212 



Professor Silvanus P. Thompson 



[May 8, 



(Fig. 13), and known as the " focus tube." It was with such a tube 

 that I showed you at the outset the fundamental experiments of 

 Eoentgen. A concave polished kathode of aluminium concentrates 

 the kathodic discharge upon a small oblique sheet of platinum, which, 

 while acting as antikathode, serves at the same time as anode. Not 

 only does the concentration of the kathodic discharge upon the metal 

 cause it to emit X-rays much more vigorously, but it also has the 

 effect of causing them to be emitted from a comparatively small and 

 definite source, with the result that the shadows cast by opaque objects 

 are darker. [Photographs were then thrown upon the screen, those 

 taken with " focus " tubes showing remarkable definition of detail. 

 Some of these were by Mr. J. W. Giffen ; others, showing diseased 

 bones, &c., taken by the lecturer, and some by Mr. Campbell-Swinton 

 and by Mr. Sydney Rowland, were also projected.] 



The objection has been taken that in these shadow photographs it 

 is impossible to distinguish the parts that are behind from those that 



/ 



II 



Fig. 13. 



are in front. In a sense that is so. But I venture to say that the 

 objection not only can be got over but has been got over. I cannot 

 show the proof of my assertion upon the screen, because I cannot put 

 upon the screen a stereoscopic view. But here in my hand is the 

 Roentgen stereograph of a dead tame rabbit. Two views were taken, 

 in which the X-rays were thrown in two different directions at an 

 angle to one another. When these two views are stereoscopically 

 combined, you observe the rabbit's body with the lungs and liver inside 

 in their relative positions. The soft organs, which cast faint shadows 

 almost indistinguishable amid the detail of ribs and other tissues, 

 now detach themselves into different planes and are recognisable 

 distinctly. I now send up for projection in the lantern the two 

 photographs that were taken at the beginning of my discourse, and 

 which have in the meantime been developed. 



Turning back to the phenomena of luminescence,* permit me to 



* This very convenient term was suggested some six years ago by Wiede- 

 mann, to denote the many phenomena known variously as fluorescence or 



