238 THE WONDERFUL HOUSE THAT JACK HAS 



casionally to gaze off as far as possible into the dis- 

 tance, for during such an interval these muscles are 

 completely relaxed. Forming this habit is one of 

 the best ways to help keep the eyes strong and vig- 

 orous, for occasional complete rest has the same good 

 effect on them that it has on other organs of the 

 body. 



Another muscular effort necessary in using the eyes 

 for close work is convergence. By this is meant turning 

 the eyeballs inward so that both may look at the same 

 object at the same time. By holding a small object 

 three or four inches from the eyes and looking at it 

 closely, the effort to turn the eyes inward can be 

 distinctly noticed, and, if continued, the act will 

 soon become painful. Whenever the range of vision 

 for close work changes, both of these sets of muscles 

 have to accommodate the form of the lens and the 

 position of the eyeballs to correspond with the size 

 and position of what is being examined. 



The ability of the eye to make these changes is 

 often spoken of as its power of accommodation. In 

 health, these muscles in a normal eye work together 

 in perfect harmony ; but if either set becomes weakened, 

 they do not work well together, and weak or painful 

 sight is the result. From these facts, it can be readily 

 seen that the usual cause of weak sight in persons 

 having otherwise normal eyes, is that these muscles 

 have been made tired and sensitive by overwork. 

 Now, the accommodation muscles in normal eyes are 



