300 History of Luminescence 



The spectra of gases, excited electrically at low pressure are made 

 up of a series of lines, a condition now known to be characteristic of 

 relative simple atoms. These lines exhibit a certain regularity and 

 have given important information regarding the constitution of 

 matter. In 1869, E. Mascart suggested that the regular distribution 

 of lines in sodium or magnesium spectra might be as important as 

 the fact that such lines were characteristic of these metals. Lecoq 

 de Boisbaudran (1869-1873) also, called attention to regularities in 

 potassium and rubidium lines, and in 1870 G. J. Stoney studied 

 hydrogen, and showed that the distribution of the line frequencies 

 were such that they might be considered overtones of a fundamental 

 frequency. On discovery of additional hydrogen lines in the stars, 

 Stoney (1880) extended his findings and later (1891, 1892) added 

 to the general knowledge of regularity. In 1871, J. L. Soret wrote 

 a paper, " On Harmonic Ratios in Spectra," dealing with mag- 

 nesium, while A. Schuster (1881) applied the harmonic ratio con- 

 cept to other elements. 



These papers represent the start of a field of spectroscopy in which 

 gas spectra have played an important part. Probably the most satis- 

 factory relationship was that noted in 1885 by Johan Jakob Balmer 

 (1825-1898) for hydrogen, now known as the Balmer series. An 

 enormous amount of research has been carried out since then, and 

 many other series observed. Such spectral lines are now regarded 

 as light of definite frequencies emitted as a result of sudden corre- 

 sponding energy changes in the atoms (or molecules) of a substance, 

 connected with shift in position of electrons from one orbit of revo- 

 lution to another— what has come to be called a transition. In this 

 way electroluminescent spectra can be interpreted in a logical man- 

 ner, although it would be presumptuous to say that the last word 

 has been written on spectral analysis. 



In addition to the colors of gases in a discharge tube, the changing 

 forms of luminescence, dark spaces and striations, also required 

 explanation.^^ They presented some baffling problems to the physi- 

 cists of the last century. These rapidly changing visual phenomena 

 depend on temperature, pressure, and electrical characteristics of 

 the discharge, just as in the case of spectral structure. They have 

 also been studied in minutest detail,®* and their character related 



®^ See J. E. H. Gordon, A physical treatise on electricity and magnetism (2 v., 

 London, 1880) , for a general account of luminescent effects in vacuum tubes. 



"* In addition to Becquerel, Crookes, de la Rive, de la Rue, Goldstein, Hittorf, 

 Wiedemann, Gassiot, Pliicker, Spottiswoode, and others previously mentioned, the fol- 

 lowing men, working in the latter part of the century, have contributed to knowledge 

 of conditions during electroluminescence in gases: E. C. C. Baly, 1893; C. Chree, 1891; 

 H. Ebert, 1899; H. Ebert and E. Wiedemann, 1893; E. S. Ferry, 1898; W. P. Graham, 



