Optical telegraphy. Historical illustration of the electric projector invented by French soldier Colonel Mangin and used by Colonel Perrier in his survey expeditions. The Mangin projector was invented in the early 1880s. Early devices used oil lamps at night, but this device has an electric light and a lens. Optical telegraphy systems used a telescopic device (top) for observing a distant signal. The range of visibility varied from 20-100 kilometres, depending on the weather. Artwork from 'Electricite' (1911) by Max de Nansouty, part of the 'Les merveilles de la science' series of 1867-1891 by Louis Figuier.