212 PHYSIOLOGY. 



similarly situated, showing how by the intervention of a concave lens, 

 G., the rays are di-verged, and the image of the arrow, F., accurately 

 converged to the retina at A. 



33. Long-sightedness is the opposite defect to this, and is 

 owing to a flattening of the cornea, and a relaxation in the 

 structure of the crystalline lens, by which its power of re- 

 fraction is lessened. ) In this case, the rays of light are car- 

 ried beyond tlie retina, and therefore do not form a distinct 

 image on it. pld persons usually are subject to this defect ; 

 the only remedy known is convex glasses.. Where persons 

 have been short-sighted in youth, as the eye grows flatter by 

 age, they at length are able to see well, without glasses of any 

 kind. This change in the shape of the eye is often denoted 

 by a tendency to hold a book at a greater distance when 

 reading. Glasses do for the eye that portion of the labour 

 of bending the rays of light, which it is not able to do for 

 itself. By adapting glasses to the successive changes which 

 age produces in the shape of the eye, the sight may be pro- 

 longed very often to the close of life. After the operation of 

 extracting the crystalline lens for cataract, or depressing it 

 by couching, very convex glasses are needed to remedy the 

 deficiency. 



34. The sight is often injured by long-protracted attention 

 to minute and near objects ; as we see in the watch-maker 

 and engraver ; also by watching for objects at a distance ; 

 as in sailors, and keepers of telegraph stations ; in the one 

 case the eye becoming near, and in the other far-sighted. 

 During the arbitrary reign of Napoleon, the young men of 

 France often produced short-sightedness voluntarily, by wear- 

 ing very concave glasses, in order that they might be exempt 

 from military service. 



35. Some persons are unable to tell one colour from 

 another. Sir David Brewster gives us the following exam- 

 ples. A Mr. Scott mistook pinJc for pale blue, and red for 

 green. His father, uncle, sister, and two sons, all mistook 

 these colours in the same way. A shoemaker named White, 



