World War 2, Western Australia, Rottnest island, Coast Artillery Instrumentation, Barr and Stroud Rangefinder

Barr and Stroud Rangefinder. The Barr and Stroud is a self-contained horizontal base rangefinder. It consists of two telescopes with one eye piece so arranged that the observer sees one partial image above the other. To operate, the instrument is traversed placing one telescope at right angles to the target. The other telescope is adjusted laterally s until the two images coincide. This creates a parallax angle at the target by the intersection of the two images from the base line. The parallax angle varies inversely as the range of the target and the range can be quickly read off from the integral calibrated range table.