A heliograph (Greek: Ἥλιος helios, meaning “sun”, and γραφειν graphein, meaning “write”) is a wireless solar telegraph that signals by flashes of sunlight (generally using Morse code) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.
Most heliographs were variants of the British Army Mance Mark V version. It used a mirror with a small unsilvered spot in the centre.
(1) The sender aligned the heliograph to the target by looking at the reflected target in the mirror and moving his head until the target was hidden by the unsilvered spot.
(2) Keeping his head still, he then adjusted the aiming rod so its cross wires bisected the target.
(3) He then turned up the sighting vane, which covered the cross wires with a diagram of a cross, and aligned the mirror with the tangent and elevation screws so the small shadow that was the reflection of the unsilvered spot hole was on the cross target. This indicated that the sunbeam was pointing at the target.
(4) The flashes were produced by a keying mechanism that tilted the mirror up a few degrees at the push of a lever at the back of the instrument.
If the sun was in front of the sender, its rays were reflected directly from this mirror to the receiving station. If the sun was behind the sender, the sighting rod was replaced by a second mirror, to capture the sunlight from the main mirror and reflect it to the receiving station.