HEINRICH HERTZ 369 



clearest possible type, which happened eleven years later. 

 It then appeared that when air is excluded, so that the ultra- 

 violet light acts only upon the metallic plate upon which it 

 falls, the light causes free electrons to leave the metals. This 

 is the fundamental process, the thorough study of which 

 then not only brought understanding of the various forms 

 of photo-electric effects, also in air and other gases, but led 

 also to a very great deal of further knowledge. 



It should first be mentioned that the phenomenon of 

 'phosphorescence,' which had been known for over a 

 century, now could be understood, after G. Stokes had 

 already recognised (1853) that these phenomena, as well as 

 that of 'fluorescence,' consist in a transformation of light 

 taking place inside the body concerned, and resulting in a 

 change in the refrangibility, that is the colour, of the light. 

 It now became evident, when phosphorescent material 

 suited to proper experiments had been made, that the excit- 

 ing light liberates electrons from the metallic atoms of the 

 phosphorescent substance, and that it is the return of these 

 electrons which causes the after-glow with change of colour, 

 which is peculiar to the metal contained in the phosphores- 

 cent substance. This rendered it probable that the pro- 

 duction and emission of light - for example also in metallic 

 atoms in the Bunsen flame - is connected quite generally 

 with displacements of electrons.^ This fact, particularly in 

 conjunction with our knowledge of the structure of atoms 

 and the quantum-like manner of their operation, very con- 

 siderably advanced our understanding of all processes of 

 light emission and its excitation. 



Furthermore, the photo-electric effect first enabled the 



^Almost at the same time, studies of the 'canal rays' brought the same 

 idea very close. I have given some historical data on an earlier occasion 

 (Ouantitatives iiber Kathodenstrahlen, new edition, 1925, pp. iv and v). 

 The English reader should consult J. J. Thomson's Conduction of Elec- 

 tricity through Gases, Cambridge, 3rd Ed., 1928; Zworykin and Wilson's 

 Photocells, London, 1930. 



Aas 



