become distinct, the first become blurred. Or if one look at 

 a system of concentric rings, such as given in Figure 174, 

 astigmatic persons will perceive certain sectors in which the 

 lines are blacker and more distinct, bordered by portions 

 where they are blurred and indistinct. 



Fig. 174. FIGURES TO DEMONSTRATE THE EXISTENCE OF ASTIGMATISM. 



To remedy astigmatism it is necessary to grind a special 

 lens of such a form that it shall exactly counteract the in- 

 distinctness of the cornea, the lens being more rounded 

 where the cornea is not rounded enough, and hollowed out 

 where the cornea possesses too much curvature. The fit- 

 ting of a lens for astigmatism must in each case be a bit of 

 special work, there being possibly no two cases of astig- 

 matism exactly alike. 



4. Cataract. Cataract is an optical defect arising 

 from the opacity of the lens. When light is no longer 

 able to penetrate this readily, a more or less complete 

 blindness results, the only remedy for which is the removal 

 of the lens from the eye and the substitution for it of an 

 artificial lens in front of the eye. 



In addition to the usual cataracts of the lens, there 

 occur sometimes slight opacities in the cornea. 



