286 ADVANCED ELECTRICITY AND MAGNETISM. 



The fluoroscope. Many substances such as barium platino- 

 cyanide 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 ren- 

 dered visible by allowing it to fall upon a photographic plate and 

 developing the plate as in ordinary photography. 



Fig.'209. 



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 209 shows a Crookes tube with 

 a concave cathode c from which the cathode rays are thrown off 

 in a convergent stream which strikes 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 denned Roentgen ray shadows may be 

 produced. The platinum plate p is usually connected as shown 

 to the aluminum anode a. An interesting feature of the Crookes 



