THE PROGRESS OF PHYSICS. 163 



high as four hundred millions of millions of vibra- 

 tions per second. Hertz and others used waves of 

 some millions per second, and showed how they could 

 transmit signals to distances without wires; these 

 invisible waves being recognised by suitable receivers. 

 The recently announced Marconi wireless telegraph 

 is much the same thing, with certain improvements in 

 detail." * 



" Hardly had the work of Hertz and others who 

 followed in his footsteps been assimilated, before the 

 truly remarkable, not to say astounding, discovery 

 by Professor Rontgen of what he called the X-rays 

 produced a profound impression not only in the 

 scientific world, but upon the general public as well. 

 The interest of the scientist had a different basis 

 from the popular one of disclosure of objects hidden 

 in opaque structures; for he saw in the discovery 

 a new weapon of attack upon the secrets of nature. 

 This weapon has already proved to be so serviceable 

 as to show that his anticipations were not unfounded. 

 The X-rays, which became at once indispensable to 

 surgery, are the results of electrical actions in certain 

 vacuum bulbs ; and the discovery is properly an elec- 

 trical one." f 



X and other Rays. It has long been known that 

 remarkable effects are produced when cathode rays 

 are passed through a highly exhausted vacuum tube. 

 The glass shows bright " phosphorescence," shadows 

 are thrown by opaque bodies, and the rays are de- 

 flected by a magnet. Crookes and Goldstein have 

 been prominent investigators of the phenomena. 



In 1893, Lenard used a tube with a thin window 

 of aluminium, and found that rays passed through 



* Ann. Rep. Smithsonian Inst., 1897, p. 135. 

 t Loc. cit., p. 138. 



