THE SENSES 



153 



FlG. 125. — Change of lens in accom- 

 modation. (Jegi.) 



Accommodation. — In order to focus the light upon the 

 retina, the lens must change shape for every change in the 

 distance of the object looked 

 at (see Fig. 125). The shape 

 of the lens can be readily 

 changed, for it is elastic and 

 has muscular fibers around 

 its edges (Exp. 17). 



Defects in the Eye. — Some 

 eyeballs are too long, and the lens brings the rays to a 

 focus before they reach the retina. Such eyes are near- 



sigJited (Fig. 126) and require 

 glasses that round inward (con- 

 cave). Some eyeballs are too 

 flat, and the rays are not brought 

 to a focus soon enough. Such 

 eyes are farsigJited and require 

 glasses that round outward 

 (convex). See Fig. 127. (Re- 

 peat Exp. 15.) 



Care of the Eyes. — Because 

 the eyes can do a large amount 

 of work without giving pain, 

 they are often abused. When 

 reading or doing intricate work, turn the eyes from the 



work occasionally and look 

 at some distant object ; stop 

 work before the eyes are 

 tired. Twilight of early 

 evening has ruined many 

 good eyes. You should 



FIG. I27.-FARSIGHTED EYE (ball r] ^^ ^^ 



too short) which needs convex lens J l 



to focus rays upon retina. the twilight begins, for the 



Fig. 126. — (1) Nearsighted 

 Eye (ball too long), which only 

 focuses rays for near objects 

 (2) when concave glasses are 

 used (3). 



