2o6 SCIENCE AND METHOD. 



we can photograph the curve which represents the 

 relation between these two deviations. This is what 

 Kaufmann has done, and he has deduced the rela- 

 tion between the velocity and the proportion of the 

 charge to the total apparent mass, a proportion that 

 we call €. 



We might suppose that there exist several kinds 

 of rays, each characterized by a particular velocity, 

 by a particular charge, and by a particular mass ; 

 but this hypothesis is most improbable. What reason 

 indeed could there be why all the corpuscles of the 

 same mass should always have the same velocity? It 

 is more natural to suppose that the charge and 

 the actual mass are the same for all the projectiles, 

 and that they differ only in velocity. If the propor- 

 tion c is a function of the velocity, it is not because 

 the actual mass varies with the velocity, but, as the 

 fictitious electro-magnetic mass depends upon that 

 velocity, the total apparent mass, which is alone 

 observable, must depend upon it also, even though 

 the actual mass does not depend upon it but is 

 constant. 



Abraham's calculations make us acquainted with 

 the law in accordance with which the fictitious mass 

 varies as a function of the velocity, and Kaufmann's 

 experiment makes us acquainted with the law of 

 variation of the total mass. A comparison of these 

 two laws will therefore enable us to determine the 

 proportion of the actual mass to the total mass. 



Such is the method employed by Kaufmann to 

 determine this proportion. The result is most sur- 

 prising : the actual mass is nil. 



We have thus been led to quite unexpected con- 



