1916] on The Spectra of Hydrogen and Helium 655 



lines conld not be proved from the available astronomical data, but 

 at the same time there was no proof of their absence. Experimental 

 evidence on this point, ho\Yever, has been obtained by Evans, who 

 succeeded after much patient work in photographing some of the 

 lines in the positions predicted by Bohr. Further observations of 

 these lines appear also to have been made very recently by Paschen, 

 but full particulars are not yet to hand. The complete Pickering 

 series, as we now know it, is shown in Fig. 6, from which it will be 

 seen that Rydberg's theory only accounted for one-half of the lines, 

 as was the case with the 4686 series. 



Other predictions, relating to the composite structure of the lines 

 of hydrogen and proto-helium, which have been madeby Sommerfeld, 

 based upon an extension of Bohr's theory, have also been partially 

 verified by Evans and Paschen independently. 



Whatever approach to the truth may be represented by Bohr's 

 theory, it has at least been brilliantly successful in its application to 

 the spectra of hydrogen and proto-helium. Prof. Nicholson has 



A 2b 



Fig. 6. 



attempted, without success, to extend the theory to the ordinary 

 spectrum of helium, but Dr. Bohr does not accept this negative result 

 as final. 



The primary spectrum of hydrogen is thus, after many years, 

 restored to its original simplicity, consisting only of the Balmer series 

 and the parallel series in the infra-red and far ultra-violet. There is 

 neither astronomical nor experimental evidence that it contains any 

 other lines than these. For once, things are what they seem, and the 

 primary spectrum of hydrogen is as simple as it looks. 



If time had permitted, I might have attempted to indicate the 

 importance of the lines now assigned to proto-helium in relation to 

 the order of celestial evolution. They are among the very few lines 

 of known origin which appear in the spectra of gaseous nebulae and 

 Wolf-Rayet stars, and although their mere presence adds nothing at 

 all to our knowledge of the chemistry of these bodies, they may 

 teach us a great deal more in view of what we have been able to 

 learn from laboratory experience as to the special conditions requisite 

 for their production. Even more important, the new work on helium 

 points the way to further research regarding the unknown lines which 



