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.