446 SCIENCE PROGRESS 



be noted that Dr. Bohr 1 has recently, using this model and 

 basing his work upon Planck's theory of the discontinuity of 

 emission of energy, obtained an explanation of Balmer's formula 

 for the positions of the lines in the hydrogen series, although 

 the discussion was not completed by finding whether the lines 

 have their proper intensities. But even if this estimate of the 

 number of electrons in an atom is not accurately true, it is cer- 

 tainly very much nearer the truth than is the number assumed 

 by Humphreys. If then instead of supposing the iron atom to 

 contain five thousand electrons we suppose it contains only 

 thirty, the atomic strength of field is only the one-four hundred 

 and fiftieth part of that necessary to account for the observed 

 separation. The conclusion is inevitable that the atomic mag- 

 netic fields are such that their mutual influence is entirely 

 negligible, and incapable of accounting for the observed pressure 

 shift. There are other considerations which justify this con- 

 clusion. As Humphreys himself states, if this theory were true 

 the lines which give large Zeeman effects should also show 

 large pressure displacements, whilst those with small Zeeman 

 effects should be shifted but little. The connection between 

 these two phenomena has been investigated by King, 2 who com- 

 pared the Zeeman separation of a large number of lines with 

 their pressure shifts as determined by Humphreys, and found a 

 complete lack of connection : for example, in the case of iron, 

 the ratio of the Zeeman displacement to the pressure shift 

 varied from 078 to 1 5*5, and in the case of several lines showing 

 large pressure displacements, no Zeeman effect could be observed 

 even with the most intense magnetic fields. 



Prof. O. W. Richardson 3 formulated another theory which 

 sought to explain the displacement by means of sympathetic 

 vibrations occurring in the surrounding atoms. To quote his 

 own words : " The fact that an atom A is emitting light shows 

 that it is surrounded by an alternating field of electric force. 

 This alternating electric field will produce forced vibrations of 

 equal period and, under certain conditions, of like phase in 

 neighbouring atoms. The electric field due to the forced vibra- 

 tions will react upon the emitting electron in the atom A and in 

 such a way — as will be shown — as to increase the period of the 



1 Phil. Mag. July and September 191 3. 



* Astroph. Journ. 31, p. 433, 1910 ; 33, p. 250, 1911. 



3 Phil. Mag. 14, p. 557, 1907. 



