VISION*. 247 



we see an object, therefore, it is because these re- 

 flected rays enter our eyes, and impinge or fall 

 upon the retina. In this way a perfect picture is 

 formed at the bottom of the eye, just as we see our 

 face reflected in a looking-glass. 



In order, however, that the image or picture 

 may be formed upon the retina, it is needful that 

 the rays of light should pass through the eye. 

 For this purpose the cornea and parts behind are 

 transparent, and permit the rays to pass freely, 

 whilst the crystalline lens refracts or bends 

 them, so that they proceed in a proper form and 

 direction. 



During this operation, the iris contracts or 

 expands, to regulate the quantity of rays which 

 the retina can bear. By this beautiful and simple 

 contrivance, the eye accommodates itself to the 

 different degrees of light to which it is exposed. 

 It is a bad thing to look for a long time at a strong 

 light, as this weakens the iris, and blunts the sen- 

 sibility of the retina, and consequently injures the 

 sight. 



It is wonderful to think what perfect instru- 

 ments the eyes are. Thus in looking at a land- 

 scape of hill, dale, and plain, even of many miles 

 in extent, the whole space, with its numberless 

 objects of all colours and sizes, is represented on 

 the bottom of the eye; and though the picture is 

 not half an inch in diameter, how accurate it is, 

 and how minute in all its details, not a line or 3 

 shade being omitted! 



