674 SPECIAL SENSES 



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 divergence of the other, and there will be 

 no amplification of the object. Newton supposed that dispersion, or 

 decomposition of light, by lenses was always in exact proportion to re- 

 fraction, 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 corresponding differences in refraction. This discovery ren- 

 dered it possible to construct achromatic lenses (Dollond, 1757). Ac- 

 cording to Ganot, Hall was the first to make achromatic lenses, 1753, 

 but his discovery was not published. 



In the construction of modern optical instruments, chromatic aberra- 

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

 ing together lenses made of different materials, 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 is combined 



with a concave or plano-concave lens of flint- 



^^~ ^^\ glass, the chromatic aberration of the convex 



--- ~^ I lens may be corrected by a concave lens with a 



curvature that will reduce the magnifying power 



Fig. 175. Achromatic lens. , v ir A j i -*.i- ^u 



about one-halt. A compound lens, with the 



spherical aberration of the convex element corrected by the curvature 

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

 curvature and in part by the greater refractive power of flint-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 aberra- 

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

 in consistence and in the refractive power of its different layers. 



FORMATION OF IMAGES IN THE EYE 



It is necessary only to call" to mind the general arrangement of the 

 structures in the eye and to apply the simple laws of refraction, in order 

 to comprehend how images are formed on the retina. 



The eye corresponds to a camera obscura. Its interior is lined with 

 a layer of dark pigment-cells, the immediate action of which is to 

 prevent the confusion of images by internal reflection. The rays of 

 light are admitted through a circular opening (the pupil), the size of 

 which is regulated by the movements of the iris. The pupil is con- 



