116 Generating Economic Cycles 



spectacular as well as humane uses in the practice of 

 medicine. The cathode rays had been observed to 

 produce luminiscence when they impinged upon the 

 walls of the Crookes' tube. Roentgen showed that the 

 rays which he discovered originated at the point where 

 the cathode rays coUided with a solid substance. Later 

 researches showed that X-rays are produced under 

 the best circumstances when a definite surface to 

 receive the cathode rays — the anti-cathode — is pre- 

 pared of polished metals. The view which is at present 

 generally accepted as to the nature of the X-rays was 

 first advanced by Sir George Gabriel Stokes. He 

 suggested that they are electro-magnetic waves having 

 their origin in the oscillation of the electrical charge 

 upon the cathode particle when the particle is brought 

 to a sudden stop by striking a solid substance. 



The principal characteristics of X-rays that we need 

 to consider are these : 



(1) The X-rays originate at the spot in which cathode 



rays strike a solid substance. 



(2) They light up a fluorescent screen. 



(3) They are not deflected in a magnetic field. 



(4) They produce an ionization of gases through 



which they pass. 



(5) The gaseous ions may become nuclei for the 



condensation of water vapor. 



The cathode rays, as we have seen, are negatively 

 charged particles moving normally from the cathode. 

 If an obstacle, say a metal Maltese cross, is placed in 

 the path of the cathode rays, two shadows of the cross 



