324,518. Barr &. Stroud, Ltd., Barr, A., and Strang, J. M. Jan. 23, 1929. Range-finders-In optical measuring instruments in which the measurement is made by the translation of a refracting prism or prisms of small angle to and fro along the path of one, or both, of the convergent beams of light, means are provided whereby the movement of the image caused by the translational movement is modified by an additional movement of an optical part, thereby modifying the scale of measurement. In one example, as shown in Fig. 2, two prisms A1 and A2 in a carrier 4 are moved in one of the two convergent beams. The rack 3 and pinion 2 give a translational motion to the prisms and the pins 5 and 6 working in slots 51 and 61 give a rotary movement. The prisms also may be situated one in each beam. In the first example the cams are not exactly similar as with prisms of finite thickness half of the image would then be displaced vertically with respect to the other half, but in the second case the two halves move together when the cams are similar. Fig. 4 shows an example using a glass plate A3 as the displacing device. The plate rotates around the axis 7 and a spring 8 holds it in contact with cam 9 placed on the shaft carrying the pinion. Fig. 5 illustrates the use of two rotating prisms A4 and A5 connected together by gearing and driven by a pin 11 working in the extension 10 of the rack 3. Also an objective D can be moved as in Fig. 6 by a lever 13 actuated by a cam 12 on the end of the rack 3.
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