Page:Proceedings of the Royal Society of London Vol 59.djvu/176

162 accuracy. The receiver is now placed at R´, and the cylinder rotated in the opposite direction till total reflection again takes place. The difference between the first and second readings is evidently equal to twice the critical angle.

To utilise only the central rays, a metallic screen with a small central opening is placed against the plane face of the semi-cylinder. In order that all the rays should undergo total reflection simultaneously, it is necessary that the rays incident on the plane of separation of the two media should be parallel. This is effected by the cylinder itself. From the approximate value of μ found from a preliminary experiment, the focal distance of the semi-cylinder is roughly calculated. The spark-gap of the radiator is placed at this focus, and the rays thus rendered very near parallel. Each subsequent experiment gives a more accurate value of μ, and from the corrected value of the focal distance thus obtained, a more accurate adjustment is made for the next experiment.

The apparatus used consists of an electric radiator emitting short waves, a large graduated circle provided with a central circular platform on which the refracting substance is placed, and a receiver, which responds to the electric radiation.

The Radiator.—I have given a detailed description of the electric radiator in my paper on polarisation of electric rays (vide Journal, Asiatic Society of Bengal, Part 2, No. 2, 1895). Electric oscillation is produced by sparking between two metallic beads and an interposed metallic sphere 1 cm. in diameter. By a single sudden break of the primary in a Ruhmkorff's coil, a flash of radiation is emitted. The spark gap is placed at one end of a brass tube 5 cm. in diameter. By a sliding arrangement, the length of the tube may be varied. The Ruhmkorff's coil is enclosed in a copper box.

The Circle.—The circle has a diameter of 45 cm., and is graduated into degrees, but one-fourth of a degree may be easily estimated. The circle, as a whole, may rotate round a vertical axis which passes thoughthrough [sic] the centre of a massive stand. There is a raised circular platform at the centre of the circle on which the refracting substance is placed. This platform carries an index, and may be rotated independently of the large circle. When the platform index is clamped, the two circles rotate together.

The Refracting Substance.—For substances which can be cast, the molten mass is poured into a cylindrical mould with a thin partition in the middle. In this way two equal semi-cylinders are obtained at each casting. Substances like wood or stone are turned, and the cylinder sawn into two equal halves. In my experiments different sized cylinders were used. I have successfully used small ones with