SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
Optical telegraphy. Historical artwork of 19th century French soldiers using a telescopic device for optical telegraphy (sending and receiving a message using light). One such early device was the Mangin projector, invented by a colonel Mangin in the early 1880s. Optical telegraphy systems used a telescopic device for observing a distant signal. The range of visibility varied from 20-100 kilometres, depending on the weather and if it was day or night. In the day, reflected sunlight was used to send the signal. The sequences of flashes was read out by the observer and recorded and decoded by the other soldiers. From La Telegraphie Historique (Alexis Belloc, 1888).
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