HOW DISCOVERIES ARE MADE 125 



the negative pole of the Plucker tube when very highly 

 exhausted could pass out of the tube through a thin 

 1 window ' of the very light and strong metal aluminium. 

 It is true they could not pass very far ; they soon became 

 scattered. Here was a discovery made with a set purpose. 

 Professor Lenard wished to decide the question whether 

 Crookes's ' rays ' were really due to a stream of corpuscles 

 or whether they were vibrations like those of light. 



Sir William had previously found that if a magnet were 

 placed near the tube the path of the rays was no longer 

 straight, but curved. And Lenard observed that if the 

 aluminium window were placed so that a ' vacuum ' (not 

 a complete, but a nearly complete one) were on both sides of 

 the aluminium window, the 'rays' could be bent out of their 

 course by the magnet after passing through the window. 



It must be remembered that these rays are not them- 

 selves visible ; it is only possible to see where they strike 

 by their causing phosphorescence. Professor Rontgen, 

 the celebrated German physicist, discovered in his turn 

 that if these rays be suddenly stopped say by falling on 

 glass or metal rays of another kind are sent on, which 

 have the power of affecting a photographic plate and of 

 rendering certain substances exposed to them phosphores- 

 cent; so that, as different kinds of matter have very 

 different powers of stopping Rontgen rays, it is possible to 

 photograph the bones of the body, although the flesh is 

 comparatively transparent to them. The bones, as it 

 were, cast their shadow ; or the shadow of the bones can 

 be thrown on a piece of card, painted with material which 

 phosphoresces and shines when exposed to the impact of 

 the rays. 



I believe that Rontgen's discovery arose from an acci- 

 dental observation that a box of photographic plates left 

 near a Crookes's tube became 'fogged/ and he too had 

 genius to follow up this clue. 



