264 



THE SPECIAL SENSES 



so much of the lens is uncovered that its defects due 

 to irregularities of curvature and of adjustment be- 

 come apparent and we can no longer see clearly. 



Eye strain. As long as the adjustments of the eye 

 are reflex and take place accurately enough to give a 

 fair degree of vision, they ordinarily give rise to little 

 or no trouble either directly or indirectly. When, how- 

 ever, they are put to severe use, the attempt to make, 

 in spite of their defects, the constant and complete 

 adjustment necessary for good vision gives rise to eye 

 strain. Eye strain seldom or never shows itself by dis- 

 comfort about the eyes, but rather causes such symp- 

 toms as nervous irritability, headache, dizziness and 

 nausea, which for years may fail of being traced to the 

 eyes. As a result, the nervous system often suffers 

 seriously, even to the point of a breakdown. 



Astigmatism. In addition to far- and near-sight, 

 another defect which causes much eye strain is astig- 



FIG. 145. If certain of the lines or portions of the circles appear blacker than 

 others, astigmatism is present. The test should be made by shutting one eye 

 and then the other. 



matism. Astigmatism results when either the cornea or 

 the lens has not a perfectly spherical surface but tends 

 to become curved in one diameter more than in another. 



