ANIMAL MECHANISM. Ill 



the long-sightedness of old age, mechanically overcome 

 by wearing convex spectacles. So gradually are the 

 changes wrought by aye, that glasses of different focal 

 distances are sought from time to time, to keep pace with 

 the progress of decay. 



The ingenuity of man is nowhere more curiously dis- 

 played, than in thus availing himself of his discovery of 

 the laws of refraction, in producing artificial lenses to 

 gratify his eye, a never failing source of enjoyment, long 

 after nature has begun to draw the blind that will ulti- 

 mately close between him and the world forever. 



CRYSTALLINE LENS.* 



As magnifying glasses of different refractive pow- 

 ers give perfection to optical apparatus, so it is with re- 

 spect to the lenses within the ball. The coats of the eye 

 are equivalent to the tubes of such ingenious instruments. 

 By crystalline lens, is simply meant a body like a button, 

 resembling pure flint glass, somewhat of the shape of a 

 common sun glass, convex on both sides. Its posterior 

 convexity is greater than its anterior, thereby bringing 

 the rays to a point a little distance behind it. Careful 

 investigation shows that this lens is made of a series of 

 plates, applied to each other like the coats of an onion : 

 the centre is firmer than the edges or space between the 

 nucleus and margin. 



As a whole, it possesses a highly refractive property, 

 but in different degrees, according to the thickness of the 

 lens, receding from the centre to the circumference. 

 Over the whole, to keep it from sliding in any direction, 

 that the centre may not get without the axis of vision, 

 is an envelope, having connexion with all the coats, where 

 they are united on the borders of the cornea, and where 

 it joins the white part of the eye. Being equally trans- 

 parent with the lens itself, it cannot be conveniently ex- 

 hibited. One of its properties is elasticity, though not to 

 the extent we should at first view be led to imagine from 

 the following remarks. 



Cataracts, the most frequent cause of blindness, origi- 



* Crystalline lens, resembling crystal or glass. 



