1902.] on X-Baijfi and Localisation, 127 



if a tube is broken and air is allowed to rush in through the ojiening, 

 there is about a million times more air in it at the ordinary pressure 

 than was the case before the opening was made. When we pass a 

 discharge through such a tube from the negative or cathode side, a 

 stream of particles starts, and anything upon which they impinge is 

 made to glow. This cathode ray has the capacity of producing 

 great heat ; it will make platinum red hot, and even melt it ; it will 

 cause substances to fluoresce and phosphoresce ; it will only travel in 

 straight lines, it will not turn round corners, and it is deflected by a 

 magnet ; it will cast shadows of any solid substances placed in its 

 path. 



All these observations had been made by Crookes, and necessarily 

 created much interest. Dr. Philip Lenard, in 1894, endeavoured to 

 bring the cathode rays outside the tube into the open, and as glass 

 was opaque to them, he endeavoured to make an opening for them by 

 introducing a small window of thin aluminium foil opposite the 

 cathode. He succeeded, for the rays which passed through his 

 aluminium window produced fluorescence. They affected a photo- 

 graphic plate. He found that they would pass through thin sheets 

 of aluminium, and even of copper ; he also found them to affect a 

 photographic plate through metal. 



Professor Eontgen of Wiirtzburg took up the subject where 

 Lenard left it. When I had the pleasure of an interview with 

 Professor Rontgen, shortly after his discovery, I asked him what 

 intention he had in his mind when he commenced the research. He 

 said that he was lookiug for any invisible rays that might be pro- 

 duced from a Crookes' tube. The next question I asked him was, 

 why had he used a screen of Barium platino-cyanide. He replied 

 tbat he had used this substance because, by its brilliant fluorescence 

 it was capable of revealing the invisible rays of the spectrum. He 

 then told me that he had covered the Crookes' tube with black paper, 

 so that when it was excited by the electric current no visible light 

 was seen, but he suddenly observed that a piece of cardboard, which 

 had been covered with crystals of Barium platino-cyanide, glowed 

 brilliantly. I asked him what he thought when he saw this. His 

 reply was, " I did not think ; I experimented." He showed me the 

 original screen with which this great discovery was made, and I 

 suggested that such an historical screen should be carefully protected 

 in a glass case, and I trust that my suggestion has been carried out. 



^He speedily found that various substances interposed between a 

 Crookes' tube and a fluorescent screen allowed the rays to pass 

 through in varying degrees, and doubtless he was immensely surprised 

 and interested to observe that when he interposed his hand, his 

 bones inside his flesh were revealed on the screen. One of the 

 earliest experiments which he tried was to take a photograph through 

 a deal door between two of his laboratories. He placed the tube on 

 one side of the door and affixed to it a strip of platinum, and at a 

 corresponding part on the other side of the door he placed a photo- 



