AND MODEH.V PHYSICS. 205 



(3) Tho identification of this electro-magnetic 

 medium with the luminiferous ether, and the con- 

 sequent view that light is an electro-magnetic 

 phenomena. 



(4) Tho view that electro-magnetic forces arise 

 entirely from strains and stresses set up in the ether ; 

 the electro-static charge of an insulated conductor 

 being one of the forms in which the ether strain is 

 manifested to us. 



(5) A dielectric under the action of electric force 

 is said to become polarised, and, according to Maxwell 

 (voL i. p. 133), all electrification is the residual effect 

 of the polarisation of the dielectric. 



Now it must, I think, be admitted that in Max- 

 well's day there was direct proof of very few of these 

 propositions. No one has even yet so measured the 

 displacement currents in a dielectric as to show that 

 the total flow across every section of a circuit is at 

 any given moment the same, though there are other 

 experiments of an indirect character which have now 

 completely justified Maxwell's hypothesis. Experi- 

 ments by Schiller and Von Helmholtz prove it is 

 true that some action in the dielectric must bo taken 

 into consideration in any satisfactory theory; they 

 therefore upset various theories based on direct action 

 at a distance, " but they tell us nothing as to whether 

 any special form of the dielectric theory, such as 

 Maxwell's or Helmholtz's, is true or not" (J. J. 

 Thomson, ' Report on Electrical Theories," BJL Re- 

 port, 1885, p. 149.) 



When Maxwell died there had been little if any 

 experimental evidence as to the stresses set up in a 



