428 MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
to bring the light spot to zero. The circuit being kept closed, a second similar piece of 
ebonite was placed at /3'. Now, if any of the lines of force had curved round the 
edge of /3, the introduction of /S' would have caused a deflection in the direction 
showing an increase of specific inductive capacity in a b. No such deflection however 
took place, which shows that no lines of force strong enough to affect the electrometer 
pass round the edge of a 7-inch dielectric. 
The experiments. 
After about five months spent in preliminary experiments and in perfecting the 
adjustments of the various instruments, the following measurements were made :— 
Optical glass .—Four beautiful slabs, each seven inches square, of perfectly homo¬ 
geneous glass, were cast in Birmingham by Messrs. Chance, and polished in London 
under the directions of Mr. Kieser. 
The glasses were— 
Double extra-dense flint. 
Extra-dense flint. 
Linht flint. 
Hard crown. 
Small prisms of each glass were also prepared in order that the refractive indices 
might l)e compared with the specific inductive capacities.'" 
The following are the details of the exjDeriments and values of K deduced from 
them :— t 
Optical glass. 
Double extra-dense flint. 
b=S47. 
a, (1) D613 (3) D615 (5) D610 
cq (2) 1-034 (4) 1-034 1*032 
a 2 -~« 1 -579 '581 "578 
Mean "5793. 
K = 3T64. 
See p. 440. 
f Oj, o 2 , and b are all given in incites. 
