SCIENCE OF COMMON THINGS. 



Explanation of near-sightedness. 



Next in order is the aqueous humor, &, e, in the middle of which is the 

 iris, d, c. Behind the pupil we have the crystalline. lens, /, and then the 

 vitreous humor, A, filling all the interior of the ball of the eye. m indi- 

 cates the retina, which is an expansion of the optic nerve, n. k is the 

 choroid coat, a membrane interposed between the retina and the sclerotic 

 coat ; it terminates in form in a series of folds or filaments, #, called the 

 ciliary ligament or processes. 



1711 Why are some persons near-sighted ? 



Because the curvature of the cornea and the crystal- 

 line lens is too great, and the rays of light which form 

 the image are brought to a focus before they reach the 

 retina or the back part of the eye. The object, there- 

 fore, is not distinctly seen. 



Fig. 79 represents the manner 

 in which the image is formed 

 upon the retina in the perfect eye. 

 The curvature of the cornea, s s, 

 and of the crystalline lens, c c, is 

 just sufficient to cause the rays 

 of light proceeding from the im- 

 age, e e, to converge to the right 

 focus, m, upon the retina. 



Fig. 78 represents the man- 

 ner in which the image is formed 

 in the eye of a near-sighted per- 

 son. The curvature of the cor- 

 nea, s 5, and of the crystalline 

 lens, c c, is so great that the im- 

 age is formed at m m in advance 

 of the retina. 



