414 History of Luminescence 



made a detailed study of the spectra emitted by the various com- 

 pounds, pointing out the importance of impurities. Finally the 

 extensive publication of Wiedemann and Schmidt (1895) indicated 

 that exposure to cathode rays would also excite liquids and render 

 many solid compounds thermoluminescent (see Chapter IX) . 

 Wiedemann (1895) called the rays which had this effect " Entla- 

 dungsstrahlen," but the most general term was cathode rays (c/. 

 P. Villard, 1899). 



One of the interesting actions of cathode rays was to cause de- 

 velopment of color in previously clear crystals exposed to their 

 action. E. Wiedemann (1880) noted the browning of barium-plati- 

 num cyanide with an accompanying change in the color of its 

 cathodoluminescence and E. Goldstein (1894, 1895) studied corre- 

 sponding changes in lithium, sodium, and potassium chloride. A 

 great deal of research on this coloration, also induced by X-rays, 

 was carried out at the turn of the century. Wiedemann and Schmidt 

 (1895) showed that the colored salts" exhibited not only thermo- 

 luminescence, but also triboluminescence, and even emitted light 

 as they dissolved in water, a lyoluminescence (see Chap. X) . 



It was abundantly evident that in cathode rays man possessed 

 perhaps the most powerful tool for exciting luminescence ^- in a 

 wide variety of materials, and in luminescence a correspondingly 

 valuable means of following the path of cathode rays. Crookes' 

 (1879) early experiments on deflection of cathode ray beams and 

 C. Swinton's (1899) study of the focus of a beam both involved 

 luminescence for detection. The invention of the " Braun tube," 

 an early type of cathode ray oscillograph, by Ferdinand Braun (1850- 

 1918) in 1897, opened a whole new field for investigation of rapid 

 electrical changes, and gave the scientific world a research instru- 

 ment of extraordinary value. 



Of the many men connected with cathode ray research, J. J. 

 Thomson and P. Lenard appear to have received most acclaim, 

 Thomson for establishing the ratio of mass to charge of the electron 

 and the general phenomena connected with gases and electricity; 

 Lenard for investigations on cathode rays and the photoelectric 

 effect. Lenard paid more attention to luminescence and should be 

 particularly mentioned for application of the electron in a theory 

 of phosphorescence, described in Chapter VIII. 



^^ Elster and Geitel (Ann. der Physik 59: 487-514, 1896) showed that these salts were 

 highly photoelectric, discharging negative electricity like a metal when exposed to 

 light. 



^^ Although the efficiency of the process is low, E. Wiedemann (1898) showed that 

 about the same small fraction of the energy of cathode rays was converted to visible 

 light as in the case of light exciting photoluminescence. 



