CHAPTEE XII. 



THE THEORY OF AETHER AND ELECTRONS IN THE CLOSING 

 YEARS OF THE NINETEENTH CENTURY. 



THE attempts of Maxwell* and of Hertzf to extend the theory 

 of the electromagnetic field to the case in which ponderable 

 bodies are in motion had not been altogether successful. 

 Neither writer had taken account of any motion of the material 

 particles relative to the aether entangled with them, so that in 

 both investigations the moving bodies were regarded simply 

 as homogeneous portions of the medium which fills all space, 

 distinguished only by special values of the electric and 

 magnetic constants. Such an assumption is evidently incon- 

 sistent with the admirable theory by which FresnelJ had 

 explained the optical behaviour of moving transparent bodies ; 

 it was therefore not surprising that writers subsequent to Hertz 

 should have proposed to replace his equations by others 

 designed to agree with Fresnel's formulae. Before discussing 

 these, however, it may be well to review briefly the evidence 

 for and against the motion of the aether in and adjacent 

 to moving ponderable bodies, as it appeared in the last decade 

 of the nineteenth century. 



The phenomena of aberration had been explained by Young 

 on the assumption that the aether around bodies is unaffected 

 by their motion. But it was shown by Stokes|| in 1845 that 

 this is not the only possible explanation. For suppose that 

 the motion of the earth communicates motion to the neighbour- 

 ing portions of the aether ; this may be regarded as superposed 

 on the vibratory motion which the aethereal particles have 



* Cf p. 288. t Cf. p. 365. I Cf. p. 116. $ Cf. p. 115. 



|| Phil. Mag. xxvii (1845), p. 9 ; xxviii (1846), p. 76; xxix (1846), p. 6. 



