SENSE ORGANS. n e 



The term longsightedness or farsightedness is some- 

 times used to express this defect. It is a misnomer.* 

 No eye can be longer-sighted than the young normal 

 eye. It can define perfectly the edge of the moon or of 

 the setting sun. Moreover, all eyes— the myopic and 

 hyperopia, as well as the normal— undergo the presby- 

 opic change with age; but the myopic eye does not 

 thereby become normal, as many suppose. 



Astigmatism— Dim-sightedness.— All other eyes 

 see distinctly at some distance, but the astigmatic eye 

 does not see distinctly at any distance. Hence the term 

 dim-sightedness. In all other eyes all the rays of light 

 issuing from a radiant point are brought to a focal point; 

 in this one they are brought together to a focal line, or 

 rather to two focal lines, one farther than the other. 

 Hence the term astigmatism.* In all other eyes the 

 curve of the lenses, and therefore their refraction, is 

 equal in all directions. In this one the curve and the 

 refractive power tip and down is greater or less, usually 

 greater, than from side to side. The remedy is, of course, 

 the use of glasses which correct the unequal refraction. 

 For example, suppose the curve and the refractive power 

 from side to side is normal, but the curve and refractive 

 power up and down is too great, then the glasses should 

 have no curve horizontally, but should be concave ver- 

 tically — i. e., should be cylindrical concave glasses, with 

 the axes of cylinder horizontal. 



The usual test for astigmatism is a large rectangular 

 cross, thus +. At a certain distance the astigmatic eye 

 sees the vertical line distinctly, but the horizontal line is 

 blurred. At a certain other distance the horizontal line 

 is distinct, but the vertical blurred. But at no distance 

 are they both distinct. 



* Not a point. 



