PEOFESSOE CLEEK MAXWELL ON THE ELECTEOMAGXET1C FIELD. 
501 
we should call the electric fluid, and select either vitreous or resinous electricity as the 
representative of that fluid, then we might have normal vibrations propagated with a 
velocity depending on this density. We have, however, no evidence as to the density of 
electricity, as we do not even know whether to consider vitreous electricity as a sub- 
stance or as the absence of a substance. 
Hence electromagnetic science leads to exactly the same conclusions as optical science 
with respect to the direction of the disturbances which can be propagated through the 
field; both affirm the propagation of transverse vibrations, and both give the same velocity 
of propagation. On the other hand, both sciences are at a loss when called on to affirm 
or deny the existence of normal vibrations. 
'Relation between the Index of Refraction and the Electromagnetic Character of the 
substance. 
(101) The velocity of light in a medium, according to the Undulatory Theory, is 
where i is the index of refraction and V 0 is the velocity in vacuum. The velocity, 
according to the Electromagnetic Theory, is 
where, by equations (49) and (71), k=^k 0 , and k 0 =iirWl. 
(80) 
Hence 
D = 
or the Specific Inductive Capacity is equal to the square of the index of refraction 
divided by the coefficient of magnetic induction. 
Propagation of Electromagnetic Disturbances in a Crystallized Medium. 
(102) Let us now calculate the conditions of propagation of a plane wave in a 
medium for which the values of k and p are different in different directions. As we 
do not propose to give a complete investigation of the question in the present imperfect 
state of the theory as extended to disturbances of short period, we shall assume that the 
axes of magnetic induction coincide in direction with those of electric elasticity. 
(103) Let the values of the magnetic coefficient for the three axes be X, v, then 
the equations of magnetic force (B) become 
^ M dy dz ’ 
0 d¥ dH 
^P = -T-— -j- 1 
«(H dG 
' A P— dz ~ dx 
( 81 ) 
dG d¥_ 
dx~ dy' 
3 Y 2 
