Photographing the Unseen 



"Now, Professor," said I, "will you tell me 

 the history of the discovery ? " 



"There is no history, " he said. " I have been 

 for a long time interested in the problem of the 

 cathode rays from a vacuum tube as studied by 

 Hertz and Lenard. I had followed their and 

 other researches with great interest, and deter- 

 mined, as soon as I had the time, to make some 

 researches of my own. This time I found at the 

 close of last October. I had been at work for 

 some days when I discovered something new. ' ' 



"What was the date?" 



"The eighth of November. " 



"And what was the discovery?" 



"I was working with a Crookes tube covered 

 by a shield of black cardboard. A piece of 

 barium platino-cyanide paper lay on the bench 

 there. I had been passing a current through 

 the tube, and I noticed a peculiar black line 

 across the paper. " 



"What of that?" 



"The effect was one which could only be pro- 

 duced, in ordinary parlance, by the passage of 

 light. No light could come from the tube, be- 

 cause the shield which covered it was impervious 

 to any light known, even that of the electric arc. " 



"And what did you think ? " 



"I did not think; I investigated. I assumed 

 that the effect must have come from the tube, 

 since its character indicated that it could come 

 from nowhere else. I tested it. In a few min- 

 utes' there was no doubt about it. Rays were 

 101 



