SHORT SIGHT AND LONG SIGHT. 



327 



retina, instead of light from the near. The power of chang- 

 ing the form of the lens according as near or distant objects 

 are looked at is called "accommodation" 



Short Sight and Long Sight. In the normal eye the range 

 of accommodation is very great, allowing light from objects 

 infinitely distant up to 

 that proceeding from 

 those only about eight 

 inches in front of the eye 

 to be brought to a focus 

 on the retina. In the 

 natural healthy eye par- 

 allel rays of light meet on 

 the retina when the mus- 

 cles controlling the crys- 

 talline lens are at rest and 

 the lens is at its flattest 

 (A, Fig. 94). Such eyes 

 are emmetropic. In other 

 eyes the eyeball is too 

 long from before back; 

 in the resting state paral- 

 lel rays meet in front of the retina (B). Persons with 

 such eyes cannot see distant objects distinctly without the 

 aid of diverging (concave) spectacles; they are short-sighted 

 or myopic. Or the eyeball may be too short from before 

 back; then, in its resting state, parallel rays are brought 



What is meant by the "accommodation" of the eyeball? 



Where do parallel rays of light which have entered a healthy eye 

 meet in a focus? Where do such rays meet when the eyeball is too 

 long from back to front? What is the result as regards vision? 

 What form of spectacle lenses do short-sighted persons require? Ex- 

 plain what is meant by a hypermetropic or long-sighted eye. What 

 sort of spectacles do long-sighted persons require ? 



FIG. 94. Diagram illustrating the path 

 of parallel rays after entering: an emme- 

 tropic 04), a myopic (B), and a hyperme- 

 tropic ((7) eye. 



