an abrupt variation. When the radiation passes from an optically denser to a lighter medium, then 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 of various substances for the electric ray, by the method of total reflection, which gives very accurate results.
The refracting substance cut out or cast in the form of a semi-cylinder, is 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 is refracted into the denser medium 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
Fig. 4. The refracting semi-cylinder and the Radiator at R and R′.
