FLUORESCENCE

an optical effect in which a body becomes luminous when placed in sunlight or when acted on by certain radiations. Substances such as paraffin oil, uranium or canary gla , and chlorophyll exhibit, in bright sunlight, a surface colour which is different from that of the body of the substance. The effect is brought out in a more striking manner by placing the substances in a dark room in the path of a beam of invisible ultra-violet rays, when these substances become luminous. They have the property of absorbing the short jnvisible waves and of emitting, in their place, longer waves capable of affecting the eye. The fluorescent light lasts only so long as the body is receiving the stimulating radiation; in the case of paraffin oil, the light is blue, that from uranium gla is green, whilst a solution of chlorophyll gives a dark-red finorescence. The term tluorescence was derived from fluorspar, one of the substances in which the effect was first observed.; Some fluorescent solids lose their optical property when in a state of solution; other substances are fluorescent in solution, but not when solid. Fluorescence can be excited by the action of radium end of X-rays. Tink See HYDRO property is employed in the fluorescent screen, a sheet of stiff paper covered with a thin layer of barium platinocyanide crystals, used in detecting short-wave radiation. Fluorescence may be cla ed under the more general term of luminescence, or the emi ion of light by a body which has not been raised to the high temperature usually a ociated with the emi ion of light.—BIBLIoGRAPHY: Winkelmann, Handbuch der Physik: S. P. Thompson, Light, Visible and Invisible.