1116 Prof. R. Whiddington on 



under the conditions of the experiments ; but already a 

 rough calculation makes it very probable that the fields, 

 due to neighbouring ions and free electrons, to which the 

 emitting atoms have been subject, have been of the order 

 of magnitude claimed by the quantum theory for the appear- 

 ance of the new lines with the intensity observed. Espe- 

 cially it must be noted, that the intensity of these fields 

 may easily have been many thousand times larger than the 

 intensity of the external electric forces present in the 

 unscreened part of the apparatus, as a direct consequence 

 of the applied potential. 



On the whole it seems that the theory of series spectra 

 based on the quantum theory allows to account in a general 

 way for the experimental evidence, and that especially the 

 relative intensity, with which under various experimental 

 conditions combination lines appear, may not be considered 

 as presenting the theory with serious difficulties, but rather 

 as providing a means of investigating the conditions under 

 which spectra are emitted. For instance, investigations of 

 the presence of combination lines may perhaps offer the 

 most direct means of estimating the density of ions in the 

 different parts of a discharge tube. As regards a more detailed 

 account of the theory of series spectra, and for references to 

 the literature on this subject, the reader of this Magazine may 

 be referred to a paper on the effect of electric and magnetic 

 fields on spectral lines, which was read by the writer as a 

 Guthrie lecture at the Physical Society of London on March 

 24th, and which will soon appear in the Proceedings of this 

 Society. 



Yours faithfully, 



University, Copenhagen, -N . BOHR. 



April 11th, 1922. 



CXVIII. X-ray Electrons. By R. Whiddington, M.A., 



D.Sc, Cavendish Professor of Physics, University of Leeds* . 



[Plate XXIV.] 

 Introduction. 



IT has been known for many years that X-rays have 

 the power of: ejecting high-speed electrons from the 

 surface of materials on which they are incident. 



The earliest successful attempts to measure the speed 

 of these X-ray electrons were made by Dorn in 1900 and 

 * Communicated by the Author. 



