A rainbow, as every schoolboy knows, is formed by the refraction (bending) of sunlight by raindrops. Optical engineers have known for some time that an artificial rainbow can be produced by passing light through certain crystals. An ingenious wartime application of this phenomenon is a rainbow gunsight.
The device, known as the "ring sight," looks something like a reading glass. When a gunner peers through it, he sees a set of rainbow-colored, concentric circles. Unlike rings painted on glass, these optical rings are projected beyond the disk and seem to lie directly on the object sighted (see cut). Easier to use and...
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