REFRACTION IN THE EYE 6/3 



indistinct. If a concave or a plano-concave lens is placed in front of 

 this convex lens, which will diverge the rays more or less, the inequality 

 of the divergence by different portions of the second lens will have the 

 following effect : As the angle of divergence gradually increases from 

 the centre toward the periphery, the rays near the periphery which 

 are most powerfully converged by the convex lens will be most widely 

 diverged by the peripheral portion of the concave lens ; so that if the 

 opposite curvatures are accurately adjusted, the aberrant rays may be 

 blended. It is evident that if all 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 is exactly opposite to the aberra- 

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

 not effected correction of every portion of powerful convex lenses in 

 this way ; but by a combination of lenses and diaphragms together, 

 highly-magnified images, nearly perfect, have been produced. Lenses 

 in which spherical aberration has been corrected are called aplanatic. 



It is evident that for distinct vision at different distances, the 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 considered hereafter. 



Chromatic Aberration. A refracting medium does not act equally 

 on the different colored rays into which white light may be decomposed ; 

 in other words, as the white ray falling on 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 is corrected, always produces more or less de- 

 composition of light. The image formed by such a lens consequently is 

 colored at its borders ; and this defect in simple lenses is called chromatic 

 aberration. 



In prisms the chromatic dispersion may be corrected by allowing the 

 colored rays from one prism to fall on a second prism, which latter is 

 inverted, so that the colors are brought together and produce white light. 

 Two prisms thus applied to each other constitute, in fact, a flat plate of 

 glass, and the rays of light pass without deviation. If this principle 

 is 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 ; 

 and in the concave lens the colored rays are diverged in the opposite 

 direction. If, then, a convex is combined with a concave lens, the white 



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