82 SPECIAL SENSES. 



the rays near the periphery, which are most powerfully con- 

 verged by the convex lens, will be most widely diverged by 

 the peripheral portion of the concave lens ; so that, if the 

 opposite curvatures be accurately adjusted, the aberrant rays 

 may be blended. It is evident that, if all of the rays were 

 equally converged by the convex lens and equally diverged by 

 the concave lens, the action of the latter would be simply to 

 elongate the focal distance ; and it is equally evident that, if 

 the aberration of the one be exactly opposite to the aberra- 

 tion of the other, there will be perfect correction. Mechanical 

 art has not enabled us to effect correction of every portion of 

 very powerful convex lenses in this way ; but, by a combi- 

 nation of lenses and diaphragms together, highly-magnified 

 images, nearly perfect, have been produced. 



It is evident that, for distinct vision at different distances, 

 the crystalline lens must be nearly free from spherical aberra- 

 tion. This is not effected by a combination of lenses, as in 

 ordinary optical instruments, but by the curvatures of the lens 

 itself, and by certain differences in the consistence of different 

 portions of the lens, which will be fully considered hereafter. 



Chromatic Aberration. We have already alluded to the 

 fact that a refracting medium does not act equally upon the 

 different colored rays into which pure white light may be de- 

 composed ; in other words, as the pure ray falling upon the 

 inclined surface of a glass prism is bent, it is decomposed into 

 the colors of the spectrum. As a convex lens is practically 

 composed of an infinite number of prisms, the same effect 

 would be expected. Indeed, a simple convex lens, even if 

 the spherical aberration be corrected, always produces more 

 or less decomposition of the light. The image formed by 

 such a lens will consequently be colored ; and this defect in 

 simple lenses is called chromatic aberration. At the same 

 time, it is evident that the centre of the different rays from 

 an object will be composed of all the colors of the spectrum 

 combined, producing the effect of white light ; but, at the 



