Mr. N. Campbell on the sEtlier. 185 



body is distributed electrostatic energy localized in the 

 a?ther ; the positions or! the portions of the rether which con- 

 tain stated amounts of energy (belonging to one and the same 

 body), relative to each other or to the charged nucleus, are 

 not changed by the motion. If the rether is the body where 

 electrical energy is localized, it seems obvious and simple 

 to identify points in the aether, as required by the definition 

 of velocity, by the amounts of energy contained in them. 

 Then the velocity of the a?ther relative to the observer would 

 be different according as one or other of the charged bodies 

 was considered, and would be in each case the same as the 

 velocity of the corresponding charged body relative to the 

 observer. 



§ G. Such, I think, is the simple and obvious view, leading 

 directly to the principle of relativity, which would have been 

 accepted without question had it not been for the use of the 

 singular word " a?ther." " If," it was said, " there is only 

 one aather, it cannot have more than one velocity relative to 

 any one observer : hence we must suppose that portions of 

 aether are not to be identified by the energy which they 

 contain, that the energy moves through the aether, being 

 transferred from one portion to another, with a velocity 

 which has nothing to do with the Aelocity of the aether itself.*' 

 This view is, I imagine, maintained by those who write of 

 the rether; let us see whither it leads us. 



§ 7. It is clear at once that if it be not permitted to 

 identify a point in the aether by the energy localized at it, 

 no other means of identification can be substituted. All 

 optical phenomena prove that the aether (outside material 

 bodies) is perfectly homogeneous, so far as the power to 

 contain energy is concerned : the velocity of radiation is 

 rectilinear and uniform in whatever direction it is propagated. 

 All portions of the aether which contain the same amount of* 

 energy are, so far as experiment can tell, perfectly similar, 

 and there is no possible means of distinguishing between 

 them ; neither have the boundaries of the aether, if there are 

 such, ever been attained. The first requisite for the applica- 

 tion to the aether of the definition of velocity, which is 

 implied in all statements concerning the velocities of material 

 bodies, cannot be fulfilled : until some other definition of 

 velocity is put forward as applicable to the aether, all pro- 

 positions about velocity of or relative to the aether are 

 meaningless. On the view of the aether which rejects the 

 identification of portions of the aether by their energy- 

 content, the first statement which is made about the velocity 

 of the n?ther must either be a definition^ or be wholly devoid 



