Page:Proceedings of the Royal Society of London Vol 59.djvu/175

1895.] For the accurate measurement of deviation, the effect produced by radiation on the receiver should undergo an abrupt variation. When the radiation passes from a dense to a light medium, at a certain critical angle of incidence, the radiation is totally reflected. From the critical angle the index of refraction is easily determined. The great advantage of this method lies in the fact that the transition from refraction to total reﬂection is very sudden.

I have determined the μ of various substances for the electric ray, by the method of total reflection.

It will be seen from the results of the experiments, that this method is capable of giving very good results.

The refracting substance is cut out or cast in the form of a semi-cylinder, and mounted on the central table of a spectrometer; the electric ray is directed towards the centre of the spectrometer, and its direction is always kept fixed. It strikes the curved surface and passes into its mass without any deviation. It is then incident on the plane surface of the semi-cylinder, and is refracted into the air beyond.

The incident angle on the plane surface is increased or decreased by rotating the central table on which the cylinder rests. In practice, it is more convenient to commence the experiment with an angle of incidence greater than the critical angle, the incident ray being then

totally reflected. The angle of incidence is slowly decreased till the critical value is reached. At this point the ray is all at once refracted into the air, making an angle of 90° with the normal to the surface. If a receiver be fixed against the side of the semi-cylinder at R, it will now respond to the refracted radiation.

The platform on which the cylinder rests, carries the usual index. By alternately rotating the platform in one direction or the other, and observing the position of the index when the receiver just responds, the reading for the critical angle is obtained with great