194 



SCIENTIFIC THOUGHT. 



56. 



Difficulties 

 of Maxwell's 

 theory. 



The theory of Maxwell had not only failed to give a 

 definite meaning to the conception of a charge of elec- 

 tricity ; it had also, in the general term " dielectric," some- 

 what obliterated the clear distinction between empty 

 space and space filled with insulating matter, such as 

 air. Empty space, i.e., space devoid of matter, was sup- 

 posed to be filled with some continuous substance, the 

 ether, which was the seat or bearer of electric and mag- 

 netic actions, the electro-magnetic field. When the only 

 clearly known property of this ether, the fact that it 

 was the carrier of radiation or the luminiferous medium, 

 was identified with its electro-magnetic nature light 

 being conceived to be an electro-magnetic disturbance 

 the new theory had to attack the great question of the 

 relation and interaction of ether and matter, in which 

 all the remaining problems of physical optics seemed 

 centred. 1 How was the electro-magnetic theory of light, 



lagen der Elektrodynamik, ' pub- 

 lished on the occasion of the un- 

 veiling at Gottingen, in 1899, of 

 the monument erected in honour 

 of Gauss and Wilhelm Weber. It 

 is interesting to see how, from ap- 

 parently quite independent begin- 

 nings, and in centres far removed 

 from each other, the ideas of the 

 atomic nature of electricity have 

 almost simultaneously become crys- 

 tallised, and have united them- 

 selves with the great experimental 

 labours emanating from Pliicker 

 and Crookes to give rise, at the 

 end of the century, to the modern 

 theory of electrons. 



1 One of the most important of 

 these problems is the question to 

 what extent the ether takes part 

 in the motion of ponderable matter 

 through it. Astronomical aber- 

 ration, discovered by Bradley, and 



easily explained by the then 

 current projectile theory of light 

 (see above, chap. vi. p. 10, note), has 

 caused great difficulty to the un- 

 dulatory theory, and even Sir 

 George Stokes, whose ideas on the 

 subject have been very generally 

 quoted and accepted, would, in his 

 Burnett Lectures on Light (1883), 

 say no more than that "according 

 to the theory of undulations . . . 

 it is not inexplicable" (ed. of 1887, 

 p. 25). That the electro-dynamic 

 view of the ether should take up 

 the problem was most natural, and 

 the discussion of it is accordingly 

 placed at the opening of Lorentz's 

 memoir of 1895 ; the effect of the 

 motion of the earth on optical 

 phenomena having already been 

 treated by him in 1887. Dr 

 Larmor treats very fully of this 

 subject in the first section of his 



