heliograph

thumb|Fig. 1: Signaling with a Mance heliograph; Alaska–Canada border, 1910.

A heliograph () is a solar telegraph system that signals by flashes of sunlight (generally using Morse code from the 1840s) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam 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 centuries. Its main uses were military, surveying and forest protection work. Heliographs were standard issue in the British and Royal Australian armies until the 1960s, and were used by the Pakistani army as late as 1975.