"One of the most successful and widely used visual signalling systems, the heliograph, did not appear until 1865, long after most visual systems were considered obsolete. The factor that established the heliograph was the existence of the Morse alphabet of dots and dashes, widely used for land telegraph and submarine cable operations. The ancients understood the principles of reflected sunlight, but no one ever got around to devising a code for the letters of the alphabet. Signal codes of some type had existed long before Morse, but none of them ever reached a level of universal acceptance, and they were mostly forgotten by the time Morse published his code.
"Early in the nineteenth century, Gauss, a German mathematician, had discovered the tremendous potential of the sun's rays reflected from a plane mirror. Through experiments he was able to demonstrate that even a small mirror one inch square could send flashes that could be seen over a distance of seven miles. The silvered glass mirror, invented in 1840 by Justin Liebeg, paved the way for the heliograph. (...)
"Like the American army, the British did not have a separate Signal Corps organization until the 1860s. The first British signal school was established at Chatham in 1865. Shortly after, a young officer named Henry Christopher Mance (1840-1926) became interested in signalling with the sun. Mance, later to be knighted for his achievements in engineering, knew of the use of mirror instruments called heliotropes in the triangulation of India. The Indian survey, one of the great engineering projects of the nineteenth century, required accurate location of high mountain peaks to serve as control points fot the ground survey. Bright fire pots were used at night and the heliotropes by day. It is not know whether any Morse code signalling was done by heliotrope, but it is certain that prearranged signals were exchanged.
(...) "The simple and effective instrument that Mance invented was to be an important part of military communications for the next 40 years. Limited to use in sunlight, the heliograph became the most efficient visual signalling device ever known. In preradio days it was often the only means of communication that could span ranges of up to 100 miles with a lightweight portable instrument.
"The Mance instrument employed tripod-mounted mirrors, with one mirror linked to a key mechanism. The key tilted the mirror enough to turn the flash on and off at the distant station in accordance with the dots and dashes of the Morse code. Range was line-of-sight, with atmospheric conditions establishing the upper limit. The British army found the Mance heliograph ideally suited to field operations in India and Afghanistan. It was used to transmit daily reports and orders to and from the remote mountain posts and for tactical communications when troops were in the field. (One hundred ten years later, TV pictures were to show Afghan guerilla units using British pattern heliographs in their conflict with the Russians.) The present Afghans have found the helio useful for the same reason as their British enemies of old; namely, a simple uncomplicated mechanism that requires no batteries or complex maintenance."