334 Mr. R. Hargreaves on Radiation 



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sum of two terms, the first of which is radiant energy as 

 viewed from a moving standpoint. The second is kinetic in 

 character, i. e. it is a composite term of kinetic energy, in 

 which the components of translation are associated with those 

 of a momentum belonging to radiation. If we have an aber- 

 rational argument Xlx — ( ^H, and look at the wave 



from a standpoint in motion (u r v'w') relative to the dielectric, 

 i.e. put x = %-\-u't, ... the argument is 



The corresponding motional term in the energy is 

 KM 



2 V 



( X/3 -Y«)(»' + 5). 



We take the expression for momentum to retain its normal 

 value, while a reduced effective value of the velocity appears 

 in the aberrational case. Thus (40) gives the momentum of 

 radiation in the dielectric when the differentiation is with 

 regard to a velocity relative to the dielectric (w f ). 



The case as regards aberration is briefly as follows : — For 

 free aether the electric and magnetic sections of energy have 

 expressions iSX 2 and \%a* ; when the radiation is viewed 

 from a moving standpoint only a part of this has the character 



of radiant energy, and there is a kinetic term 2^ (X/3 — Yu) . 



In a dielectric ^2X 2 and JS« 2 are weighted with the coeffi- 

 cients K and M respectively. Is the kinetic term to be 

 weighted or not ? If it is not weighted we have the 

 aberrational effect, i. e. the latter amounts to the retention of 

 the same expression for the part of the formula which denotes 

 kinetic energy. If it is weighted, with a coefficient KM, 

 the meaning corresponds to an alteration of standpoint in 

 viewing a wave which is proceeding with a velocity V/fi. 

 It is here supposed that this case is actual, that a wave 

 originating in a dielectric does travel with constant velocity, 

 and that the translation in the formula is a motion relative 

 to the dielectric. 



There is I think a prima facie case for a real difference 

 between the cases which are here separated; i. e. it is reason- 

 able to expect a difficulty in boarding, or gaining foothold in, 

 the moving medium ; and the fact of aberration shows that a 

 wave in such circumstances does not obtain a grip of the full 

 propagating power of the medium. 



It is a consequence of this separation of cases that no 



