GB541948A - Improvements in or relating to theodolites - Google Patents

Abstract

541,948. Theodolites. COOKE, TROUGHTON& SIMMS, Ltd., and TAYLOR, E. W. Aug. 14, 1940, No. 12993. [Class 97 (ii)] In an optical theodolite having vertical and horizontal circles of transparent material bearing graduations thereon, and optical systems adapted to produce images of both graduations in a common focal plane, a transparent index setting plate is provided through which light rays from the vertical circle pass, the index setting plate being angularly adjustable about an horizontal axis for the purpose of displacing the image of the graduations on the vertical circle laterally into accurate registration with a datum line or pointer in order to obtain accurate readings. As shown, light reflected from a mirror 1 forms, adjacent to a field lens 13, images of the graduations on the vertical and horizontal circles 5, 24 by means of the optical systems described in Specification 541,947. These images, together with an horizontally adjustable fine reading scale 12, are viewed through an eye piece 19 and the graduations are read off against pointers carried by an apertured masking plate. The rays form ing the image from the vertical circle 5 pass through a parallel plate of glass 8 capable of rotation about an axis VI ... VI by means of an adjusting screw, in order to displace the vertical circle image within its masking aperture. To correct the datum reading, the vertical circle image is displaced by rotating the adjusting screw so as to bring the appropriate graduation into accurate registration with the datum pointer in the mask aperture. In a modification the datum setting of the vertical image is effected by levelling an altitude bubble 60, Fig. 4, in the line of sight. The bubble is carried by a frame 61 rocked by means of a setting screw 62 which, through a ball connection 64, also imparts a rocking movement to the index setting plate 8 and thereby deflects the vertical circle image the appropriate amount. The eyepiece assembly 55 is detachably secured to the instrument by a threaded ring 56 which permits angular adjustment and ready detachment for stowage in its box.