THE 
LONDON, EDINBURGH, ayn DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
[FIFTH SERIES.] 
JUNE 1885. 
XLV. On the Electromagnetic Wave-surface. 
By OLIver HEAVISIDE*. 
) Geen showed (Electricity and Magnetism, vol. ii. 
art. 794) that his equations of electromagnetic disturb- 
ances, on the assumption that the electric capacity varies in 
different directions in a crystal, lead to the Fresnel form of 
wave-surface. There is no obscurity arising from the ignored 
wave of normal disturbance, because the very existence of a 
plane wave requires that there be none. In fact, the electric 
displacement and the magnetic induction are both in the 
wave-front, and are perpendicular to one another. The mag- 
netic force and induction are parallel, on account of the 
constant permeability ; whilst the electric force, though not 
parallel to the displacement, is yet perpendicular to the mag- 
netic induction (and force) ; the normal to the wave-front, 
the electric force, and the displacement being in one plane. 
The ray is also in this plane, perpendicular to the electric 
force. There are of course two rays for (in general) every 
direction of wave-normal, each with separate electromagnetic 
variables to which the above remarks apply. 
It is easily proved, and it may be legitimately inferred 
without a formal demonstration, from a consideration of the 
equations of induction, that if we consider the dielectric to be 
isotropic as regards capacity, but eolotropic as regards per- 
meability, the same general results will follow, if we translate 
capacity to permeability, electric to magnetic force, and elec- 
* Communicated by the Author. 
Phil. Mag. 8. 5. Vol. 19. No. 121. June 1885. 2E 
