CHROMATIC ABEEKATION. 83 



borders, the different colors will be separate and distinct, and 

 an image produced by a simple convex lens will thus be sur- 

 rounded 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 to- 

 gether 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 ; 

 while, in the concave lens, the colored rays are dispersed in 

 the opposite direction. If, then, we combine a convex with a 

 concave 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 of the object. 



In the construction of optical instruments, the chromatic 

 aberration is corrected, with but slight diminution in the am- 

 plification, 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 concave lens with a curvature which will take 

 but little from the magnifying power. A compound lens, 

 with the spherical aberration of the convex element corrected 

 by the curvature of a concave lens, and the chromatic aberra- 

 tion 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 dis- 

 persion of light is not sufficient to interfere with distinct 



