PEOFESSOE CLEEK MAXWELL ON THE ELECTEOMAGNETIC FIELD. 
503 
are given by the equations 
.'f + ”G'+“H'=0, (91) 
^(5>-A)+^(A-^)+^(aV-5»=0, (92) 
(105) The velocities along the axes are as 
Direction of propagation . 
Direction of the electric displacements -j 
Now we know that in each principal plane of a crystal the ray polarized in that 
plane obeys the ordinary law of refraction, and therefore its velocity is the same in 
whatever direction in that plane it is propagated. 
If polarized light consists of electromagnetic disturbances in which the electric dis- 
placement is in the plane of polarization, then 
a 2 =J 2 =c 2 (93) 
If, on the contrary, the electric displacements are perpendicular to the plane of pola- 
rization, 
X=p=v (94) 
We know, from the magnetic experiments of Faraday, Plucker, & c., that in many 
crystals a, v are unequal. 
The experiments of Knoblauch * on electric induction through crystals seem to show 
that a, b and c, may be different. 
The inequality, however, of X , p , v is so small that great magnetic forces are required 
to indicate their difference, and the differences do not seem of sufficient magnitude to 
account for the double refraction of the crystals. 
On the other hand, experiments on electric induction are liable to error on account 
of minute flaws, or portions of conducting matter in the crystal. 
Further experiments on the magnetic and dielectric properties of crystals are required 
before we can decide whether the relation of these bodies to magnetic and electric 
forces is the same, when these forces are permanent as when they are alternating with 
the rapidity of the vibrations of light. 
* Philosophical Magazine, 1852. 
follows : — 
X 
y 
z 
d 2 
d 2 
X 
— 
— 
V 
b 2 
b 2 
y 
V 
A 
c 2 
c 2 
z 
A 
