[Illustration: FIG. 3. Spectrum of zinc sulphide phosphorescence (_after Ives and Luckiesh_). Photographs were taken by a special device one minute (middle) and fifteen minutes (bottom) after exposure to the light of the mercury arc and compared with a helium spectrum (top). In the middle photograph, the mercury exciting lines are visible. It will be noted that the narrow band of phosphorescent light does not shift its position during decay of phosphorescence.] Other facts show that the light of luminous animals is in no sense a phosphorescence and is quite independent of previous illumination of the animal. Luminous bacteria will continue to luminesce although they are grown in the dark for many weeks. Indeed strong light has a bactericidal action on these forms similar to that with ordinary bacteria. With some marine forms light has an inhibiting effect. They lose their power of luminescence during the day and only regain it at dusk or when kept in the dark for some time. Indeed, ordinary light never has the effect of causing luminescence in the same sense as it causes phosphorescence of CaS. Fluorescence is most efficiently excited by the cathode rays of a vacuum tube. They not only cause the residual gas in the tube to glow (_electroluminescence_) by which their path may be followed with the eye, but also a vivid fluorescence of the glass walls of the tube, yellow green with sodium glass, blue green with lead and lithium glass. LiCl_{2} in the path of cathode rays gives off a blue light; in the path of anode rays a red light; NaCl a blue cathodoluminescence and a yellow anodoluminescence. The spectrum of the latter is a line spectrum of Li or Na, showing the characteristic red or yellow lines similar to those observed where Li or Na is held in the bunsen flame. The spectrum of the salts under excitation of cathode rays is a short continuous one in the blue region. Fluorescent spectra in general are of this nature, made up of short bands of light in one or more regions. Diamonds, rubies and many minerals fluoresce brilliantly in the path of cathode rays. Some specimens of fluorite (CaF_{2}) show the phenomenon especially well, whence the name _fluorescence_. Fluorescent screens of barium platinocyanide, willemite (Zn_{2}SiO_{4}), Sidot blend (ZnS) or Scheelite (Ca tungstate) are frequently employed to render visible X-rays. The luminous paint most used at the present time is ZnS containing a trace of radium salt. The rays of the radium continually emitted cause a steady fluorescence of the ZnS. Indeed, if one examines the paint on the hands of a watch with a lens the flash of light from the impact of alpha particles on the ZnS can be distinctly seen, as in the _spinthariscope_.