228 SCIENCE AND METHOD. 



accelerations must occur as if the electron underwent 

 Lorentz's deformation. 



V. 



Kaufmann's Experiment. 



Two theories are thus presented to us : one in 

 which the electrons are undeformable, which is Abra- 

 ham's ; the other, in which they undergo Lorentz's 

 deformation. In either case their mass grows with 

 their velocity, becoming infinite when that velocity 

 becomes equal to that of light ; but the law of the 

 variation is not the same. The method employed by 

 Kaufmann to demonstrate the law of variation of the 

 mass would accordingly seem to give us the means of 

 deciding experimentally between the two theories. 



Unfortunately his first experiments were not suffi- 

 ciently accurate for this purpose, so much so that he 

 has thought it necessary to repeat them with more 

 precautions, and measuring the intensity of the fields 

 with greater care. In their new form they have shown 

 Abraham's theory to be right. Accordingly, it would seem 

 that the Principle of Relativity has not the exact value 

 we have been tempted to give it, and that we have no 

 longer any reason for supposing that the positive elec- 

 trons are devoid of real mass like the negative electrons. 



Nevertheless, before adopting this conclusion some 

 reflexion is necessary. The question is one of such 

 importance that one would wish to see Kaufmann's 

 experiment repeated by another experimenter.* 



* At the moment of going to press we learn that M. Bucherer h.is 

 repeated the experiment, surrounding it with new precautions, and that, 

 unlike Kaufmann, he has obtained results confirming Lorentz's views. 



