XVIII 



THE SPECIAL SENSES AND SENSATIONS 



273 



The lens may be too nearly flat or the eyeball too short, 



so that the rays of light focus behind the retina ; this 



is called "farsightedness" The 



book must be held far from 



the eyes because objects near 



by cannot be seen well. The 



cornea may not curve uniformly 



in all directions, so that the 



rays are not all focused on the FlG - 148. - Parallel rays focused 



on the retina (Zuppke). 



same surface ; this is called 



astigmatism. The muscles of the two eyes may not 



work as they should in focus- 

 ing upon near and far ob- 

 jects or in moving the two 

 eyes together. 



As a matter of fact, very 

 few eyes are perfect in 

 every respect. But most of 

 the defects, if not too serious, 



are overcome by the wonderful power of accommodation 



which the eyes of the young 



possess. As long as a person 



is in good health or does little 



reading or studying, he may 



not know that his eyes are 



defective. When, however, 



general health fails, or much 



indoor work requiring close 



application of the eyes is attempted, the eye strain may 



be so severe as to cause serious discomfort and lead to 



FlG. 149. Parallel rays focused in 

 front of the retina (Zuppke). 



FlG. 150. Parallel rays focused 

 behind the retina (Zuppke) . 



