In the early 1800s, a German mathematician, Carl Friedrich Gauss, demonstrated that even a tiny mirror with a surface area of 1 square inch (6.5 square centimeters) could reflect flashes of sunlight that could be seen over a distance of 7 miles (11 kilometers) [source: Coe].
In the 1860s, a British Army officer named Henry Christopher Mance, who was stationed in India, got the idea of using Gauss' discovery to send coded messages. He developed the heliograph, a special tripod-mounted mirror with a lever that tilted it, so that it could transmit flashes of light of different lengths.
That enabled a mirror to be used to send Morse code, the same system of dots and dashes developed to transmit messages over telegraph wires. While a flash of light from a mirror couldn't be transmitted as far as a telegraph message, Mance found this an ideal way to communicate in the rough, mountainous areas of northern India and Afghanistan. By the late 1800s, the U.S. Army was using relay networks of heliograph stations to send messages through the vast desert expanses of Arizona and New Mexico [source: Coe].
By the early 1900s, radio had taken the place of the heliograph for most routine military communication [source: Coe]. But mirror communication didn't go away. During World War II, U.S. Navy pilots carried mirrors in their survival kits, so that if they crashed or were shot down, they could get the attention of rescuers [source: Flying Magazine]. In 1960, a Life magazine article by Mercury astronaut Alan Shepard noted that in addition to a battery-operated radio transmitter, he carried a signal mirror to help recovery ships locate him in the ocean [source: Shepard]. Today, signal mirrors are still recommended to military personnel and adventurers alike as a device to always have on hand.
In the next section, we'll tell you how to find a good signal mirror.