How did a heliograph send military messages using sunlight?

In the context of military communication, the heliograph was a visual signaling instrument that utilized two adjustable mirrors to capture and reflect sunlight in any desired direction.1

To transmit messages, the device incorporated a key-operated shutter positioned in front of the reflected light beam. By opening and closing this shutter, an operator could interrupt the beam to create short and long flashes, which corresponded to the dots and dashes of Morse Code.1

This method was particularly effective in regions with consistent sunshine, such as the American Southwest and India, where it was utilized by the U.S. and British armies. However, because the system relied entirely on favorable weather conditions, it was never widely adopted by armies across continental Europe.1

Note: The term "heliography" also refers to an early photographic process developed by Nicéphore Niépce, which used sunlight to create permanent images on light-sensitive plates; this is distinct from the military signaling apparatus.2