Radioluminescence 415 



Anodoluminescence 



Not long after the recognition of cathode ray phenomena, it was 

 apparent that another type of ray existed, the anode rays or canal 

 rays or positive rays. They were discovered by E. Goldstein (1850- 

 1931) in 1886, and first called canal rays because, arising from the 

 anode, they can be easily separated from cathode rays by allowing 

 them to pass through a small opening or canal in the cathode, 

 whence they travel in a region away from the electric field between 

 anode and cathode. 



After considerable experimentation, anode rays or canal rays were 

 identified as streams of positively charged atoms (ions) of various 

 kinds. It was W. Wien (1898) , who showed that they are positively 

 charged and deflected in strong electric and magnetic fields, a prop- 

 erty made use of in the mass spectrograph. A. Wehnelt (1899) dis- 

 cussed their relation to the glow observed in vacuum tubes. Like 

 cathode rays they excite luminescence in a wide variety of materials, 

 in modern terminology an ionoluminescence. 



This anodoluminescence or ionoluminescence excited by posi- 

 tively charged canal rays did not at first arouse as much interest as 

 cathodoluminescence. Its study has been mostly confined to the 

 twentieth century. Wien (1901) noticed the luminescence of some 

 metallic oxides when thoroughly dry, and both Wien (1902) and 

 Goldstein (1902) studied luminescence of the glass. J. Tafel (1903) 

 and G. C. Schmidt (1902, 1904) observed the light from oxides and 

 salts of various metals. It was soon established that the spectra of 

 anodo- and cathodoluminescence are not by any means alike. The 

 same substance can be readily compared by constructing a vacuum 

 tube in such a way that material placed in a holder may be bom- 

 barded either by anode rays or by cathode rays merely by changing 

 the sign of the charge of the electrodes, that is by making one first 

 an anode and later a cathode. Striking differences in the color of 

 anodo- and cathodoluminescence are apparent. Lithium chloride, 

 for example, luminesces bright red with canal rays, showing the red 

 line of lithium, and a steely blue with cathode rays, giving a faint 

 continuous spectrum but no red Li lines. A similar behavior was 

 early observed with glass. Instead of the gieen cathodolumines- 

 cence, soda glass bombarded by canal rays emits the yellow lines of 

 sodium. 



In addition to the luminescence of solids which are struck by 

 cathode and anode rays, Goldstein (1886) noticed that the path of 

 both types of rays can be observed in highly evacuated tubes by 

 the luminescence of the trace of residual gas through which they 



