CH. x.] MAGNETISED RADIATION 111 



further condition is quite consistent with those 

 imposed by dynamics. 



Every frequency of rotation will correspond to a 

 definite line in the spectrum. But if this be its real 

 cause, radiation must be susceptible to magnetic 

 influence, for a revolving electric charge consti- 

 tutes a circular current ; and if a magnetic field be 

 started into existence with its lines threading that 

 circuit, it must, while it is changing in intensity, cause 

 the speed either to increase or to decrease, and so 

 will either raise or lower the refrangibility. If 

 electrons are revolving in every direction, and if a 

 magnetic field is applied to them, then during the rise 

 of the field the pace of some will be increased and of 

 some decreased ; and this increase or decrease will not 

 stop until the magnetic field is destroyed again. 



Hence it would appear that if a source of radiation 

 is put into a magnetic field, and its lines examined 

 with a spectroscope, they should be affected, either 

 by way of shift or broadening, or in some other way. 

 It happened, however, that when Dr. Larmor theo- 

 retically perceived this, and estimated the order of 

 magnitude of the effect to be expected, he made the 

 assumption natural in 1895 that an electron was 

 comparable in mass with a hydrogen atom. On this 

 assumption, knowing what he did about the massive- 

 ness of an atom, he calculated that the effect would 

 be too small to see ; indeed, Faraday had, with 

 imperfect appliances many years ago, looked for some 

 such effect not then guided by theory, but simply 

 with the object of trying all manner of experiments 

 and had failed to see anything ; Prof. Tait also had 

 been moved by theory in the same direction, but no 

 fresh experimental attempt to examine the question 

 was initiated. Nor was the matter publicly referred 



