Page:Collected Physical Papers.djvu/47

Rh obtained, the experiment was modified to secure a higher degree of accuracy.

Two equal semi-cylinders P and Q were taken and placed on the rotating table face to face, with an air space between. A metallic plate with a narrow rectangular opening was also interposed between the

semi-cylinders to serve as a diaphragm to cut off all but the central rays. When the spark gap is placed at O, the principal focus of P, the rays emerge parallel into the air space, and are then focussed by the second cylinder at an equal distance ƒ on the other side.

The spark gap is placed at O, and the receiver at O′, OO′ being extremities of a diameter passing through the centre of the circle. The air space is for convenience placed parallel to the index.

The platform carrying the cylinders is now rotated, say to the left. The angle of incidence is thus gradually increased, till the rays just undergo total reflection.