556 STUDIES IN ADVANCED PHYSIOLOGY. 



page closer to his eye. The increase in the intensity oi the 

 light" temporarily increases the clearness, but the difficulty 

 is soon exaggerated and so frequently a second attempt to 

 see more clearly is made by moving the object still closer. 

 Teachers especially should be on the lookout for such cases 

 since the confounding of far-sight with near-sight might 

 lead to undesirable results. Far-sightedness also very gen- 

 erally induces inflamed conditions of eyes and eyelids, and 

 seems a predisposing cause in the production of styes. 



3. Astigmatism. The distinctness of an object, as any 

 one will readily understand who has handled lenses of any 

 kind, depends upon an even curvature and density of the 

 lens in question. If the surface of a lens be made un- 

 even, a greater or less distortion of the image occurs. If, 

 for instance, a drop of water is allowed to run across the 

 front of a pair of spectacles, objects seen through them are 

 at once distorted. The water in this case having about the 

 same density as glass, acts really like a bit of extra glass 

 put on the spectacles, and so the surface of it is made un- 

 even. The spectacles under these circumstances are astig- 

 matic. 



Exaggerated cases of astigmatism are also observable on 

 nearly all window panes, for seldom (except in the case of 

 plate glass) is a window pane so even as not to distort the 

 objects seen through it, while sometimes the distortion is 

 so great as to transform a straight object seen through it 

 into a series of zig-zags. 



It is not necessary that a lens should have such exagger- 

 ated cases of unevenness to be astigmatic. It may, in fact, 

 be quite smooth, but if the curvature in one direction differs 

 from the curvature in another direction, a distortion or rela- 

 tive change of focus for the various parts of the lens will 

 arise. Rays of light passing through the lens where it has 

 a greater curvature will come to a focus sooner than rays 

 of light passing through portions of a lens having a lesser 

 curvature. 



