644 Professor A. Fowler [March 31, 



WEEKLY EYENINCx MEETING, 



Friday, March 31, 1916. 



Edward Pollock, F.R.C.S., Yice-President, m the Chair. 



Professor A. Fowler, F.R.S., Sec. R.A.S. 



The Spectra of Hydrogen and Helium. 



The investigation of the spectra of hydrogen and heHuoi is inti- 

 mately connected with problems apparently so diverse as the 

 constitution of the atom and the order of celestial evolution. 

 Hydrogen is remarkable as yielding the simplest of all known 

 spectra, and, though helium ordinarily presents greater complexity, 

 it is now believed to be capable of emitting another spectrum which 

 rivals in simplicity that of hydrogen itself. These simple spectra 

 naturally make a special appeal to those who are attempting to 

 calculate the spectra of the elements from theoretical considerations 

 relating to the structure of atoms, for here the problem is reduced to 

 its simplest terms. The two elements are also of exceptional interest 

 to astronomers, for hydrogen shows itself in nearly every self- 

 luminous celestial body which has been observed, while helium is 

 characteristic of an important special class of stars. Indeed, 

 helium, as everyone knows, received its name from the occurrence 

 of its yellow line D3 in the sun's chromosphere, long before it was 

 discovered on earth by Ramsay ; and the spectrum of hydrogen is 

 more completely displayed in some of the stars, and in the chromo- 

 sphere, than in any laboratory experiment which has yet been made. 

 It may therefore be of interest to bring together some of the 

 principal results which have been obtained in connexion with the 

 spectra of these two elements. 



Let me first recall the spectrum of hydrogen, which we can 

 readily obtain from a hydrogen vacuum tube. If we take a tube at 

 random we shall most likely see two spectra superposed. One of 

 them consists of many hundreds of faint lines, constituting what is 

 called the secondary spectrum, while the other includes the well- 

 known lines of the so-called primary spectrum, beginning with Ha in 

 the red, and following on with H^g in the blue-green, H^ in the blue, 

 and so on. Either of these spectra can be suppressed, or nearly so, 

 at will. Generally speaking, narrow tubes or strong discharges 

 favour the primary spectrum, while the secondary is best developed 

 in wider tubes or with weaker discharges. 



Although for some time considered as possibly due to impurities, 



