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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1871 Excerpt:...and Testing Reflecting Telescopes.--To adjust the specula of a Cassegrainian or Gregorian instrument, procure a Eamsden's eye-piece, which will render an object visible in the compound focua of the two lenses of which it is composed; then hold this eye-piece in front of the Huyghenian eye-piece of the telescope, and, by varying the distance, find the position in which the image of the large speculum is seen, well defined through both eye-pieces, and, if the image of the small speculum is seen precisely on the centre of the large one, the metals may be considered as rightly placed; but, if not, the proper screws must be used in succession, till the required position is determined. When the face of the large metal stands at right angles to the length of the tube, the adjustment may generally be finished without disturbing it; and, when the bed that receives it has once been properly finished, it will be advisable not to alter it, unless some accident should render such alteration indispensable. To try whether the figures of the metals are adapted for each other: let the instrument be directed to some luminous point, as a white disc on a black ground, or, what is better, to a star: then having adjusted for distinct vision, firstly observe if the disc or star is well defined, and free from irradiations; secondly, carrying the small speculum short distances beyond, and short of, the place for distinct vision, examine if the disc or star enlarges alike in similar changes of position: if the result be satisfactory, the metals may be considered as well placed, and well adapted for each other. To try whether the large speculum partake of the parabolic form, let the aperture be partially covered, first at the central part, and then round the circumference, by tin, pa...