A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . the expense of light. The achromatism is judged of by pointing the telescope tosome bright object, as the moon or Jupiter, and alternately push-ing in and drawing out the eye piece from the place of most per-fect vision : in the former case, if the lens is good, a ring of purplewill appear round the edge of the image, in the la
A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . the expense of light. The achromatism is judged of by pointing the telescope tosome bright object, as the moon or Jupiter, and alternately push-ing in and drawing out the eye piece from the place of most per-fect vision : in the former case, if the lens is good, a ring of purplewill appear round the edge of the image, in the latter, a ring ofpale green (which is the central color of the prismatic spectrum);for these appearances show that the extreme colors of the spec-trum, red and violet, are corrected. 11. Achromatic eye pieces.—The eye pieces now most commonlyused are of two kinds: the Hiiygenian and the Ramsdeii. The Hiiygenian eye piece consists of two plano-convex lenses of crown glass, A and B^^s- 6. (Fig. 6), the convex sur- faces of both being turnedtowards the object. Thefirst lens A receives theconverging rays Sa, Sb,coming from the objectglass, before they havereached the principal fo-cus _Fof the object glass,and brings them to a focus F half-way between the two lenses. EYE PIECES. 21 A and B. The focal length of the lens jB being made equal toBF\ the image formed at F is distinctly visible to an eye be-hind B. Since this eye piece is adapted to rays already converg-ing, instead of diverging rays, it is commonly called the negativeeye piece. The Ramsden eye piece is shown in connection with the tele-scope in Fig. 5. It also consists of two plano-convex lenses;but the plane surface of the lens nearest the object is turnedtowards the object. The diverging rays from an image i^arerendered less divergent by the first lens, and finally parallel bythe second lens; after emerging from the latter, therefore, theyare adapted for distinct vision to an eye placed behind it. Thiseye piece being ad
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Keywords: ., bookcentury1800, bookdecade1, booksubjectastronomicalinstruments
