GB200002A - An improved alidade or sight rule - Google Patents

Abstract

200,002. Potter, A. J. Oct. 6, 1922. Sighting - devices; clinometers; crossstaves.-Relates to the use of a right-angled triangular prism in various sighting - devices. In the application to a sight-rule a base e, Fig. 1, provided with a straight-edge or fiducial line d is fitted with a prism c having the plane of its hypotanuse face b arranged parallel to a plane which is normal to the base and contains the line d, the distance between the planes being preferably such that light incident in the direction of the line d emerges in prolongation of that line after refraction and total reflection in the prism. The prism may be shielded from extraneous light by a sheet-metal hood formed with openings at the front and rear to enable an object to be sighted through and over the prism. For sighting a line at a definite angle to the fiducial line a prism is arranged with its hypotenuse face disposed at that angle to the straight edge of the rule as shown in Fig. 3. In a cross-staff, Fig. 4, a base y carries two prisms w, x with their hypotenuses disposed in planes at right-angles, and a reflector 13, of greater height than the prisms, is secured to the face of the prism w which is directed towards the prism x. To set out a line at rightangles to that joining two stakes, the optical centre of the instrument is placed over one stake, a sight line on the other is taken through and over the prism w, a second sight line is taken through and over the prism x and is observed in the reflector 13, and a third stake is set up by an assistant to be observed in this second sight line. Fig. 6 shows a sight-rule mounted for use as a sighting clinometer. The rule is pivoted at a point 14 in the fiducial line d to a vertical graduated scale 15, and the plumb position of the scale is indicated either by a plumb line or by observing the bubble of a spirit level 16 through the prism c as shown. The level 16 is axially adjustable for this purpose. In the application to a telescope two prisms 20, 21 are arranged in front of the object glass 19 as shown in Fig. 8, or four prisms 27, 28, 29, 30 are arranged as shown in Fig. 12, and the telescope is correctly sighted on a point when the two or four images of the point coincide. The apical portions of the prisms may be removed.