362 RICHARDSON— DYNAMICAL EFFECTS OF [April 22, 



not to be expected. At present the balance of evidence seems to be 

 decidedly against the corpuscular view. 



I am inclined to think that the primary Roentgen rays originate 

 largely as the result of secondary actions due to the stirring up 

 of the electrons in the atoms of the anti-cathode by the rapidly mov- 

 ing cathode rays which impinge on them. On this view the con- 

 stituent frequencies of the rays would be, to a considerable extent, 

 a matter of the atoms in which they originate ; and it may be that the 

 gap in electro-magnetic radiations between ultra-violet light and the 

 Roentgen rays, which exists at present, may never be filled up ; as 

 there may be no atoms which have natural periods in the neighbor- 

 hood of these frequencies. 



Recent years have seen the accumulation of a very large quantity 

 of material relating to optical efl^ects which are produced by a mag- 

 netic field. It is impossible, within the limits of this discussion, to 

 attempt to show the enormous usefulness of the electron theory in 

 the development of the science of magneto-optics ; but there is one 

 phenomenon which we cannot afford to pass by entirely, if only on 

 account of its historical importance. I refer to the Zeeman effect. 

 This effect was called after its discoverer, who showed that the 

 spectral lines which are emitted by all gaseous substances under 

 suitable conditions of excitation, were slightly displaced by a very 

 strong magnetic field. The true explanation of this phenomenon 

 was at once given by H. A. Lorentz. He pointed out that if the 

 monochromatic light was emitted by vibrating electrons the fre- 

 quency of the vibrations would be altered if the atom which con- 

 tained the electrons found itself in a magnetic field. This change 

 in the frequency, of course, corresponds with a change in the wave- 

 lengths of the emitted light. He also predicted that the emitted 

 light would be polarized in a certain v/ay and this was confirmed by 

 experiment. Lorentz showed, in addition, that the value of the 

 electric charge of an electron, divided by its mass, could be calculated 

 from the displacement of the spectral lines in the magnetic field. 

 The results of these calculations showed that the value of this ratio 

 was the same as that found by Sir J. J. Thomson and Wiechert for 

 the cathode rays in a discharged tube. Thus the Zeeman effect and 

 the cathode rays were the first two phenomena which aff'orded ex- 



