REFRACTION IN THE EYE. 697 



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 com- 

 posed 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 dis- 

 tinct, and an image produced by a simple convex lens will thus be surrounded 

 by a circle of colors, like a rainbow. 



In prisms the chromatic dispersion may be corrected by allowing the 

 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 constitute, 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 ; and in the concave lens the colored 

 rays are diverged in the opposite direction. If, then, a convex be combined 

 with a concave lens, the white light decomposed by the one will be recom- 

 posed 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 con- 

 vergence by the one will be neutralized by the divergence of the other, and 

 there will be no amplification of the object. Newton supposed that dis- 

 persion, or decomposition of light, by lenses was always in exact proportion 

 to refraction, so that it would be impossible to correct chromatic aberration and 

 retain magnifying power ; but it has been ascertained that there are great 

 differences in the dispersive power of different kinds of glass, without corre- 

 sponding differences in refraction. This discovery rendered it possible to con- 

 struct achromatic lenses (Dollond, 1757). According to Ganot, Hall was the 

 first to make achromatic lenses, in 1753, but his discovery was not published. 

 In the construction of modern optical instruments, the chromatic aberra- 

 tion is corrected, with a certain diminution in the amplification, by cement- 

 ing together 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, a convex lens of crown-glass be combined with a concave or plano- 

 concave lens of flint-glass, the chromatic aberration of the convex lens may 



be corrected by a concave lens with a curvature 

 which will reduce the magnifying power about 

 one-half. A compound lens, with the spherical 

 aberration of the convex element corrected by the 



FLINT GLASS 



FIG. 255. Achromatic lens. curvature of a concave lens, and the chromatic 

 aberration corrected in part by the curvature, 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 ; but the chromatic aberration 

 is practically corrected in the crystalline lens, probably by differences in the 

 consistence and in the refractive power of its different layers. 



