124 ANIMAL MECHANISM. 



a direction which would form the image at some distance beyond 

 the retina, as at B. But, by the convex glass A, which, for 

 example, is the spectacle worn by aged people, the direction of 

 the rays of light is so corrected, that the image falls accurately on 

 the bottom of the ye, or retina. 



When the convex lens is interposed between the eye 

 and object, as represented in the above diagram, the rays 

 are made more converging, so that the picture strikes 

 exactly and distinctly on the nerve. People slide their 

 spectacles on the nose unconsciously till the true focus is 

 procured. 



A REASON WHY NEAR-SIGHTED PERSONS SER INDIS- 

 TINCTLY. 



Either the crystalline lens, but more generally the 

 cornea, is too prominent converging the light too sud- 

 denly ; that is, converging the luminous rays at an un- 

 natural place within the vitreous humor. An indistinct 

 outline of the object is the effect of their great divergency, 

 after decussating before they arrive at the retina. The 

 following diagrams will illustrate the subject far better 

 than a whole volume of written explanations. 



FIG. 12. 



Explanation of Figure 12. 



In this figure, the convexity of the cornea, or the focal powers of 

 the lens, being too great for the length of the axis of the eye, the 

 image is formed at A, before the rays reach the surface of the retina t 

 or inner box, illustrated in Fig. 2, letter c; and after coming accu- 

 rately to the point, they again begin to diverge ; which diverging 

 rays, striking the surface of the retina, give the indistinct vision of 

 the near-sighted individual. But as this indistinctness of vision 



Sroceeds from no opacity, but only the disproportion of the convex- 

 ;y of the eye to the diameter, the defect is corrected by a concave 

 glass, represented in the next figure. 



Concave glasses are the restoratives of the near-sighted 

 eye, by separating the rays, and carrying the image so 



