GB563872A - Improvements in optical instruments for the precise measurement of angles - Google Patents

Abstract

563,872. Theodolites &c. COOKE, TROUGHTON & SIMMS, Ltd., and TAYLOR, E. W. April 2, 1943, No. 5322. [Class 97 (ii)] In an optical system for the measurement of angles such as in theodolites as described in Specification 288,416, [Class 97 (ii)], in which light from the sky &c. is reflected by a mirror 3 into two coaxially arranged prisms 1, 2, thence through two further prisms 4, 5, through apertures 6, 7 into the mounted glass circle plate 8, back thence to the prisms 4, 5 through objectives 13, 14, deflecting prisms or plates 19, 20, field prisms 11, 12 and thence to the eye-piece, it has been found that due to the use of materials of different coefficients of expansion in the reading system, whereby a symmetrical displacement of the image of the circle graduations takes place, though no reading error, is thereby introduced, whereby the sensitivity of the setting is impaired or means of adjustment had to be provided for correction. Accordingly, each objective mount is provided with means controlling the change of distance with temperature of the centre of each objective relatively to the base line 24 so that it conforms closely to corresponding changes affecting the distance of the focal plane from the same base line 24. As shown in Fig. 2, the mount 15 of each objective 13 is secured to the metal prism plate 16 by studs 17, 18 passing through a distance-piece 22 of low coefficient of expansion, such as the alloy known under the Registered Trade Mark " Invar," and a similar piece 23 of high coefficient of expansion, such as the material known under the Registered Trade Mark " Perspex," the arrangement being such that on change of temperature the mount 15 rocks on the distance-piece 22.