A DYNAMICAL THEORY OP THE ELECTROMAGNETIC FIELD. 577 



PART VI. 



ELECTROMAUNETIC THEORY OF LIGHT. 



(91) At the commencement of this paper we made use of the optical 

 hypothesis of an elastic medium through which the vibrations of light are 

 propagated, in order to shew that we have warrantable grounds for seeking, 

 in the same medium, the cause of other phenomena as well as those of light. 

 We then examined electromagnetic phenomena, seeking for their explanation in 

 the properties of the field which surrounds the electrified or magnetic bodies. 

 In this way we arrived at certain equations expressing certain properties of 

 the electromagnetic field. We now proceed to investigate whether these pro- 

 perties of that which constitutes the electromagnetic field, deduced from electro- 

 magnetic phenomena alone, are sufficient to explain the propagation of light 

 through the same substance. 



(92) Let us suppose that a plane wave whose direction cosines are I, m, n 

 is propagated through the field with a velocity V. Then all the electro- 

 magnetic functions will be functions of 



z Vt. 



The equations of Magnetic Force (B), p. 556, will become 



dH dG 

 U.CL = m j -- n -; , 

 aw aw 



B=n -l- 



^ dw dw ' 



__ 

 ' " dw dw ' 



If we multiply these equations respectively by I, m, n, and add, we find 



lfia + m^/3 + niJ.y = Q ............................... (62), 



which shews that the direction of the magnetization must be in the plane of 

 the wave. 



VOL. I. 73 



