THE property of fluorescence in a substance is its ability to emit radiation of one quality when stimulated by radiation of another quality. A familiar example of fluorescence is the luminous paint used on the dials of watches and other instruments to render them visible in the dark. The fluorescent body is a sulphid of zinc or some other metal. It transforms the invisible radiation of the trace of radio-active material present in the paint into visible radiation or ordinary light. Closely allied to fluorescence is the phenomenon of phosphorescence. In a fluorescent substance the transformation of energy takes place almost instantaneously, whereas a phosphorescent substance continues to emit radiation for an appreciable time after the stimulating radiation has ceased. An excellent example of a phosphorescent material is an intensifying screen showing lag, about which more will be said later. Probably the most extensive application of the phenomenon of fluorescence is in the fluoroscopic and intensifying screens which every radiologist and technician uses daily. The use of fluorescent substances in X-ray work dates from the discovery of X-rays in 1895, for it was by means of a fluorescent screen of barium platino-cyanide that they were discovered by Roentgen. The sudden shining of a fluorescent screen in a darkened room in which X-rays were being generated, unknown to this experimenter, led to one of the greatest discoveries of modern times. It was soon appreciated by the early workers who attempted to take radiographs that only a small part of the energy of the rays was utilized by the photographic plate, for the bulk of the energy passed through unabsorbed. In order to make use of a part of this waste energy, the first intensifying screen was devised. In this case a considerable proportion of the energy of the X-ray beam is absorbed by the screen and by means of its fluorescence is converted into actinic or ordinary photographically effective light. This light adds its effect on the plate to the effect caused directly by the X-rays, thereby reducing several fold the exposure required to produce a given blackening of the plate. The present form of the double screen used in conjunction with a photographic film coated on both sides is a further development of the same idea. The earliest intensifying screens used calcium tungstate as the active fluorescent material and this chemical is still used in practically all makes of intensifying screens. Nevertheless, the first screens were quite crude compared to the present-day product. They were very fragile, easily soiled and scratched, and not capable of being cleaned.