Heliograph
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A heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signalling code.
The first recorded use of a heliograph was in 405 BC when the Ancient Greeks used polished shields to signal in battle, described in Xenophon's Hellenica.
The unpopular Roman emperor Tiberius used a heliograph to rule the Empire from a villa on the island of Capri - sending orders each day to the mainland, eight miles away.
In 1810, the first formal design for a heliograph was proposed by German professor Carl Friedrich Gauss of the University of Göttingen. It was designed for surveying, but was used by the French, British and American armies as a field telegraph, using Morse code.
The heliograph remained standard equipment for military signallers in the Australian and British armies until the 1960s, where it was considered a "low probability of intercept" form of communication. As recently as the 1980s, heliographs were used by Afghan forces during the Soviet invasion of Afghanistan. They are still included in survival kits for emergency signalling to search and rescue aircraft.