614 APPLIED MATHEMATICS 



receiver at the moment when the energy reaches it, and recoils in its 

 turn when the perturbation quits it? That would save the principle 

 of Newton, but that is not true. 



If energy in its diffusion remained always attached to some ma- 

 terial 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. This is what Michelson and Morley have since 

 confirmed. 



One may suppose also that the movements of matter, properly 

 so called, are exactly compensated by those of the ether; but that 

 would lead us to the same reflections as just now. The principle so 

 extended would explain everything, since whatever might be the 

 visible movements, we should always have the power of imagining 

 hypothetical movements which compensated them. 



But if it is able to explain everything, this is because it does 

 not permit us to foresee anything; it does not enable us to decide 

 between different possible hypotheses, since it explains everything 

 beforehand. It therefore becomes useless. 



And then the suppositions that it would be necessary to make 

 on the movements of the ether are not very satisfactory. 



If the electric charges double, it would be natural to imagine 

 that the velocities of the divers atoms of ether double also, and for 

 the compensation, it would be necessary that the mean velocity of 

 the ether quadruple. 



This is why I have long thought that these consequences of 

 theory, contrary to the principle of Newton, would end some day 

 by being abandoned, and yet the recent experiments on the move- 

 ments of the electrons issuing from radium seem rather to confirm 

 them. 



I arrive at the principle of Lavoisier on the conservation of masses : 

 in truth this is one not to be touched without unsettling all mechanics. 



And now certain persons believe that it seems true to us only 

 because we consider in mechanics merely moderate velocities, but 

 that it would cease to be true for bodies animated by velocities com- 

 parable to that of light. These velocities, it is now believed, have 

 been realized; the cathode rays or those of radium may be formed 

 of very minute particles or of electrons which are displaced with 

 velocities smaller no doubt than that of light, but which might be its 

 one tenth or one third. 



These rays can be deflected, whether by an electric field, or by 

 a magnetic field, and we are able by comparing these deflections, to 

 measure at the same time the velocity of the electrons and their mass 

 (or rather the relation of their mass to their charge). But when 

 it was seen that these velocities approached that of light, it was 

 decided that a correction was necessary. 



