Experiments showinga rapid and reversible change ofcolor s eem likemagic and are always fascinating. The process involved, photochromism, has a few real and many potential applications. Photochromic glasses thatdarken int he s unlight (protecting eyes from excessive light intensity) and bleach ind im lighta re today a part ofe v eryday life. Organic photochromic compounds in plastic ophthalmic lenses, more comfortable to wear, are now competing with silversalts in glasses, despite the longer lifetime oft he inorganic system. This successful commercial application has given a new impetus to...