MYOPIA. PRESBYOPIA. 169 



at hand. Short-sightedness is thus much more common in 

 towns than in the country. Sailors, mountaineers, and in- 

 habitants of deserts are generally very long-sighted; the 

 habit of looking at great distances doubtless develops this 

 faculty. 



Myopia, presbyopia. The range of sight at which we read 

 or write is, in a normal condition, about 12 to 14 inches; 

 this point in myopia is much nearer, and in presbyopia 

 much farther off; but for the latter, distinct vision does not 

 go beyond 28 to 32 inches, that is, about double the dis- 

 tance considered normal; in myopia, on the contrary, this 

 distance may diminish to within an inch. This condition 

 of the sight is the result of modifications of the media of 

 the eye. In myopia the cornea or the crystalline is more, 

 and in presbyopia less, convex than in the normal condition. 

 In myopia, therefore, the focus is in front of the retina 

 for objects which, not being very near the eye, send to it 

 rays which diverge but slightly; in presbyopia, on the con- 

 trary, the slight refraction caused by the flattening of the 

 cornea, or the crystalline, tends to place the focus behind 

 the retina, the point of convergence of rays coming from 

 objects near at hand. The faculty of accommodation is 

 rather limited in myopia, as well as in presbyopia, and is 

 necessarily almost entirely wanting in the very near-sighted. 



To remedy these modifications of the eye, the short- 

 sighted person requires double concave glasses, which increase 

 the divergence of the rays in proportion to the refraction by 

 the media of the eye; the long-sighted person requires 

 double convex glasses, which produce the opposite effect. 



It is not uncommon to find persons who can only read 

 and write at a very short distance, but who can notwith- 

 standing see objects at a distance perfectly well. In this 

 case only one eye is myopic, the other is normal. A slight 

 inequality in the eyes is very common, and often unper- 

 ceived. This is undoubtedly the reason that many persons 

 use but one eye even when looking with both, without being 

 conscious of it, and this inequality, which is either the cause 

 or effect of this exclusive action, can only be augmented 

 by it. 



