Fluorescence, in physical Optics, means a special transformation of light-waves when received into an absorbing medium. Sunlight, falling on a dark object, usually raises its temperature, and causes it to emit heat. It does not send back light waves again, but it has effected the transformation of light-waves of short period and wave-length into heat waves of longer period and wave-length. Energy is absorbed in the form of short waves and emitted in long waves - a process of degradation that occurs continually in nature. There is another form of degradation of the same sort, when light waves are transformed into other light-waves but of lower refrangibility. Violet rays may be absorbed by a substance and emitted as green or red light; the new modification is still appreciable to the eye, whereas in the former case it was only appreciable to the touch. This latter transformation is known as fluorescence, and was first observed by Brewster to be effected by an alcoholic solution of chlorophyll; sunlight, passing into the green solution, exhibited a fine red track through the liquid. Stokes has shown that many substances are more or less fluorescent, such as bone, white paper, cotton wool, etc., and that the rays transformed may be of such high refrangibility as to be beyond the limit of vision when in the incident beam. The transformation of long waves into those of shorter period is termed calorescence (q.v.).