Radiation and the Structure of the Atom. 397 



angular momentum in the discussion o£ atomic systems in 

 relation to Planck's theory was first pointed out by 

 J. W. Nicholson *. 



In paper I. it was shown that the above assumptions lead 

 to an interpretation of the Balmer formula for the hydrogen 

 spectrum, and to a determination of the Rydberg constant 

 which was in close agreement with the measurements. In 

 these considerations it is not necessary to make any assump- 

 tion about the degree of excentricity of the orbit of the 

 electron, and we shall see in the next section that it cannot 

 be assumed that the orbit is always circular. 



So far we have considered systems which contain only one 

 electron, but the general validity of the assumptions A and B 

 seems strongly supported by the fact that they offer a simple 

 interpretation of the general principle of combination of 

 spectral lines (see IV. p. 507). This principle was originally 

 discovered by Ritz to hold for the ordinary series spectra of 

 the elements. It has recently acquired increased interest 

 by Fowler's work on the series spectra of enhanced lines 

 emitted from many elements when subject to a powerful 

 electric discharge. Fowler showed that the principle of 

 combination holds for these spectra although the laws 

 governing the numerical relation between the lines at an 

 important point (see section 3) differed from those of the 

 ordinary series spectra. There is also, as we shall see in 

 section I, some indication that the principle holds for the 

 high frequency spectra revealed by interference in crystals. 

 In this connexion it may also be remarked that the assump- 

 tion A recently has obtained direct support by experiments 

 of A. Einstein and J. W. do Haas f, who have succeeded in 

 detecting and measuring a rotational mechanical effect pro- 

 duced when an iron bar is magnetized. Their results agree 

 very closely with those to be expected on the assumption 

 that the magnetism of iron is due to rotating electrons, and 

 as pointed out by Einstein and Haas, these experiments 

 therefore indicate very strongly that electrons can rotate in 

 atoms without emission of energy radiation. 



When we try to apply assumptions, analogous with C and 

 D, to systems containing more than one electron, we meet 

 with difficulties, since in this case the application of ordinary 



* Nicholson, Mouth. Not. Hoy. Astr. Soc. lxxii. p. 679 (1912). 



t Einstein and Haas, T'crh. d. D. Phi/s. Ges. xvii. p. 152 (1915). That 

 such a mechanical rotational effect was to be expected on the electron 

 theory of magnetism was pointed out several years ago by 0. W. 

 Kichardson, Phys. Review, xxvi. p. 248 (1908). ' Richardson tried to 

 detect this effect but without decisive results. 



