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Description of the two feet and half achromatic telescope
Made by P. and J. Dollond, in St. Paul's Church-Yard, London. The object-glass of this Telescope is composed of two glasses, one convex of crown glass, and the other concave of white slint glass; the diameter of the aperture is two inches. The plate represents the Telescope on its stand: AA is the wooden tube 2 1/2 feet long. The tube BB, that contains four eye-glasses, to be used for land-objects; and also the tube C, that contains two eye-glasses for astronomical purposes, are to be serewed into the end of the brass tube D. By turning the button a, this tube moves out of the wooden tube to adjust the eye-glasses to the proper distance from the object-glass, so as to render the object distinct. The magnifying power with the eye-glasses for land objects is near 50 times, and with those for astronomical uses it is 80 times. The stained glass b is applied to the eye-tube C to guard the eye in observing the spots on the sun. This glass must be taken off when the eye-tube is used for any other purposes. The vertical and horizontal motions are given to the Telescope by means of the joints at c and d. Description de la lunette Achromatique de Deux Pieds et Demi, Fait pair P. et J. Dollond, dans la Cimitiere de St. Paul a Londres -
Description of the improved achromatic telescope
made with brass sliding tubes, by P. and J. Dollond, opticians