December 29, 1916] 



SCIENCE 



911 



hydrogen is greatly intensified by the pres- 

 ence of helium. It may be added that Mer- 

 ton has concluded, from a study of the 

 width of 4686, that it is due to an atom 

 smaller than that of helium. 



Some light may be thrown on this prob- 

 lem by observations such as those made by 

 Wright and others on the distribution of 

 materials in nebulas, as indicated by the 

 length of the nebular lines. Wright finds 

 that usually 4686 is confined to the nu- 

 cleus; helium lines extend further, and 

 those of nebulum and hydrogen still 

 further. These results favor the view that 

 the elements distribute themselves accord- 

 ing to their atomic weights and that 4686 

 is due to an atom at least as heavy as that 

 of helium. But this is not conclusive, be- 

 cause a high temperature line of hydrogen 

 might be found only in the hot nucleus, 

 if we grant the possibility of a higher de- 

 gree of ionization for hydrogen. 



Fundamental questions which are of im- 

 portance to physicists and astronomers 

 alike are involved in this problem, but it 

 is evidently an elusive one. Curiously 

 enough, as Fowler has proved by compari- 

 son with other spectra, general series rela- 

 tions would permit us to assign the dis- 

 puted series to hydrogen or to helium im- 

 partially, and it seems possible that both 

 elements may give the same spectrum- 

 under appropriate conditions. Bohr has 

 also concluded, from the formula derived 

 from the assumption of the return of an 

 electron to a lithium atom which has lost 

 three electrons, that lithium would emit 

 lines close to the Balmer series. Bohr has 

 not yet succeeded in applying his method 

 to the case where an electron returns to a 

 singly charged helium or lithium atom,- 

 and hence has not been able to account for 

 the known helium lines, which are assigned 

 by Stark to singly charged atoms. Nor 

 has he taken account of atomic magnetic 

 fields, which, as Humphreys, Allen and 



others have shown, may exercise an ap- 

 preciable influence. 



One of the most fascinating fields of re- 

 search is that of fluorescence and reso- 

 nance spectra, in which much work has re- 

 cently been done, particularly by Wood. 

 He has found that white light will excite 

 the complete band and line resonance 

 spectrum of sodium or iodine, but that a 

 single exciting line will cause the emission 

 of a line of the same length and also of a 

 number of lines approximately equally 

 spaced which may not always coincide in 

 position with one of the absorption lines. 

 Thus the vapor is caused to emit forced vi- 

 bration, giving a spectrum not its own. As 

 Wood has suggested, this method enables 

 us to strike one key of the complex vi- 

 brating system of the atom, instead of the 

 whole keyboard at once. Time does not 

 permit a detailed account of this remark- 

 able work, but it is evident that it may 

 render great service in the study of the 

 mechanism of the atom. Nor is there 

 time to even mention any of the results ob- 

 tained in the field of absorption spectra. 



After reviewing the work of the past 

 decade, we may feel encouraged by the 

 progress that has been made both in the 

 perfecting and application of spectroscopic 

 methods of research and in the discovery of 

 new phenomena. Some of these discover- 

 ies have led to fundamental revisions of 

 our notions of atomic structure. The 

 Rutherford atom has definitely displaced 

 that of Thomson. In some respects this 

 has seemed to make the problem more diffi- 

 cult, but it has at least defined it more pre- 

 cisely. Many attempts have been made to 

 represent an atomic structure which would 

 satisfy the necessary mathematical condi- 

 tions, most of them so impossible as to be 

 absurd or so speculative that they suggest 

 no experimental tests of their validity. 

 The great merit of Bohr's hypothesis is 

 that it does lend itself to such tests, and 



