Patent GB-415941-A (GB)
Title: Improvements relating to tacheometers
IPC: G01C3/30
Filed: 1934-01-15
Abstract: 415,941. Tacheometers. BAROT, J. H., 20, Rue de Varenne, Paris. Jan. 15, 1934, Nos. 1442 and 1444. Convention dates Jan. 17, 1933 and Sept. 28, 1933. [Classes 97 (i) and 97 (ii).] In an instrument for measuring distances reduced to the horizontal, the deviating system for producing a parallactic angle varying with the angle of sight comprises a prism, rigidly connected to the telescope and producing a deviation equal to half the parallactic angle corresponding to a horizontal sight and two other prisms each having an angle half that of the first prism and deviating in the same sense, these two prisms being horizontal when the line of sight is horizontal and being rotated about the optical axis of the telescope in opposite directions each through an angle twice the inclination of the sight line, when the latter is inclined. In the form shown, the light incident on the telescope C, D, E, passes undeviated through a peripheral zone A of the object glass but the part passing through the central zone is deviated to form the parallactic angle by a fixed prism P<3> and prisms P<2>, P<1>. The mounts for the prisms P<2>, P', are rotatable about the telescope axis and are provided with teeth Q engaging a pinion F, geared to a toothed wheel F<11> fixed to support V, so that the parallactio angle is varied as the telescope turns about the axis Y. The fixed prism P<3> may be replaced by several prisms or a reflecting system and disposed in any order relative to the prisms p<1>, P<2>. Alternatively the deviation produced by the prism P<3> may be impressed in the appropriate direction on the light passing through the peripheral zone A. In order to measure exactly fractions of a space on the observed measuring staff, a parallel-sided glass plate L is rotatably mounted in front of the telescope and is adjusted through gearing N, K, W, I from the graduated drum H. To modify the effect of the plate L in accordance with the inclination of the telescope the axle of the plate L is supported in a rotatable collar Z which is toothed and driven through pinions U, S from the teeth Q. In a modification the drive for the prisms P<2>, P<1>, and the collar Z may be obtained through a level or pendulum. In a modification the error due to lack of parallelism between the telescope axis and the bisector of the parallactic angle may be corrected automatically by suitable optical means. Such means may comprise a pair of oppositely rotatable prisms arranged with their edges perpendicular to the prisms P<2>, P<1>, to which they may be cemented if desired. The Specification as open to inspection under Sect. 91 describes modifications in which the bisector of the parallactic angle is made to coincide with the axis of the telescope. Each of the two beams forming the parallactic angle may pass through a deviating system, such as p<3>, p<2>, P<1>, producing a maximum deviation of half the maximum parallactic angle. Alternatively one of the beams may be passed through a deviating system P<3>, P<2>, P<1>, producing a maximum deviation equal to the maximum parallactic angle and then the two beams together passed through a deviating system producing half the above deviation in the opposite direction. The full deviation may be applied to the central zone by one set of prisms, and the half deviation to both the peripheral and central zones by a second set of prisms. The corresponding prisms of each set may be cemented together in pairs. This subjectmatter does not appear in the Specification as accepted.