Tony Moss wrote: > In my impecunious searches of WWII 'surplus' stores back in the > 1950s I came across a "Portable Heliograph Set' in a pouch. It > was simply a mirror about four inches across with a sighting hole > in the middle. A length of cord attached it to a short rod with > a bead on top. > > In use the mirror was held in one hand near to the operator's > eye. The cord was then stretched tight and the 'bead' used to > 'sight' the target. If the mirror was then rotated until a > sunray coincided with the bead above the other outstretched hand > a flash of sunlight would be directed at the target.
I learned a different method in Boy Scouts: While looking through the hole at the target, you will also see an image of your face in the back side of the mirror. There will be a spot of light on your face where the sun shines on it through the hole in the mirror. If you tilt the mirror until the image of the spot coincides with the hole in the mirror, the sun will be reflected toward the target. This method might be considered less intuitive than the stick-string-and-bead method, but I actually find it simpler. I am fairly certain it is also more accurate. It also takes less equipment, so it can be carried out without preparation with any two-sided reflecting surface. And while we're on the subject, I would be interested in learning how the heliograph in Peter Mayer's jpeg is aimed. It's not as easy as aiming a laser or a search light because information on the position of the sun as well as that of the target is needed. --Art
