JAMES CLERK MAXWELL 325 



the incomprehensible ether of Fresnel, with its transversal vibra- 

 tions. 



EXPEHLMENTAL VERIFICATION 



We thus obtain all the results of the theory of waves. Yet this 

 was not enough to decide the physicists to adopt the ideas of Maxwell. 

 It was a seductive hypothesis ; but physicists consider hypotheses 

 which lead to no distinct observational consequences as beyond the 

 borders of their province. That province, so defined, no experimental 

 confirmation of Maxwell's theory invaded for twenty-five years. 



What was wanted was some issue between the two theories not too 

 delicate for our coarse methods of observation to decide. There was 

 but one line of research along which any experimentum crucis was to 

 be met with. 



The old electro-dynamics makes electro-magnetic induction take 

 place instantaneously ; but according to Maxwell's doctrine it prop- 

 agates itself with the velocity of light. 



The point was then to measure, or at least to make certain, a velocity 

 of propagation of inductive effects. This is what the illustrious Ger- 

 man physicist Hertz has done by the method of interferences. 



The method is well known in its application to optical phenomena. 

 Two luminous rays from one identical center interfere when they 

 reach the same point after pursuing paths of different lengths. If the 

 difference is one, two, or any whole number of wave lengths, the two 

 lights re-enforce one another so that if their intensities are equal, that 

 of their combination is four times as great. But if the difference is 

 an odd number of half wave lengths, the two Hghts extinguish one 

 another. 



Luminif erous waves are not peculiar in showing this phenomenon ; 

 it belongs to every periodic change which is propagated with definite 

 velocity. Sound interferes just as light does, and so must electro- 

 dynamic induction if it is strictly periodic and has a definite velocity of 

 propagation. But if the propagation is instantaneous there can be no 

 interference, since in that case there is no finite wave length. 



The phenomenon, however, could not be observed were the wave 

 length greater than the distance within which induction is sensible. 



