GB128311A - Improvements in Telescopic Sighting Instruments such as Sextants Rangefinders or the like. - Google Patents

Abstract

128,311. Taytor, A., and Gray, P. W. July 31, 1917. Sextants. - Telescopic sighting-instruments such as range-finders and sextants comprise a pair of similar telescope systems directed into an eyepiece, one telescope being directly in line therewith, and the other reflecting light into it by a pentagonal prism or optical square arranged permanently between the object-glass and the eyepiece and usually occupying half the field of view; the emergent face of the prism may be in, or approximately in, the focal plane. In the arrangement shown in Fig. 1 for an instrument of the metrometer type, a mask in a distant instrument will be observed directly through the objective 1 and eyepiece 3, 2, half of the field of which is occupied by a pentagonal prism 4 receiving light from the range-object through the objective 5. A roof-prism 6 is placed between the field-lens 3 and the eye-lens 2; or an ordinary four-lens erecting eyepiece may be used. Any known means, such as a sliding-prism, may be added to the instrument for range-finding purposes. In Figs. 4 and 5 is shown a short homebase range-finder in which the telescope objective 9 receives light from the end prism 8, light from an objective 11 at the other end of the base being received by the pentagonal prism 12 occupying half the field of view of the eyepiece 10. The emergent surface of the prism 12 is preferably at or near the focal plane of the objectives. In another form of short-home-base range-finder, light received at one end of the base is reflected inwards by a pentagonal prism and received by a second reflecting- means consisting of a prandl prism, which covers one half of the field of view of an eyepiece receiving the light from this second reflecting-means, and also receiving in the other half-field light direct from an objective. For a sextant, an eyepiece is provided with a prandl prism (or optical square) covering half its field, and is arranged to view the horizon directly through a telescope objective, while the pivoted reflecting-surface of the sextant reflects light into another objective and then into the prandl prism &c., the focal plane of the objectives being coincident, or substantially so, with the emergent face of the prandl prism &c.