Page:On the Determination of the Index of Refraction of Glass for the the Electric Ray.djvu/3

Rh There is thus a serious difference between the two views of the variation of K (and therefore of μ) with the frequency of vibration. In a previous paper, I alluded to the probability of the variation of μ with the frequency of vibration. The value of μ may at first undergo a diminution with the increase of frequency, reach a minimum, and then have the value augmented when the frequency rises above the critical rate. The result obtained by Lecher is, however, too divergent from the others to be explained by such a supposition.

The direct determination of μ for glass for electric oscillations of high frequency, seemed to me of interest, as throwing some light on the controversy; so, on the conclusion of my determination of the index for sulphur, I commenced an investigation for the determination of μ for glass. This was, however, greatly delayed by repeated failures to cast glass here, and by my long absence from India. I have now obtained from England two semi-cylinders of glass, with a radius = 12·5 cm. and height = 8 cm.

The method of experiment followed is the same as that described in my previous paper. The radiator is placed at the principal focus (obtained from a preliminary experiment) of one of the semi-cylinders. The cylinder mounted on the platform of a spectrometer is rotated till the rays are totally reflected. From the critical angle the value of μ is deduced.

I shall here describe some modifications introduced in the apparatus, which have been found to be great improvements. One of the principal difficulties met with was in connexion with the disturbance caused by stray radiation. It is to be remembered that the receiver is extremely sensitive. Comparatively long waves are found to possess very great penetrative power; shielding the receiver then becomes very difficult. Even after the receiver, the galvanometer, and the leading wires had been screened, disturbances were met with which it was difficult to localise. Part of the disturbance may have been due to that set up by the generating coil. A double box made of soft iron and thick copper removed this difficulty. But the greatest immunity from disturbance was secured by using short waves. In this case it was not at all necessary to take very special precautions to shield either the galvanometer or the leading wires, the sensitive layer in the receiver alone being affected by the radiation. I exposed the bare leading wires to the strong action of the radiator by putting them in close proximity to the source of radiation, and yet no response was observed in the galvanometer. This freedom from disturbance is not due to the opposite action on the two wires, for a single wire may be exposed to the radiation without any action on the receiver. Y2