52 



bind the retina, as illustrated in figure 15 — H. In other words, 

 the parallel rays which enter the eye come to a focus behind 

 the retina. As a rule, the axis of the eve or the diameter 

 from before to behind is too short and the cornea may appear 

 less convex or flatter than normal. Removal of the crystal- 

 line lens (as is sometimes done in cataract) produces farsight- 

 edness. Convex glasses are used in hypermetropia in man, 

 but are impractical with animals. Distant objects may be 

 seen distinctly but the images of objects at a short distance 

 are blurred and sometimes distorted into frightful forms. 

 Hence farsighted horses are frequently frightened, or are 

 caused to shv as a result of indistinct vision. 



\\"~.-i 



Fig. 15. 



Diagramatic section of an eye (after Mteller) to show : 



That the parallel raj's of light, which enter the normal eye, con- 

 verge or focus on the retina, making a distinct image. 



That, in the farsighted eye, the parallel rays focus at H behind the 

 retina, forming a blurred image on the retina. 



That, in the shortsighted eye, the parallel rays converge at M in 

 front of the retina, forming a very indistinct image on the retina. 



Myopia or shortsightedness is a condition in which the 

 refractive index of the eye is too great or the axis of the 

 eye is too long ; the parallel rays come to a focus in front of the 

 retina (as in fig. 15 — M. ) ; or the principal focus falls in front 

 of the retina. In shortsightedness the cornea may appear 

 very convex or conical as it frequently appears in cattle. 



