194 SCIENTIFIC THOUGHT. 



56. The theory of Maxwell had not only failed to give a 



Difficulties 



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- ; easily explained by the then 



lished on the occasion of the un- I current projectile theory of light 



veiling at Gottingen, in 1899, of j (see above, chap. vi. p. 10, note), has 



the monument erected in honour : caused great difficulty to the un- 



of Gauss and Wilhelm Weber. It dulatory theory, and even Sir 



is interesting to see how, from ap- ! George Stokes, whose ideas on the 



parently quite independent begin- subject have been very generally 



nings, and in centres far removed quoted and accepted, would, in his 



from each other, the ideas of the \ Burnett Lectures on Light (1883), 



atomic nature of electricity have ' say no more than that " according 



almost simultaneously become crys- j to the theory of undulations . . . 



tallised, and have united them- [ it is not inexplicable " (ed. of 1887, 



selves with the great experimental 

 labours emanating from Pliicker 

 and Crookes to give rise, at the 

 end of the century, to the modern 



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 



theory of electrons. placed at the opening of Lorentz's 



1 One of the most important of memoir of 1895 ; the effect of the 



these problems is the question to motion of the earth on optical 



what extent the ether takes part , phenomena having already been 



in the motion of ponderable matter treated by him in 1887. Dr 



through it. Astronomical aber- Larmor treats very fully of this 



ration, discovered by Bradley, and ! subject in the first section of his 



