Through a Glass Darkly

The glass looked as transparent as the standard windowpane, but scientists at the Manhattan meeting of the American Physical Society were impressed by its strange qualities. On brief exposure to sunlight the glass turned grey. Back in the shade, or in ordinary artificial light, it promptly turned clear as window glass again.

Chemist S. Donald Stookey of Corning Glass Works explained that the strange "photochromic" glass, which he had invented along with Dr. William H. Armistead, contains submicroscopic crystals of silver halide, 128 million billion of them per cubic inch. They do not affect its color or transparency, but strong visible or ultraviolet light turns the crystals to metallic silver, which absorbs light and makes the glass look grey. The same thing happens to the silver halide particles in photographic film, but their darkening is permanent. The silver atoms in the glass are held so tightly that they cannot move away from the place where they were formed. When the light that creates them is cut oft, they reunite promptly with halide atoms from which they were separated, and the glass becomes transparent again. It can repeat the trick indefinitely; some samples have already darkened and cleared many thousands of times without getting tired.

Corning says that its dark-light glass is still in the laboratory stage, but it is no mere scientific stunt, and Corning men are working hard on practical applications. Some possibilities: windows to keep solar light and heat out of air-conditioned buildings, auto windshields with an upper section treated to fend off glare by day and turn transparent when the sun is not shining, and sunglasses able automatically to adjust their density to light conditions.

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