Page:Proceedings of the Royal Society of London Vol 59.djvu/178

164 leading wires from the coherer are enclosed in a double coating of tin-foil.

I first tried to determine the index of refraction of sulphur. The material used was ordinary commercial sulphur. A semi-cylinder was made, and the two positions for total reflection determined by the method which has already been described. The difference of readings found for the two positions varied from 69° to 71° and the value of the critical angle would from these experiments seem to lie between 34·5° and 35°. This approximate value for the critical angle having been obtained, the experiment was modified to secure a greater amount of accuracy.

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

the semi-cylinders to serve as a diaphragm and 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 film, 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 film is for convenience placed parallel to the index.

The platform carrying the cylinders is now rotated, say to the left.