404 Flux of Energy in the El ectro dynamic Field. 



field are the electric force and magnetic induction vectors, and 

 any fields which are identical when defined in terms of these 

 vectors must be physically identical in spite of any difference 

 in the mathematical formulation of their relations, and the 

 identity must surely cover the transfer of energy in them. 



This discussion can be immediately extended to cover the 

 more general cases of motion of individual electrons, but no 

 essentially new results are brought to light. 



11. To sum up, we may say that the generalized form of the 

 new theory of the transfer of energy in the electromagnetic 

 field leads to conclusions which are wholly incompatible 

 with our usual preconceptions of the physical circumstances 

 of radiation problems, although in itself it appears perfectly 

 consistent as a new mathematical and physical theory. On 

 the other hand, Macdonald's own form of this theory is 

 apparently inconsistent as regards both the physical or 

 dynamical foundations on which it is based and the results 

 which are deduced from it ; and although it avoids some of 

 the fundamental difficulties of the more general theory, 

 it leads to results not entirely free from criticism in certain 

 general cases where these difficulties might be expected to 

 present themselves in tangible form. 



Under the circumstances there seems to be no other 

 alternative but to reject both forms of the theory. This 

 throws us back on Poynting's theory as being the simplest 

 and most definite theory which is consistent with the 

 mathematical relations of the field and the physical pro- 

 cesses operative therein. Of course, our physical conception 

 of radiation processes is derived mainly from Poynting's 

 theory, so that it is hardly fair to bring forward its con- 

 sistency in this respect as an argument in its favour ; 

 nevertheless, I think this theory cnn stand as by far the 

 most satisfactory yet proposed. 



Of course no definite proof of the absolute inadequacy of 

 the alternative forms of the theory can be given ; and if 

 ever we may decide to revise our fundamental notions of 

 these things in general and radiation in particular, it may be 

 as well to remember that Poynting's theory is not the only 

 form of theory mathematically consistent with our electro- 

 magnetic scheme. 



