174 THE HUMAN KODY. 



hear object, the shape of the lens is changed a little by 

 an effort of the eye. Too long-sighted eyes have to 

 make more effort to do this than eyes with natural sight. 

 The strain tires and irritates them, and may make them 

 very weak. This trouble can be entirely relieved by 

 glasses. 



TOO SHORT SIGHT. 



13. Short-sighted persons do not commonly have ach- 

 ing and inflamed eyes. Their eyes do not have to make 

 the effort that those of long-sighted people do to see 

 near objects. Reading does not tire them. But short- 

 sightedness is liable to increase. It makes the vision of 

 every thing more than a few feet away indistinct. It may 

 be remedied by glasses, but they are an inconvenient 

 necessity. 



14. Too short sight is very common among students. 

 It is found, that, when children begin to go to school, few 

 of them are short-sighted. In each higher class, there are 

 more short-sighted pupils; and the number increases so 

 fast, that we infer that there is something in the habits 

 of school-children that makes them short-sighted. 



CAUSES OP TOO SHORT SIGHT. 



15. If we inquire what this cause of short sight is, we 

 find that it is not any one thing, but many things. Every 

 thing that tires and strains the eyes of school-children, 

 tends to make them short-sighted. 



Causes of short sight are, 



1. Too much use of the eyes. 



2. Bad light. 



3. Wrong positions when reading. 



