A heliograph is a semaphore that signals by flashes of sunlight using a mirror, often in Morse code. The flashes are produced by momentarily pivoting the mirror or by interrupting the sunlight with a shutter. The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. The main uses were for the military, survey and forest protection work. Heliographs were standard issue in the British and Australian armies until the 1960s and were used by the Pakistani army as late as 1975.Major J. D. Harris WIRE AT WAR – Signals communication in the South African War 1899–1902. Retrieved on 1 June 2008. Discussion of heliograph use in the Boer War.
