A wedge in a rangefinder or heightfinder used to displace the image produced by one telescope so that it coincides with that produced by the other telescope, thus affording a measurement of the parallactic angle between the line of sight of the two telescopes. There are two principal types of measuring wedges. The sliding wedge is mounted on slides parallel to the optical axis, between the objective lens of one telescope and its focal plane. It forms an image displacement equal to the product of the deviation of the wedge and its distance from the focal plane. Compensating wedges rotate simultaneously through equal angles in opposite directions. The pair of equal compensating wedges, mounted in front of one telescope, is equal to a wedge having a variable angle of deviation in the plane of triangulation, but none at right angles to it.