Science: Rainbow Gunsight

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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 more accurate than most optical sights, the ring sight is especially helpful against moving targets, because the regularly spaced outer rings give a gunner a means of measuring how much he must lead the target.

The rainbow sight is made by sandwiching a natural crystal or a synthetic sodium nitrate crystal between two layers of polarizing glass. Optical engineers of the Polaroid Corp., manufacturers of the sight, envision many uses for it. Camera fans have already found that it makes an excellent view finder.