PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD. 497 
a quantity essentially positive ; so that, when the primary electrification is in one direc- 
tion, the secondary discharge is always in the same direction as the primary discharge*. 
PART VI.— ELECTROMAGNETIC 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 
show 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 pheno- 
mena, 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 properties 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 l , m, n is propa- 
gated through the field with a velocity V. Then all the electromagnetic functions will 
be functions of 7 , , Tr , 
w=lx-\-my-\-nz— Yt. 
The equations of Magnetic Force (B), p. 482, will become 
dH 
dG 
-n 
dw ’ 
ujfi=.n 
dF 
dH 
dw 
-l 
dw ’ 
dG 
dF 
7—1 
dw 
-771 
dw 
If we multiply these equations respectively by /, m, n, and add, we find 
, (62) 
which shows that the direction of the magnetization must be in the plane of the wave. 
(93) If we combine the equations of Magnetic Force (B) with those of Electric 
Currents (C), and put for brevity 
rfF rfG rfH 
dx'dy' dz 
T , d* , d* , d* 
J, and S +^s+^i=V 
4 W '=|-V’F, 
4 W=|-vg, 
(63) 
(64) 
* Since this paper was communicated to the Royal Society, I have seen a paper by M. Gaugain in the Annales 
de Chimie for 1864, in which he has deduced the phenomena of electric absorption and secondary discharge 
from the theory of compound condensers. 
