PRINCIPLES OF MATHEMATICAL PHYSICS 613 



themselves in every direction with the velocity of light, and in turn 

 other electrons, originally at rest, are made to vibrate when the 

 perturbation reaches the parts of the ether which touch them. 



The electrons, therefore, act upon one another, but this action is 

 not direct, it is accomplished through the ether as intermediary. 



Under these conditions can there be compensation between action 

 and reaction, at least for an observer who should take account 

 only of the movements of matter, that is to say, of the electrons, and 

 who should be ignorant of those of the ether that he could not see? 

 Evidently not. Even if the compensation should be exact, it could 

 not be simultaneous. The perturbation is propagated with a finite 

 velocity; it, therefore, reaches the second electron only when the 

 first has long ago entered upon its rest. 



This second electron, therefore, will undergo, after a delay, the 

 action of the first, but certainly it will not react on this, since around 

 this first electron nothing any longer budges. 



The analysis of the facts permits us to be still more precise. Imagine 

 for example, a Hertzian generator, like those employed in wireless 

 telegraphy; it sends out energy in every direction; but we can 

 provide it with a parabolic mirror, as Hertz did with his smallest 

 generators, so as to send all the energy produced in a single direction. 



What happens, then, according to the theory? It is that the 

 apparatus recoils as if it were a gun and as if the energy it has 

 projected were a bullet; and that is contrary to the principle of 

 Newton, since our projectile here has no mass, it is not matter, it 

 is energy. 



It is still the same, moreover, with a beacon light provided with 

 a reflector, since light is nothing but a perturbation of the electro- 

 magnetic field. This beacon light should recoil as if the light it 

 sends out were a projectile. What is the force that this recoil should 

 produce? It is what one has called the Maxwell-Bartholdi pressure. 

 It is very minute, and it has been difficult to put it into evidence 

 even with the most sensitive radiometers; but it suffices that it exists. 



If all the energy issuing from our generator falls on a receiver, 

 this will act as if it had received a mechanical shock, which will 

 represent in a sense the compensation of the recoil of the generator; 

 the reaction will be equal to the action, but it will not be simulta- 

 neous; the receiver will move on but not at the moment when the 

 generator recoils. If the energy propagates itself indefinitely with- 

 out encountering a receiver, the compensation will never be made. 



Do we say that the space \vhich separates the generator from 

 the receiver and which the perturbation must pass over in going 

 from the one to the other is not void, that it is full not only of ether, 

 but of air; or even in the interplanetary spaces of some fluid subtle 

 but still ponderable; that this matter undergoes the shock like the 



