790 SPECIAL SENSES. 



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 oppo- 

 site to the aberration 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 combination 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 aberration. 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 con- 

 sidered 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 

 decomposed ; 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 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 borders, the different colors will be separate and distinct, and an image pro- 

 duced by a simple convex lens will thus be surrounded by a circle of colors like a rain- 

 bow. 



In prisms, the chromatic dispersion may be corrected by allowing tbe colored rays 

 from one prism to fall upon a second prism, which is inverted, so that the colors will be 

 brought together and produce white light. Two prisms thus applied to each other con- 

 stitute, in fact, a flat plate of glass, and the rays of light pass without deviation. If this 

 law be applied to lenses, it is evident that the dispersive power of a convex lens may be 

 exactly opposite to that of a concave lens. By the convex lens, the colored rays are 

 separated by convergence and cross each other ; while, in the concave lens, the colored 

 rays are dispersed in the opposite direction. If, then, we combine a convex with a con- 

 cave lens, the white light decomposed by the one will be recomposed by the other, and 

 the chromatic aberration will thus be corrected. But, in using a convex and a concave 

 lens composed of the same material, the convergence by the one will be neutralized by 

 the dispersion of the other, and there will be no amplification ofrthe object. 



In the construction of optical instruments, the chromatic aberration is corrected, with 

 but slight diminution in the amplification, by combining lenses made of different material, 

 as of flint-glass and crown-glass. Flint-glass has a much greater dispersive power than 

 crown-glass. If, therefore, we use a convex lens of crown-glass combined with a concave 

 lens of flint-glass, the chromatic aberration of the convex lens may be corrected by a con- 

 cave lens with a curvature which will take but little from the magnifying power. A com- 

 pound lens, with the spherical aberration of the convex element corrected by the curvature 

 of a concave lens, and the chromatic aberration corrected by the curvature, in part, and 

 in part by the superior refractive power of flint-glass over crown-glass, will produce 

 a perfect image. 



Although the eye is not absolutely achromatic, the dispersion of light is not sufficient 

 to interfere with distinct vision. We can understand how the chromatic aberration is 

 practically corrected in the crystalline lens, when we remember that its various layers 

 are of different consistence and of different refractive power. 



