220 JAMKS CLKUK MAXWEU. 



"electric displacement" 1ms been given, the nature of 

 magnetic induction would be known. The dilliculties 

 in the way of any mechanical explanation are, it 

 is true, very great; assuming, however, that some 

 mechanical conception of "electric displacement" is 

 possible, Maxwell's theory gives a consistent account 

 of the other phenomena of electro-magnetism. 



Again, we have, it is true, an electro-magnetic 

 theory of light, but we do not know the nature of the 

 change in the ether which atVects our eyes with the 

 sensation of light, Is it the same as electric displace- 

 ment, r as magnetic induction, or since, when electric 

 displacement is varying, magnetic, induction always 

 accompanies it, is the sensation of light due to the 

 combined etVect of the two ? 



These questions remain unanswered. It may be 

 that light is neither electric displacement nor magnetic 

 induction, but some quite ditVrent periodic change of 

 structure of the ether, which travels through the 

 ether at the same rate as these quantities, and obeys 

 many of the same laws. 



In this respect there is a material difference be- 

 tween the ordinary theory of light and the electro- 

 magnetic theory. The former is a mechanical theory ; 

 it starts from the assumption that the periodic change 

 which constitutes light is the ordinary linear dis- 

 placement of a medium the ether having certain 

 mechanical properties, and from those properties it 

 deduces the laws of optics with more or less success. 



Lord Kelvin, in his labile ether, has devised a 

 medium which could exist and which has the 

 necessary mechanical properties. The periodic linear 



