156 HUMAN PHYSIOLOGY 



far-sighted eyes, and supposing that glasses are not used, 

 we see that the near-sighted person does close work with 

 a minimum and the far-sighted with a maximum of 

 strain. 



Astigmatism. This is a general term covering all 

 defects of vision due to the departure of any of the re- 

 fracting surfaces from the spheric curvature which 

 such surfaces should have. The most common type 

 for which glasses are prescribed has been simply and 

 clearly described as "spoon-shaped cornea.' 7 In the 

 language of geometry the cornea is ellipsoidal instead 

 of spheric. The spoon bowl which stands for the 

 astigmatic cornea must be thought of as held with its 

 long axis horizontal in most cases. It will then be 

 evident that such a cornea curves more decidedly up 

 and down than from left to right. 



The optic results of astigmatism are difficult to ex- 

 plain in detail. The images can never be wholly satis- 

 factory. When certain features are sharp others will 

 be blurred. Thus in looking at a window sash one who 

 has ordinary astigmatism will not see the upright and 

 the cross pieces with equal distinctness. The set par- 

 ticularly attended to will be clearer than the others. 

 This leads to a restless tendency to experiment with 

 the accommodation but whenever good definition for 

 one system of lines has been achieved the perpendicular 

 system will have become indistinct. The strain of this 

 uneasy process may work harm to the general health. 



Glasses to correct astigmatism must have the astig- 

 matic character themselves but in a sense opposite to 

 that in the eyes for which they are intended. If, as 

 usual, the excessive curvature of the cornea is up and 

 down the lenses must curve less along that meridian 

 than from left to right. The best skill is needed for 

 the designing and mounting of such glasses. If a lens 

 made for this purpose is turned one-quarter way (90) 

 round, it will double the defect which, in its proper 

 position, it corrects. 



