VETEEINAKY OPHTHALMOLOGY. 11 



refracted than those wl^ich pass near the circumference ; 

 so, you see, the nearer the eircumference, the more 

 the amount of refraction. This is called spherical ab- 

 ferration, which may be corrected as follows : — ^Increase 

 the density of the central part of the leiis, which will 

 cause it to act more strongly on the rays. Now see : 

 this is just what the lens (crystalline) does in the eye, 

 as it is more dense in its center than periphery. Or, 



Fig. 8. 

 The rays passing through the "edges, of the lens have a sho'rter' focal dis- 

 .• ; . tanoe than those passing, nearer to the center. 



placing a difiphragm between the object of which the 

 ■ima^e is to be- formed and the lens, : thus cutting off 

 those rays which pass through' the peripheral portion 

 of the lens, the image therefore being formed by the 

 rays passing through the center. This is also a con- 

 dition existing in the eye, for have we not the iris, and 

 what is the iris but a diaphragm, which is capable of 

 raodiflcation? Now, light, after all, is a composite 



