444 MR, J. B. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
For double extra-dense flint we have, using Hopkinson’s numbers, 
A=l-672. 
For the D line the equation becomes 
1-710 = 1 - 672 +^ 
or 
B=3-471(l-710 —1-672) 
= •1319 
Putting /r= V /K=1’778, we have 
\=3'527 
In Draper’s photograph line N is 3'528 
For the D line we have 
ExTRA-dense flint. 
B 
1-650=1-620 + 
3-471 
or 
B=3-471(-030) = -1041 
Putting jx— V / / K= 1'747, we have 
1 - 747 = 1 - 620 +^ 
A," 
X=2'8G2 
The following table compares the various refractive indices of each dielectric with 
the square root of the specific inductive capacity:— 
Table comparing /x and +K. 
Dielectric. 
+K. 
/R=oo- 
Md- 
/bv 
^/ A = \/#c* 
Double extra-dense flint 
glass . 
1-778 
1-672 
1-710 
1-757 
3"527, the wave 
length for N in 
ultra violet. 
Extra-dense flint glass . 
1-747 
1-620 
1-650 
1-688 
2-862 
Light flint glass* . 
1-734 
1"555 
1-574 
1-601 
Hard crown glass . 
1-763 
1-504 
1-517 
1-533 
Paraffin. 
1-4119 
l-4220f 
Sulphur. 
1-606 
Bisulphide of carbon . 
1-345 
1-611 
Common plate glass . 
1-801 
1-543 
* In the abstract of the paper published in ' Proc. Roy. Soc.,’ No. 191, the refractive indices for hard 
crown and light flint were interchanged. The mistake, which has only just been discovered, was caused 
by the polisher to whom the prisms were entrusted having interchanged the “H.C.” and “L.F.” labels.— 
June 17, 1879. 
j Gladstone and Clerk Maxwell, ‘Maxwell’s Electricity,’ § 789, vol. ii., p. 389. The melting point 
of my paraffin was 68° C., that of Dr. Gladstone’s was less than 57° C. 
