Closing Years of the Nineteenth Century. 431 

 or (neglecting 



C fjC - 1 



y - _ + - ^, 

 P M" 



which is the formula of Fresnel.* The hypothesis of Fresnel, 

 that a ponderable body in motion carries with it the excess of 

 aether which it contains as compared with space free from 

 matter, is thus seen to be transformed in Lorentz' theory 

 into the supposition that the polarized molecules of the 

 dielectric, like so many small condensers, increase the dielectric 

 constant, and that it is (so to speak) this augmentation of the 

 dielectric constant which travels with the moving matter. One 

 evident objection to Fresnel's theory, namely, that it required 

 the relative velocity of aether and matter to be different for 

 light of different colours, is thus removed ; for the theory of 

 Lorentz only requires that the dielectric constant should have 

 different values for light of different colours, and of this 

 a satisfactory explanation is provided by the theory of 

 dispersion. 



The correctness of Lorentz' hypothesis, as opposed to that of { 

 Hertz (in which the whole of the contained aether was supposed to 

 be transported with the moving body), was afterwards confirmed 

 by various experiments. In 1901 E. Blondlotf drove a current 

 of air through a magnetic field, at right angles to the lines of 

 magnetic force. The air-current was made to pass between the 

 faces of a condenser, which were connected by a wire, so as to be 

 at the same potential. An electromotive force E' would be 

 produced in the air by its motion in the magnetic field ; and, 

 according to the theory of Hertz, this should produce an 

 electric induction D of amount (e/47rc 2 ) E' (where t denotes the 

 specific inductive capacity of the air, which is practically 

 unity) ; so that, according to Hertz, the faces of the condenser 

 should become charged. According to Lorentz' theory, on the 

 other hand, the electric induction D is determined by the 

 equation 



47rc 2 D = E + (e - 1) E' 



* Cf. p. 117. t Comptes Rendus cxxxiii (1901), p. 778. 



