6 Prof. Gr. Quincke on the Constant of Dielectricity 
reflection electrometer and measured by the screw-electro- 
meter. 
These fluid condensers, alone or with a Bahinet compensator 
(employed by me in previous investigations*), placed between 
Nicol prisms, permitted the electric double refraction found 
by Mr. Kerrf to be measured. 
If we name d the difference of rate, measured in wave- 
lengths, by which the light polarized parallel is accelerated 
with respect to the light polarized perpendicular to the electric 
lines of force, P the electric difference of potential (in the 
C.Gr.S. system) of the plane electrodes, I their length, and a 
the distance between them, then 
d=B 'm^ ne ^ (5) 
The quantity B diminishes as the wave-length increases, 
and is in general found to be smaller in proportion as the 
length of the electrized liquid is greater. Slight mixtures of 
foreign substances, even water condensed from the air, have 
a considerable influence on B. 
As the means of a rather large number of series of experi- 
ments with plane electrodes of nickel of 10, 20, 46 centim. 
length and at from 0*1615 to 0*3230 centim. distance, I found 
for light corresponding to Fraunhofer's line D the following 
values of the constant B : — 
Amount of the Electric Double Refraction for different Fluids 
between Plane Nickel Electrodes, measured by a Babinet 
Compensator. (Fraunhofer's line D.) B.10 6 . 
X. 
Sulphide of carbon (Kahlbaum) .... 32*798 
„ „ (Heidelberg) . . . 31*984 
3 vol. sulphide of carbon + 1 vol. ether . 27*252 
1 „ „ „ +1 „ „ . . 19*476 
1 „ „ „ +5 „ „ . . 4*422 
Heavy benzol (from coal-tar) .... 4*460 
Pure benzol (from benzoic acid) . . . 3*842 
Light benzol 2*970 
Oil of turpentine 0*109 
Colza oil —2*273 
Ether (fresh distilled) -6*400 
„ (having stood over calcined marble) —6*685 
Sulphur dissolved in sulphide of carbon ~| „ pr . 
(19*5 per cent.) j dJ *°' 
* Pogg. Ann. cxxvii. p. 206, pi. 1. figs. 4, 5 (1866). 
t Phil. Mag. [4] 1. pp. 337, 446(1875) ; [5] viii. pp. 85, 229 (1879), ix. 
p. 159 (1880). 
