Applications: Heliography

Signalling with mirrors, called heliography, has a long and proud history. Records of mirrors used for signalling purposes include Tacitus' Annals, to the journals of conquistadors observing Native Americans. Mirror signalling is still taught in a variety of institutions including the Army, and the Boy Scouts. Mirrors are still useful in this manner today. According to the U.S. Army Survival Handbook, signals in ideal conditions have been known to travel over 150 miles. Contrary to popular belief, mirrors can be used on a cloudy day, though they are less visible.

According to the 1^st edition of the Encyclopedia Britannica, “HELIOGRAPHY is the name applied to the method of communicating between distant points in which visual signals are obtained by reflecting the rays of the sun from a mirror or combination of mirrors in the required direction. This method can of course be only employed to advantage in places where the sky is free from clouds and the atmosphere clear for considerable periods of time, and the fact that an atmospheric change may indefinitely delay the transmission of a message is an insuperable objection to the establishment of permanent heliographic stations in most climates. In favorable localities, however, heliography possesses important advantages for military signaling over other methods, the principal being the portability of the apparatus, the great distance to which messages can be sent without retransmission, and the fact that the signals are visible to those only who are on the direct line of signaling.”

This is more than one method to signal with a mirror, but this is one of the more universally applicable. To use a mirror, bring the signaling mirror to the eye. Point the hand in the direction the signal should be sent. Tilt the mirror until the reflected light is just above the hand. Flash back and forth in sets of three (for an emergency signal; more complicated codes for other uses, such as Morse Code), with a few second gap, where the mirror is covered. Make the three flashes very distinct, to distinguish them from equipment glinting or muzzle flashes. When nothing is in sight to signal, sweep the mirror back and forth along the horizon.