A YEAR OF THE X RAYS. 657 



especially strong in producing it. The substance employed by 

 Dr, Rontgen was barium platino- cyanide, which is expensive. 

 Experiment soon showed that other substances were more effi- 

 cient as well as cheaper, hundreds having been tested under the 

 rays for this effect. The best are tungstate of calcium, tungstate 

 of zinc, barium platino- cyanide, and potassium platino-cyanide, 

 the first named being at present the most common, though the 

 last named has been a favorite with English experimenters. A 

 screen of cardboard covered with a layer of fine crystals of either 

 of these substances and exposed to the rays in a dark room, imme- 

 diately lights up under their action, and a body impervious to the 

 rays, when placed before the screen, is seen upon it as a shadow. 

 If this screen is the front end of a light-proof box, into the other 

 end of which the eyes can look while all light is excluded, we then 

 have the fluoroscope, by which examinations can be made in a 

 lighted room. Probably few X rays pass through and beyond 

 the fluoroscopic screen. The effect of radiant energy upon a 

 body is determined not by the rays that pass through it, but by 

 those that are absorbed by it. It is difficult, therefore, to under- 

 stand how the light which the blind have been said to see on 

 peering into a fluoroscope can be really due to X rays. 



The invention and improvement of the fluoroscope constitute 

 an important part of the progress that has been made. The effect 

 of the rays on photographic plates is heightened by similar means. 

 The sensitive plate to be exposed to the rays is itself carefully in- 

 closed in a wrapper so as to shut out every trace of light. If, be- 

 fore thus wrapping up the plate, a fluoroscopic screen is placed 

 with its surface of crystals directly in contact with the photo- 

 graphic film, then where the rays penetrate to this crystalline 

 surface it becomes luminous, and the light immediately affects 

 the sensitive plate except in those spots where the object inter- 

 cepts the X rays, and where consequently they do not cause 

 fluorescence of the screen. This device has greatly reduced the 

 time needed to obtain a photographic impression. Fig. 1 is an 

 illustration of such action. A photographic plate was partly 

 covered by such a screen, and the hand was placed partly over 

 the screen and partly over the plate not covered by the screen. 

 An exposure of twelve seconds was more than sufficient to pro- 

 duce a strong picture of the interior of the hand, under the 

 screen, the flesh almost disappearing from view, while the effect 

 upon the other portion of the plate (the dark part) is much less 

 pronounced. The line of demarcation is very sharp. 



Also photographic plates or films have been adapted to this 

 particular use by preparing them so as to absorb the energy of 

 the rays. X-ray plates are now used of which the mode of prep- 

 aration is of course the manufacturer's secret, but which are 



VOL. L. 49 



