Patent GB-190301462-A (GB)
Title: Improvements in Rangefinders and other Optical Instruments.
Inventor: BARR ARCHIBALD
Filed: 1903-01-21
Granted: 1904-03-21
Abstract: 1462. Barr, A., and Stroud, W. Jan. 21. Range finders; optical squares; prisms and the like.-Relates to (1) prisms and prism-combinations for functioning as optical squares ; (2) prisms and reflectors for the eyepieces of range-finders ; (3) mountings; (4) range-indicating arrangements ; and (5) methods of adjusting. Several combinations of a pair of right-angled or other prisms are shown, for turning light through a right-angle. Fig. 3 shows one arrangement, and Fig. 4 another. The ordinary pentagonal or Prand'l prism is modified in various ways. In one, only half the height of the entrance and exit faces are utilized, with the object of avoiding the doubling of the image due to want of accuracy in the working of the roof faces. Fig. 8 shows a form in which one of the roof faces has been replaced by a silvered one ; or, as shown by Figs. 9 and 9<a>, both roof faces may be replaced by flat faces at which total internal reflection occurs by fitting the entrance and exit faces with prisms, as shown. Fig. 10 shows a substitute for the usual rectangular prism, in which the hypotransal face has been replaced by two others. Economy of material is thus effected. To prevent egress of the light at the surface of a prism, a thin plate of a highly-refracting glass may be cemented on to it. In order that the optical axis of the eyepiece of a range-finder may be inclined to the plane of virtual triangulation, and also for rectifying the image, the light, before entering the eyepiece, may be reflected upwards, as shown by Fig. 13<c> ; or the same object may be effected by giving a convergence to the light from the two objectives passing through the prisms r<1>, r<2> respectively. This may be done by making the incidence faces of the prisms r<1>, r<2> inclined to the vertical. The prism described in Specification No. 3172, A.D. 1901, may be made in two parts and have flats to facilitate binding together. When the two partial images are seen in the way described in Specification No. 13,507, A.D. 1893, the requisite inclination of the light rays, as they approach the dividing-edge, may be secured by various dispositions of prisms or by modifying the inclinations of the prism faces. Fig. 25<d> shows such a compound eyepiece prism in perspective. Figs. 25<e> and 25<f> show detached elements. The prism is built up of three parts r<1>, r<2>, r<3>. The inoperative faces are shown hatched. Figs. 23 and 23<a> show, in sectional plan and elevation respectively, a range-finder. Optical squares B<1> are mounted at the ends of an outer tube A within which a second tube D is mounted, at one end by a gimbal joint and ring I and at the other by a ball joint H. A prism N, as described in Specification No. 9520, A.D. 1888, operated by a screw P and nut Q, is employed to adjust the two images. A pinion S, on the screw P, engages with a spirally-disposed rack on a drum U which carries the distance scale. The pinion may be disengaged from the rack to effect adjustment of the scale. The halving adjustment is effected by a parallel plate x, which is adjusted by rotation about pivots y by means of a screw v. The distance scale may be helical, as described in Specification No. 18,273, A.D. 1902. Portability may be secured by telescoping or folding the outer tube.