The excited states of many organic molecules are sufficiently long lived so that excitation gives rise to a change in the absorption spectrum that persists for seconds or even longer. The author is unaware of ally perfect crystal for which this is true. Vacancies are essential to the appearance of photchromism in alkali halides and sodalite crystals. Changes in absorption spectrum have accompanied charge transfer reactions between two impurities in titanate and calcium fluoride crystals. Photochromism has been observed in suspensions in glass of crystals too small to scatter light as well as in homogeneous glasses.