GB615183A - Improvements in scale reading devices for instruments for measuring angles - Google Patents

Abstract

615,183. Surveying instruments. SOC. D'OPTIQUE ET DE MECANIQUE DE HAUTE PRECISION. Aug. 1, 1946, No. 23011. Convention date, Oct. 18, 1943. [Class 97 (ii)] An optical system for reading the diametrically opposed graduations of a circular scale of a theodolite or like surveying instrument is characterized by the provision of a..single translationally movable, deviating member, whereby fractional parts of the angular reading may be measured. Light rays from the two diametrically-opposed graduations of the glass scale 1, Fig. 3, pass through lens and prisms systems 2, 3, 4, 5, 6, 10, 8 and 2<1>, 31, 41, 5, 6<1> 10, 8 to the eye-piece 9, Fig. 1, in which the two real images of the graduations formed on either side of the vertical edge 7 of the prism 8 are viewed. The optical paths are so arranged that the increasing directions of the images of the graduations are in reversed relationship. In order to read the fractional parts of the angle being measured the deviating prism 10 is moved until the images of the two graduations separated by 180 degrees coincide when the fractional part may be read off on a glass scale 11, viewable in the eye-piece 9, and which is attached to and moves with the prism 10. Prisms 10 and 8 are such that movement of prism 10 causes both images to move in the increasing sense of their numbering. By rotation of a prism 18 the graduations of the vertical scale 13 may be read off in a similar manner. A modification in which the prism 8 is formed of two prisms with silvered faces is described.