ACTINIC RAYS, ultra-violet rays which produce chemical changes. The term is sometimes popularly applied to the ultra violet rays which are invisible, but can be detected by their action on a photographic plate and by rendering certain substances fluorescent (see FLUORESCENCE and PHOSPHORESCENCE). They are, to a certain extent, absorbed by the atmosphere, and, in mod erate doses, have a beneficial effect on man. (See LIGHT AND RADIATION IN RELATION TO HEALTH.)