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Fluorescence (n.) That property which some transparent bodies have of producing at their surface, or within their substance, light different in color from the mass of the material, as when green crystals of fluor spar afford blue reflections. It is due not to the difference in the color of a distinct surface layer, but to the power which the substance has of modifying the light incident upon it. The light emitted by fluorescent substances is in general of lower refrangibility than the incident light.