246 History of Luminescence 



had to await the development of electronic techniques of the next 

 century. The spectra of luminous animals were fully investigated 

 and the absence of sharp lines established. 



Survey 



It is hardly possible to summarize the truly overwhelming mass 

 of information collected during the nineteenth century concerning 

 every conceivable type of luminescence. Even the chapters of Parts 

 II and III of this book, on the history of specific types of lumines- 

 cence, can do little more than summarize. It will suffice to say that 

 many important facts were known, but that integration and expla- 

 nation were lacking. 



Rather than attempting to summarize the advances covered in 

 this chapter, it is more valuable to look ahead and note how the 

 nineteenth century influenced later luminescence research. Perhaps 

 the discoveries which had the greatest eff^ect in changing the inter- 

 pretation of luminescence and setting a trend for present lumines- 

 cence theory came at the end of the nineteenth or the beginning of 

 the twentieth century. They were, in order of publication: (1) The 

 final identification of the electron in the paper on " Cathode Rays " 

 by J. J. Thompson in 1897, (2) the quantum theory of light of Max 

 Planck ^« (1900) as modified by Einstein ^^ (1905) and (3) the new 

 theory of the structure of the atom ^° with electronic energy levels 

 as the basis of spectral lines, advanced by Niels Bohr in 1913. 



This new physics led to the conception of energy levels in atoms 

 and molecules and the mechanism of emission of light quanta of 

 definite energy, connected with a particular frequency. Incandes- 

 cence played less of a role in the new conceptions of atomic struc- 

 ture, than did the luminescence of gases, with their spectra of sharp 

 discrete lines arranged in mathematically predictable order. The 

 theories derived from study of electroluminescence were applied to 

 other luminescences with a broader distribution of spectral energies, 

 even to the chemiluminescences, and hence to bioluminescence. 



From energy changes of matter in bulk, attention turned to 

 energy changes in atoms and molecules. The Einstein (1905,*^ 

 1912) ^^— Stark " (1908) photochemical equivalence law taught that 



*« M. Planck, Verhandl. deut. physik. Gesell. 2: 202-204, 1900. The theory was de- 

 veloped to account for the radiation from heated black bodies, spoken of as tempera- 

 ture radiation or incandescence. 



"A. Einstein, Ann. d. Physik 17: 132-148, 1905. 



^° N. Bohr, On the constitution of atoms and molecules, Phil. Mag. 26: 1-25, 676-502, 

 857-875, 1913. 



"A Einstein, Ann. d. Physik (4) 37: 832-838; 38, 881-884, 1912. 



^2 J. Stark, Physikalische Ztschr. 9: 889-894, 1908. 



