Elements of natural philosophy (Volume 2-3) . must be concave, the other convex; and the powers of the lenses being inversely ELEMENTS OF OPTICS. 307 as the focal distances, we conclude, that chromatic aber-™* *>* ,» 7 77*7* 77, 77 constructing an ration, as regards red and violet, may be destroyed by achromaticlen3uniting a concave with a convex lens, the principal focalfor re(l andlengths being taken in the ratio of their dispersive powers.^ The usual practice is to unite a convex lens of crownglass with a concave lens of flint glass, the focal distance usualof the first being to that of
Elements of natural philosophy (Volume 2-3) . must be concave, the other convex; and the powers of the lenses being inversely ELEMENTS OF OPTICS. 307 as the focal distances, we conclude, that chromatic aber-™* *>* ,» 7 77*7* 77, 77 constructing an ration, as regards red and violet, may be destroyed by achromaticlen3uniting a concave with a convex lens, the principal focalfor re(l andlengths being taken in the ratio of their dispersive powers.^ The usual practice is to unite a convex lens of crownglass with a concave lens of flint glass, the focal distance usualof the first being to that of the second as 33 to 50,com ina lon;these numbers expressing the relative dispersive pow-ers as determined by experiment; (see Table §116). Theconvex lens should have the greater power, and, there-fore, be constructed of the crown-glass; otherwise, theeffect of the combination would be the same as that of convex lens should have the a concave lens with which it is impossible to form a x greater power; real image of a real object. Fig. Illustration ; To illustrate : let parallel rays be received by the lensA ; its action alone would be, to spread the differentcolors over the space VE, whose central point m, is dis-tant from A, 33 units of measure, (say inches), the violetbeing at V, and the red at R\ the action of the lens B,alone would be, to disperse the rays as though they pro-ceeded from different points of the line V B\ whose Explanation ofcentral point m, is distant from B, 50 inches, the violet ^JJ^J^appearing to proceed from V\ and the red from R\ and theeffect of their united action would be, to concentrate thered and violet at F, whose distance from the lens isequal to the value of F, deduced from the formula 1 -| 1 Example; 1 _ _ j - = — inches. 2T - 33 ^ 50 97 , 06 308 NATURAL PHILOSOPHY. Point in which q\*red and violetwould be united; F = — 97 , 06 inches. Geometricalillustration ; Fig. 84.
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