558 STUDIES IN ADVANCED PHYSIOLOGY. 



5. Spherical Aberration. Spherical aberration is not 

 so much .a real defect of the eye as it is a somewhat uni- 

 versal defect of all lenses, and it is therefore in common 

 with all lenses shared by the crystalline lens. All the rays 

 of light passing through a lens do not meet at the same 

 point, but the rays of light passing near the edge of the 

 lens are brought to a focus sooner than those passing nearer 

 the middle, and consequently when the edge of the lens is 

 in focus the center is blurred, and vice versa. Instances of 

 such spherical aberration are frequently noticeable on or- 

 dinary opera-glasses, where when objects seen through the 

 middle of the lens are in focus the field around the edge of 

 the lens seems blurred. There would be two ways to remedy 

 this defect. One to cut the light out of the edge of the lens 

 by placing a circular shutter over it. Such is, in fact, very 

 usual in optical instruments, and photographers frequently 

 interpolate a shutter having a small hole in it through 

 which the light is able to reach the center of the lens only. 

 A similar arrangement exists in the eye where the iris cuts 

 off the entire outer portion of the crystalline lens and per- 

 mits the light passing through the pupil to reach the cen- 

 ter of the lens only. V 



A second way to remedy spherical aberration would be 

 to increase somewhat the density of the middle of the lens, 

 for as the rays of light passing through the edge of the lens 

 are bent more an increase of the density of the middle of the 

 lens would bend the rays passing through it more, and so 

 the foci would coincide. Both of these plans are followed 

 in the eye, the function of the iris having just been pointed 

 out, and in addition to this the lens has a slightly greater 

 density in the middle, the result of these two agencies being 

 to reduce this defect in most eyes to an imperceptible mini- 

 mum. 



6. Chromatic Aberration. A second almost universal 

 defect of lenses is chromatic aberration. This defect arises 

 from the unequal refraction which the different colors suffer 

 while passing through a lens. It will be remembered that the 



