The Heliograph is basically a mirror that uses the light from the sun to flash a message across a distance, often using Morse code. They were widely used in the 19th and early 20th centuries for military and also for surveying purposes. Advanced models came with a tripod.
Signals could routinely be sent over distances of 30 miles or more. The equipment was light and did not need any electrical power. However anyone in or near the line of sight could potentially read the message! Although they are no longer in common use, they are frequently supplied as part of emergency survival kits, allowing the user to signal to a distant ship or airplane.
This 4 inch metal Heliograph is light and strong, and being made of metal will not easily break. Apart from being useful in emergencies, the mirror is also useful for shaving (as soldiers have long known!) or adjusting make-up. This Heliograph is from one of the UK suppliers of emergency survival kits, and the fascinating thing for us is the simple physics involved in the aiming mechanism.
If you simply use a mirror to reflect light for a short distance you will be able to see the bright reflection, and direct it where you want. However at any distance, this will not work, and an aiming mechanism is needed.
The Heliograph has a small hole in the centre of the mirror, with a series of concentric circles arranged around the hole. There is a small sighting device, attached by a piece of string, and this has a small hole in one end. The user holds the mirror up to their eye, and holds the sighting device as far away as the string will allow. Line up the hole in the sighting device with the place where you want to shine the light, and then adjust the mirror so that the small black circle caused by the viewing hole in the mirror, exactly lines up with the hole in the aiming device. You will now be shining the reflection of the sun's light in exactly the direction you have chosen. Hopefully rescue will not take too long! You will see how to aim the Heliograph in the video.
The mirror is 4 inches square, and it has instructions on the reverse side.
The traditional signalling device...
A 4 inch square mirror, plus a 'foresight', attached to the mirror by a cord to prevent it getting lost.
The mirror has a small hole in the centre. Hold this hole up to your eye, and hold the foresight up as far as the cord allows. Make sure you can see the target you are signalling to through the hole in the mirror and the hole in the foresight. Make sure that the mirror is adjusted so that the little black dot in the middle of the light beam - caused by the presence of the hole you are looking through - lines up with the hole in the foresight. This will make sure that the reflected beam of sunlight is aimed directly at the desired target.