SOME coxTF.MPtm-.ihy .ii>r.ixcF.s ix riiysics riii 427 



that if it wiTf the i>nly series in ixislfiue. no (inc would .iit.n h any 

 particular iniix)rtanre to it." 



Ti) the physicists of a Kt^'itTi*''"" i>K'>. wht) reKarded the spertrum 

 fre(|iien»ies as natural \il)ralion-fre(|ucnries of the atom, and tried 

 hard to invent a nieehanieal niode! of which the vihration-frequencips 

 should conform to the formula ('.i) or the more general formula (o), 

 the character of these formulae was an insurmountable obstacle, 

 l-^lsewhere '" I have given a brief account of the \ain attempts to con- 

 trive such a nuxlel. Bohr abandoned this procedure altogether; and 

 taking equation (3), he multiplied both sides of it by Planck's con- 

 stant /; I =()..")•;■ 10 -■"). 



h, = l,R(\-\). (6) 



The significance of this act depends on the meaning of //. Planck 

 hail found it expedient, in tleveloping an adequate theory of radiation, 

 tt> assume that soliti hot bodies are popul.ited willi multitudes of 



Kii;. I I'riiicip.il scries of helium (singlet systini . 1. l.yni.in, A^trnphyiiiat 

 Journal) 



oscillating electrons of all the various frecpiencies, possessing a very 

 curious and inexplicable property; this being, that an oscillator 

 vibrating with frequency v can emit radiant energy of that same fre- 

 quency V only in units or quanta of amount hv. Kinstein had found 

 it exjiedicnt, in describing the photoelectric effect and other phe- 

 nomena, to assume that radiant energy of the frequency v goes about 

 in units or quanta of the amount hv, emitted integrally, absorbed 

 integrally, travelling integrally. Suppose then that we assume that 

 the quantity hv, standing on the left-hand side of the equation (6), 

 represents the amount of radiant energy emitted by the hjdrogen 



'• As a matter of fact, the series-limit is not generally so obvious to the eye that 

 it can be l<x-ate«l at once: it is determined after and by means of a careful choice 

 of the most suital)lc form for the function /(i). This is one of the difficulties of the 

 spectroscopist's task. 



'• In the seventh article of this series (footnote 9). 



