THE DISCOVERY OF X-RAYS 31 



would come to think of making such a request. 

 No one at that time had ever conceived the possi- 

 bility of being able to do a thing which was uncom- 

 monly like seeing through a door. It would have 

 been classed with vision of that penetrating kind 

 if it had been proposed ; and considered as incom- 

 patible with known facts. So indeed it was, if 

 its accomplishment was to be some new act of 

 ordinary vision; but the solution came in an 

 entirely different fashion. The problem was solved 

 indirectly, as so often happens ; and for this very 

 reason its solution could not have been expected 

 or asked for. Not only may it be asked why this 

 particular request should be chosen in preference 

 to any other of a thousand requests which could 

 be imagined; but it may be asked how it is to be 

 expected that the request should be conceived 

 at all. There was nothing to make anyone think 

 of asking for such a thing as X-rays. 



It is impossible to conceive of progress being 

 made in this order, which is the reverse of the 

 natural course. What happened actually was that 

 Rontgen was one day performing an experiment 

 with a spark passing through a tube very perfectly 

 exhausted of air. As there was a certain amount 

 of light from the spark which was interfering with 

 his work, he covered the tube with opaque material. 

 He found that in spite of his precaution something 

 was issuing from the tube which was able to affect 

 photographic plates in the neighbourhood. The 



