GB387776A - Apparatus for reading a divided circle - Google Patents

Abstract

387,776. Theodolites &c. BAKER, T. Y., Fircroft, Burton's Road, Hampton Hill, Teddington, Middlesex. Nov. 7, 1931, No. 30887. [Class 97 (ii).] When reading the divided circle of a theodolite &c., an image of a scale marking 180‹ from the marking it is desired to read is formed in proximity to and is then compared with the latter. Thus one marking is always brought to an image during the optical transmission once more than the other. In the form shown in Fig. 1, the graduated circle comprises a fixed glass cylinder 1. Alternatively a transparent or opaque disc or conical member may be used. A spindle 5 rotatable with the telescope carries an arm 4 supporting a micro. scope 3 opposite the marking to be read and lenses 6 adapted to form an image adjacent the latter of the diametrically opposite marking. Cylindrical lenses 7 on arms 8 are provided to counteract the optical effect of the cylinder 1. The microscope 3 is provided with a mask 11 provided with an aperture enabling the degree markings to be read and with two adjacent apertures separated by a strip serving as a datum line. In use the mask is moved by a micrometer until the datum line is midway between the two images. In the theodolite shown in Fig. 5, the marking it is desired to read and an image of the opposite marking obtained by light from the mirror 21 and lenses 6 is transmitted to the microscope 3 by lenses 22, 23, 24 and prisms 25, 26, 27. A mask 11 is provided with an aperture 12 (Fig. 7), indicating degree markings and two apertures 13, 14, forming a datum line 16. In use, the two images of the markings are moved together until the datum line is midway therebetween by a parallel sided plate glass 33 included in the optical path and mounted on a pivoted plate 28 rocked by the knob 30. The plate 28 is connected by a pin and slot to a slide 35 carrying a transparent screen 37 adapted to indicate the correction for centreing error in the microscope 3 through a further aperture 38 in the screen 11. A similar graduated circle 1a is provided for the motion of the telescope 19 about the horizontal axis and a prism 41 may be adjusted when desired to reflect the indications thereof into the optical system for reading.