FLUOROSCOPIC SCREE XS. 43 



gator found that, if a small aluminum window was fitted into 

 the end of a Crookes tube opposite to the kathode, and if the 

 tube was excited in the usual manner by means of an induction 

 coil, certain materials would show fluorescence when they were 

 placed within a distance of six centimeters from the aluminum 

 window. Although this effect may not have been due to 

 Rontgen rays — which had not then been discovered — yet the 

 experiment showed that certain phenomena relating to Crookes 

 tubes could be advantageously studied by the use of screens of 

 fluorescent material. 



The '* X-rays" which were discovered by Professor Rontgen, 

 and which were emitted from an ordinary Crookes tube, were 

 observed to effect fluorescent substances as far distant as two 

 meters. The room in which Professor Rontgen conducted the 

 experiments was darkened, and the Crookes tube which was 

 used was covered with black paper. Then, when the fluores- 

 cent substances were brought near to the tube and in the path 

 of the X-rays, the fluorescence mentioned above was observed. 

 It will be seen, therefore, that the appliances which are used now 

 to observe X-ray shadowgraphs and fluorescence are simply 

 improvements of the methods used by Professor Rontgen. 



Department of Physics, 



Columbia University. 



