250 THE HUMAN MECHANISM 



securing distinct retinal images, the student will see that this 

 defect must seriously interfere with clear vision. The optics 

 of astigmatism are too complicated to be explained in an 

 elementary work, but the defect reveals itself generally in 

 an inability to see with equal clearness lines running in dif- 

 ferent directions. Thus some of the lines in Fig. 98 will be 

 sharply defined and black while one is looking with one 

 eye at the white center, and others will be blurred and 

 lighter in color. 



Astigmatism is of special importance in reading, because 

 the lines of printed letters run in different directions. The 

 effort to see clearly the printed page is often severe, and 

 results in headaches and other general disturbances of health, 

 the true cause of which is often unsuspected. The trouble 

 m&y usually be corrected by the use of so-called " cylindri- 

 cal " glasses ; that is, glasses which compensate the defects 

 of curvature in lens and cornea. 



12. Accommodation and "near" work. The above-described 

 defects of the eye as an optical instrument may usually be 

 successfully corrected by the use of proper glasses, which 

 should, generally speaking, be prescribed by a good oculist 

 and not by an optician. Glasses may be used for various 

 reasons as a matter of convenience, as where a person 

 with slight myopia wears them merely to see distant objects 

 clearly; or of necessity, as when the myopia is more pro- 

 nounced; or they may serve the much more important pur- 

 pose of relieving the muscle of accommodation of undue 

 work in reading or sewing, and thus of avoiding " eye- 

 strain." A hypermetropic eye should always be provided 

 with glasses, since otherwise its muscle of accommodation 

 cannot be rested by looking at distant objects. But since it 

 is near work which requires the greatest effort of accom- 

 modation, it is in reading, writing, drawing, sewing, etc. 

 that the eyestrain is apt to be greatest. As this kind of 

 work is constantly increasing in modern life, the need for 



