. Astronomy for students and general readers . tist found that a concave lens offlint glass could be combined with a convex lens of crown ofdouble the curvature in such a manner that the dispersivepowers of the two lenses should neutrahze each other, beingequal and acting in opposite di-rections. But the crown glasshaving the greater refractivepower, owng to its greater cur-vature, the rays would be broughtto a focus without is the construction of the Fig. 31.—section of object-achromatic objective. As now glass. made, the outer or crown glass lens is double convex ; theinner o
. Astronomy for students and general readers . tist found that a concave lens offlint glass could be combined with a convex lens of crown ofdouble the curvature in such a manner that the dispersivepowers of the two lenses should neutrahze each other, beingequal and acting in opposite di-rections. But the crown glasshaving the greater refractivepower, owng to its greater cur-vature, the rays would be broughtto a focus without is the construction of the Fig. 31.—section of object-achromatic objective. As now glass. made, the outer or crown glass lens is double convex ; theinner or flint one is generally nearly 21 shows the section of such an objective as madeby Alvan Clark & Soxs, the inner curves of the crownand flint being nearly equal. *By the refractiixpowev of a glass is meant its power of bending therays out of tlxeir course, so as to bring tliem to a focus. By its disfper-sive power is meant its power of separating the colors so as to form aspectnim, or to produce chromalic C2 ASTRONOMY. A great advantage of the aeliromatic objective is that itmay be made to correct the spherical as well as the chro-matic aberration. This is effected by giving the propercurvatuie to the various surfaces, and by making suchslight deviations from perfect sphericity that rays passingthrough all parts of the glass shall come to the same focus. The Secondary Spectrum.—It is now known that thechromatic aberration of an objective cannot be perfectlycorrected with any combination of glasses yet the best telescopes the lu-ightest rays of the spectrum,which are the yellow and green ones, are all brought tothe same focus, but the red and blue ones reach a focusa little farther from the objective, and the violet ones afocus still farther. Hence, if we look at a bright starthrough a large telescope, it will be seen surrounded by ablue or violet light. If we push the eye-piece in a littlethe enlarged image of the star will be
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