440 MR, J. B. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
any serious effect, as the experiments were made on many different days during which 
the temperature and dew-point varied very considerably. 
The effect of the film of moisture, if any, would have been to decrease a 2 —cq. Now 
as each set of determinations took about 20 minutes, the film, if it existed, would have 
dried gradually during this time, and in the last experiments of a set, a 2 — a 1 would 
always have been greater than in the earlier ones. An examination of the details of 
the experiments does not show that this was the case.* 
The close agreements of the values of K for paraffin found by me and by Messrs. 
Gibson and Barclay, who worked by an entirely different method, shows that there 
cannot be any serious defect either in their method or in mine. 
Measurement of the refractive indices of the transparent dielectrics. 
In order to see how far the results of these experiments bore out Professor 
Maxwell’s electromagnetic theory of light, the square roots of the specific inductive 
capacities of the various transparent dielectrics were compared with the refractive 
indices given in works on physics. Wherever an approximate agreement was found 
the refractive index was carefully re-determined. It was not, however, thought 
necessary to make experiments on the refractive indices of the dielectrics when either 
there was an obvious and wide difference between /x and fK, or when, as in the case 
of paraffin, a previous determination, probably more trustworthy than any I could 
make, was available. 
Optical glass. —In order to see whether Dr. Hopkinson’s! determinations of the 
refractive indices of Chance’s optical glass could be compared with those obtained by 
me, the specific gravities of the specimens used by me were taken and compared with 
the specific gravities of those used by Dr. Hopkinson. 
The following were the results:-— 
The name given to the glass 
by Messrs. Chance. 
Specific gravity of 
Ur. Hopkinson’ s 
specimen?. 
Specific gravity 
of G-oedon’s 
specimens. 
Double extra-dense flint . 
4-42162 
4-421 
Extra-dense flint. 
3-88947 
3-884 
Ligbt flint. 
3-20609 
3-198 
Hard crown. 
2-48664 
2-484 
* September 25, 1879.—Some later experiments on the effect of wetting the plates bave shown that no 
error could bave been introduced accidentally by tbe presence of a film of moisture.—Rep. Brit. Assoc., 
1879, p. 250. 
| ‘ Proc. Roy. Soc.,’ 1877, p. 290. 
