302 DISCOVERY CH. 



Marburg University, had discovered rays which would 

 pass through opaque substances, such as wood or the 

 human body, more easily than through a glass, it was 

 scarcely believed. The observation was, however, only 

 a natural consequence of scientific work done by earlier 

 investigators. 



It had been known for many years previously that 

 when an electric discharge is caused to take place 

 through a sealed glass tube or bulb containing only 

 extremely rarefied air, the tube becomes filled with a 

 beautiful luminosity. Sir William Crookes made a 

 number of experiments with vacuum-tubes of this kind 

 after reducing the air in them to the highest degree of 

 rarefaction. He found that electrified particles seemed 

 to be shot out from one of the two slips of metal sealed 

 into the tube for connection with the machine which 

 produced the electricity. Diamonds, rubies and other 

 substances, when placed in the tube so that these par- 

 ticles or rays fell upon them, became phosphorescent, 

 and emitted a shimmering light. When a pattern was 

 cut out of a sheet of mica and placed in the path of the 

 rays, it stopped them, the result being that a dark 

 shadow of the pattern was seen at the end of the tube 

 opposite that from which they were projected, while 

 all round the shadow was phosphorescent light. 



Hertz, who first demonstrated the existence of electric 

 waves, showed that the rays would pass through thin 

 sheets of certain metals ; hence, a pattern made of one 

 of these metals, and placed in a vacuum-tube in the 

 same way as the mica pattern, produced no shadow, 

 for, though opaque to light, they were transparent to 

 Crookes's rays. This was a decided step in advance, 

 and the next was taken by Prof. Lenard, in 1894, who, 

 by using a vacuum-tube having an aluminium end or 



