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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. 1856 Excerpt:...or spider-lines at the place of the image in the field of view, for the purpose of accurately measuring the position of an object, at the time of observation, or to apply an apparatus, called a micrometer, for measuring the dimensions of an image, the Huyghenian eye-piece can no longer be employed. We have then recourse to Ramsden's Eye-piece, called a Positive Eye-piece (p. 79). This consists of two lenses of equal focal lengths, one plano-convex, and the other convexo-plane, so that the convex sides are turned towards one another, the interval between them being equal to two-thirds of the focal length of either. When the telescope is arranged for distinct vision of a distant object, the field-lens /, is placed at a distance from the object-glass o, greater than the focal length of this glass by one-fourth of its own focal length. The focus of the object-glass is then also the focus of the entire eye-piece, and the rays proceeding from the image at i, emerge from the eye-lens e, parallel, or in the condition best adapted for distinct vision. This eye-piece is not achromatic, but the spherical aberration is less with it than with the Huyghenian eye-piece. Whether the eye-piece be positive or negative, a diaphragm is placed at the place of the image so as to intercept all the extraneous light. With the eye-pieces of which we have been speaking, the object appears inverted, which is no inconvenience when this object is one of the heavenly bodies. These eye-pieces are consequently called celestial eye-pieces. For the convenient observation of stars near the zenith, a plane reflector or prism is placed in the eye-piece, by which the directions of the pencils are turned, so that the axis of the eye-lens is at right angles to the axis of the instrument. Such an e...