MECHANICS AND OPTICS. 231 



electron like a wake. This energy, therefore, can only- 

 increase the inertia of the electron, as the agitation 

 of the liquid increases that of the body immersed in 

 a perfect fluid. And actually the electrons, at any 

 rate the negative electrons, have no other inertia but 

 this. 



In Lorentz's hypothesis, the vis viva, which is 

 nothing but the energy of the ether, is not propor- 

 tional to z/^. No doubt if v is very small, the vis 

 viva is apparently proportional to v^, the amount of 

 momentum apparently proportional to v, and the two 

 masses apparently constant and equal to one another. 

 But when the velocity approaches the velocity of light, 

 the vis viva, the amount of momentum, and the two 

 masses increase beyond all limit. 



In Abraham's hypothesis the expressions are some- 

 what more complicated, but what has just been said 

 holds good in its essential features. 



Thus the mass, the amount of momentum, and the 

 vis viva become infinite when the velocity is equal to 

 that of light. Hence it follows that no body can, by 

 any possibility, attain a velocity higher than that of 

 light. And, indeed, as its velocity increases its mass 

 increases, so that its inertia opposes a more and more 

 serious obstacle to any fresh increase in its velocity. 



A question then presents itself Admitting the 

 Principle of Relativity, an observer in motion can have 

 no means of perceiving his own motion. If, therefore, 

 no body in its actual motion can exceed the velocity 

 of light, but can come as near it as we like, it must be the 

 same with regard to its relative motion in relation to 

 our observer. Then we might be tempted to reason 

 as follows : — The observer can attain a velocity of 



