Telescope Book PDF, ePub eBook

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Book Descriptions:
This historic book may have numerous typos, missing text or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1861. Not illustrated. Excerpt: ... OF THE METHODS PRACTISED IN THE ACTUAL CONSTRUCTION OF LENSES AND SPECULA. (84.) Having thus described the principal combinations of refracting and reflecting surfaces, which have been devised for the construction of telescopes, it remains to say something of the practical means which have been adopted for the actual manufacture of the specula and lenses. They require a delicate and difficult art, especially as regards the former, where parabolic, elliptic, or hyperbolic figures have to be communicated to the polished surfaces in the very act of polishing them. In the case of lenses, as we have seen, all the requisites of a perfect telescope can be attained by the use of truly spherical surfaces, and these it is comparatively easy to form, by reason of the natural tendency of two surfaces which grind each other by equable rubbing over every part, to work each other into a spherical concavity and convexity exactly fitting. Practically speaking, when it is intended to work a glass surface to a sphere of an exactly assigned radius, the nearly approximate spherical form of the right curvature is first communicated to it by grinding in a metal basin, of the proper concavity, which may be done to some considerable nicety by turning the tool in a lathe, to fit a guage. The lens is then connected, or otherwise firmly attached, to the lower end of a vertical rod of wood or metal AB (fig. 23), the upper end of which terminates in a steel ball F, working in a cup c, to which it has been accurately fitted by smooth grinding, so that every point of the surface A of the lens, when made to oscillate or revolve conically by a motion given to the rod by a hand grasping it at B (where it is enveloped in woollen cloth or felt, to prevent the communication of warmth), will move in ...

Telescope by John F. W Herschel

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