( 52 ) 
magnetic principal axes; we shall suppose the direction of the two 
sets of principal axes to be perfectly arbitrary with respect to 
each other. 
4. In the case of a non-condactive medium, the equations of the 
electromagnetic field become, if a direct system of axes is used: 
I 
Ss 
a), EEE 
ar rn ANN 
Ss 
A) en 
nn en 
te ” 
hert co 
zakken 7 
where v represents the ratio of the magnetic and electric units of 
electricity. From equations (19), (20) and (21) one deduces 
a/fof, og. heh 
a Geers = 
dt \dw dy oz 
Let us suppose the medium to be uncharged, then: 
Of » (09... BA 
eN 
DE ade 
If the medium is charged, this makes only this difference that 
we have a superposition of a variable field and a statie field. 
5. Let us now investigate whether a plane polarized wave of 
light can satisfy equations (19) to (26). For this we substitute: 
