Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/420

can be said to be determined as yet with such a degree of accuracy as to enable us to assert that the one is greater or less than the other. It is to be hoped that, by further experiment, the relation between the magnitudes of the two quantities may be more accurately determined.

In the meantime our theory, which asserts that these two quantities are equal, and assigns a physical reason for this equality, is certainly not contradicted by the comparison of these results such as they are.

788.] In other media than air, the velocity $V$ is inversely proportional to the square root of the product of the dielectric and the magnetic inductive capacities. According to the undulatory theory, the velocity of light in different media is inversely proportional to their indices of refraction.

There are no transparent media for which the magnetic capacity differs from that of air more than by a very small fraction. Hence the principal part of the difference between these media must depend on their dielectric capacity. According to our theory, therefore, the dielectric capacity of a transparent medium should be equal to the square of its index of refraction.

But the value of the index of refraction is different for light of different kinds, being greater for light of more rapid vibrations. We must therefore select the index of refraction which corresponds to waves of the longest periods, because these are the only waves whose motion can be compared with the slow processes by which we determine the capacity of the dielectric.

789.] The only dielectric of which the capacity has been hitherto determined with sufficient accuracy is paraffin, for which in the solid form M. M. Gibson and Barclay found^[1]

$K=1.975.$

(12)

Dr. Gladstone has found the following values of the index of refraction of melted paraffin, ${\text{sp. g. 0.779}}$, for the lines A, D and H:―

Temperature	A	D	H
54℃	1.4306	1.4357	1.4499
57℃	1.4294	1.4343	1.4493

from which I find that the index of refraction for waves of infinite length would be about ${\text{1.422.}}$

The square root of $K$ is ${\text{1.405.}}$

The difference between these numbers is greater than can be ac

1. Phil. Trans. 1871, p. 573.