. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. n, and wesee. 422. In order that we may see, it is essential that therays should enter the eye parallel or nearly so. Hence theuse of the common magnifying-glass. We bring the glassclose to the eye, and place the object to be magnified in itsfocus,—that is, at c in Fig. 93; the rays which divergefrom the object are rendered parallel by the lens, and weare enabled to see the object, which appears
. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. n, and wesee. 422. In order that we may see, it is essential that therays should enter the eye parallel or nearly so. Hence theuse of the common magnifying-glass. We bring the glassclose to the eye, and place the object to be magnified in itsfocus,—that is, at c in Fig. 93; the rays which divergefrom the object are rendered parallel by the lens, and weare enabled to see the object, which appears large becauseit is brought so close to us. 423. Refraction by Concave Lenses.—If, instead of ar-ranging the prisms as shown in Fig. 92, with their bases together, weplace thempoint to point, itis evident thatthe rays fallingupon them willno longer con-verge ; they willin fact separate,or diverge. Wemay suppose a lens formed of an infinite number of prisms,joined together in this way. Such a lens is called a bi-concave lens. Its shape and action on parallel rays areshown in Fig. 95. 424. Achromatic Lenses.—A lens being equivalent, aswe have seen, to a combination of prisms, we would natu-. FlG -Bi-concave Lens, causing Parallel Eatsto diverge. principle on which we see. 422. In order that we may see, what is essential ?Explain the principle of the common magnifying-glass. 423. Explain the actionof a bi-concave lens. 424. What kind of an image would we naturally expect a 234 ASTKONOMICAL INSTRUMENTS. rally expect it to throw a colored image. This it does;and unless we could get rid of the colors, it would be im-possible to make a large telescope worth using. By com-bining, however, two lenses of different shapes, and madeof different kinds of glass, we cause the color to disappear,thus forming what is called an Achromatic Lens (from theGreek a, without, and %pw/*a, c°Z°r 425. We are able to get rid of color in the image inconsequence of the varying dispersive powers (Art.
Size: 2018px × 1239px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, booksubjectastronomy, bookyear18
