26 THE NATURE OF ANIMAL LIGHT 



Phosphorescence and Fluoeesoencb. — Althougli the 

 word phosphorescence has been used in a very loose way 

 to indicate all kinds of luminescence, and particularly that 

 of phosphorus or of luminous animals, to the physicist 

 it has a very definite meaning, namely, the absorption of 

 radiant energy by substances which afterwards give this 

 off as light. Phosphorescence does not strictly apply to 

 the light of white phosphorus. If the radiant energy is 

 light (visible or ultra-violet) we speak of photolumines- 

 cence, if cathode rays we have cathodoluminescenoe, if 

 anode rays, anodoluminescence, and if X-rays (Eontgen 

 rays) we have radioluminescence. Inasmuch as the a, j8, 

 and y rays of radium correspond to the anode, cathode, 

 and X-rays, respectively, radium radiation also produces 

 luminescence in many kinds of material. If the material 

 gives off the light only during the time it is radiated we 

 speak of fluorescence; if the light persists we speak of 

 phosphorescence. The distinction is perhaps a purely 

 arbitrary one, as there are a great many substances 

 which give off light for only a fraction of a second (1/5000 

 sec. in some cases) after being illuminated {photolumines- 

 cence.) Some substances also, which fluoresce at ordinary 

 temperatures, will phosphoresce at low temperatures. 

 Phosphorescence is exhibited chiefly by solids, fluores- 

 cence also by liquids and vapors. 



Special means must be used to observe a phosphores- 

 cence of short duration. E. Becqucrel has devised an 

 apparatus for doing this, a phosphoroscope. It consists 

 of revolving disks with holes in them between which tlie 

 object to be examined is placed. The holes are so arranged 

 that the object is first illuminated and then completely 

 cut off from light. The observer looking at it through 



