THE VALUE OF SCIENCE 347 



these conditions can there be compensation between action and reac- 

 tion, at least for an observer who should take account only of the move- 

 ments of matter, that is, 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 will certainly not at that moment react upon 

 it, since around this first electron nothing any longer budges. 



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

 agine, for example, a Hertzian oscillator, like those used 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 oscillators, 

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

 happens then according to the theory? The apparatus recoils, as if 

 it were a cannon and the projected energy a ball ; and that is contrary 

 to the principle of Newton, since our projectile here has no mass, it is 

 not matter, it is energy. The case is still the same, moreover, with a 

 beacon light provided with a reflector, since light is nothing but a 

 perturbation of the electromagnetic field. This beacon light should 

 recoil as if the light it sends out were a projectile. What is the force 

 that should produce this recoil? It is what is called Maxwell-Bar- 

 tholdi pressure. It is very minute, and it has been difficult to put it in 

 evidence even with the most sensitive radiometers; but it suffices that 

 it exists. 



If all the energy issuing from our oscillator 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 oscillator's recoil; the reaction will 

 be equal to the action, but it will not be simultaneous; the receiver 

 will move on, but not at the moment when the oscillator recoils. If 

 the energy propagates itself indefinitely without encountering a re- 

 ceiver, the compensation will never occur. 



Shall we say that the space which separates the oscillator 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 subtile but still pon- 

 derable; that this matter undergoes the shock like the receiver at the 

 moment when the energy reaches it, and recoils in its turn when the 

 perturbation quits it? That would save Newton's principle, but that 

 is not true. If energy in its diffusion remained always attached to 

 some material substratum, then matter in motion would carry along 

 light with it, and Fizeau has demonstrated that it does nothing of the 

 sort, at least for air. Michelson and Morley have since confirmed this. 



