222 SCIENCE AND METHOD. 



III. 

 The Principle of Reaction. 



Let us see what becomes, under Lorentz's theory, 

 of the principle of the equality of action and reaction. 

 Take an electron, A, which is set in motion by some 

 means. It produces a disturbance in the ether, and 

 after a certain time this disturbance reaches another 

 electron, B, which will be thrown out of its posi- 

 tion of equilibrium. Under these conditions there 

 can be no equality between the action and the re- 

 action, at least if we do not consider the ether, but 

 only the electrons which are alone observable, since 

 our matter is composed of electrons. 



It is indeed the electron A that has disturbed the 

 electron B ; but even if the electron B reacts upon A, 

 this reaction, though possibly equal to the action, 

 cannot in any case be simultaneous, since the electron 

 B cannot be set in motion until after a certain length 

 of time necessary for the effect to travel through the 

 ether. If we submit the problem to a more precise 

 calculation, we arrive at the following result. Imagine 

 a Hertz excitator placed at the focus of a parabolic 

 mirror to which it is attached mechanically ; this 

 excitator emits electro-magnetic waves, and the mirror 

 drives all these waves in the same direction : the 

 excitator will accordingly radiate energy in a particular 

 direction. Well, calculations show that tlie excitator 

 zvill recoil like a cannon that has fired a projectile. 

 In the case of the cannon, the recoil is the natural 

 result of the equality of action and reaction. The 



