| If a point source of any disturbance, wave-like
or particle-like, is placed at the focal point of a parabolic reflector,
the disturbance, after reflection from the mirror, will be propagated in
a collimated beam.
This effect is usually used with sound, but can also be used with any electromagnetic wave, such as infra-red radiation. A hot coal is placed in the holder at the focal point of one reflector, and a small piece of phosphorus or guncotton is at the other. Turning the reflectors toward each other, even with a considerable distance between them, will ignite the detecting material.
The pair of reflectors from Benedictine College in Atchison, Kansas, was made by Max Kohl of Chemnitz, German, ca. 1900, and cost about 80 Marks (about $20).