Newcomb, Simon, 1835-1909, Holden, Edward Singleton, 1846-1914, joint author · New York, H. Holt · leaf 80
he outer or crown glass lens is double convex ; theinner or flint one is generally nearly plano-concave.Fig. 21 shows the section of such an objective as madeby Alvan Clark & Sons, the inner curves of the crownand flint being nearly equal. *By the refractive power of a glass is meant its power… … Fig. 21.—section of object GLASS. 62 ASTRONOMY. A great advantage of the achromatic objective is that itmay be made to correct the spherical as well as the chro-matic aberration. This is effected by giving the propercurvature to the various surfaces, and by making suchslight deviations from perfect…
Astronomy
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