416 History of Luminescence 



pass. Positive rays cause a yellowish luminescence of air, a rose 

 luminescence of hydrogen, bright red with neon, and yellowish with 

 helium. Cathode rays often excite very different colors, for example 

 a pale blue with neon and a green with helium, thus making a 

 sharp contrast in a tube filled with the appropriate gas, and so 

 constructed that both types of rays can be observed at the same 

 time. This luminescence of the residual gas in the path of cathode 

 and anode rays is to be considered an electroluminescence, since 

 it depends on the same mechanism of excitation of light as in any 

 gas discharge tube through which current flows. Only when cathode 

 and anode rays strike solids or liquids should the light be classified 

 among the radioluminescences. 



Radioluminescence Proper 



FROM ROENTGEN RAYS 



The well-known story of X-ray discovery began oh November 8, 

 1895. On that date, Roentgen noticed that a barium platinocyanide 

 screen lying near a vacuum tube with which he was working became 

 brightly fluorescent whenever current was turned on, despite the 

 fact that the tube was enclosed in an opaque black paper shield. 

 Roentgen (1895, 1896) himself, who had seen the fluorescence due 

 to cathode rays outside a tube, took pains to demonstrate that his 

 new X-rays were not cathode rays but much more penetrating. They 

 would pass through a book and even through metals, if not too thick. 

 It is when cathode rays are stopped suddenly, on hitting glass or 

 metallic plates that X-rays or Roentgen rays are generated. While 

 anode rays pass through matter with great difficulty and cathode 

 rays only pass through very thin sheets of metal, the great penetra- 

 tion of X-rays and their absorption by matter in proportion to its 

 density aroused the profound interest of contemporary scientists. 

 The ability to render visible bone and other structures in the human 

 body particularly excited the medical profession and the public. 

 News of the new sensation quickly spread throughout the world. 

 Photographs of various parts of the body were obtained by Roentgen 

 in 1895, and a photograph of a human hand injured by a buckshot 

 wound was made in the United States by Michael Pupin (1858- 

 1935) on February 2, 1896. This particular photographs^ is of 

 unusual interest because Pupin then used his method of shortening 

 the exposure for X-rays photographs, now universally adopted, of 

 combining a fluorescent screen and a photographic plate. The lumi- 



^^ Charles Henry (1896) reported augmenlation of the photographic effect of X-rays 

 at a meeting of the French Academy, February 10, 1896, 



