THE PHENOMENA OF ELECTROSTATICS. 



2 3 I 



anide and calcium tungstate become luminescent under the action 

 of Roentgen rays. This effect is utilized in the fluoroscope which 

 consists of a cardboard screen covered with a layer of barium 

 platinocyanide. When the Roentgen ray shadow of an object, 

 such as the hand, falls on this screen the shadow becomes visible ; 

 where the Roentgen rays have been greatly reduced in intensity 

 by the bones of the hand the screen remains dark, where the 

 Roentgen rays have been slightly reduced in intensity by the 

 flesh the screen is moderately luminous, and where the rays have 

 not been reduced at all in intensity the screen is highly luminous. 

 The Roentgen ray shadow of an object may be rendered visible 

 by allowing it to fall upon a photographic plate which is after- 

 wards developed like an ordinary photographic negative. Thus, 

 Fig. 162* is a reproduction of a shadow photograph of a wrist. 

 The focusing tube. In order that a shadow may be sharply 

 defined the radiation which produces the shadow must emanate 

 from a very small source. Figure 163 shows a Crookes tube 



Fig. 163. 



with a concave cathode c from which the cathode rays converge 

 and strike a small spot on a platinum plate p. This small spot is 

 the source of the Roentgen rays. Such a Crookes tube is called 

 a focusing tube, and, by the use of such a tube, very sharply 



*From a negative by Dr. E. W. Caldwell, President (1908) of the American 

 Roentgen Ray Society. 



