THE EYE 



353 



People who wear flat lenses must look through the center of the lens 

 to make objects appear clear and sharp. This, undoubtedly, is 

 the reason why people turn their heads frequently when wearing 

 glasses, or look over the tops of their glasses. The eyes are not 

 stationary. They rotate in their sockets, and one moves them across 

 the printed page, or whatever the field of vision may be, without 

 moving the head. A person wearing flat lenses will notice a hazy 

 and distorted appearance at the margin of the lenses. Either the 



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i ;t:i 



t i it 



FIG. 234. These illustrate the effect of looking obliquely through a lens. 

 The clear one was taken with the lens in its proper position; the second 

 with the lens tilted so that the light from the letters passed obliquely 

 through it. Lenses worn on a tilt cause eye strain. 



object is magnified too much or too little. The optical scientist 

 calls this astigmatism and distortion. 



Eig. 232 will show the effect of photographing a church through 

 a strong flat eye lens which was used in place of a camera lens. 

 With the meniscus lens a different kind of picture was obtained, 

 as shown in Fig. 233. 



The meniscus lens has spherical surfaces. It is called toric if 

 one surface has two different curvatures. The use of this lens allows 

 a clear image to be seen at almost any angle to which the eyes are 

 shifted, without moving the head, thus allowing a more normal 

 vision. Sometimes the superiority of the deep curved lens is claimed 



