sometimes possible to work continuously for about two hours; but greater weight should of course be given to the first sets of observations, which are taken at a time when the receiver is most sensitive.
It is superfluous to add that special precautions should be taken to guard against the disturbance due to stray radiations. The walls of the room, the table, even the person of the experimenter himself may act as reflectors, scattering the rays in all directions. I spent a considerable time in trying to find a substance that will act as a good absorber. Lamp black is useless, as it reflects copiously. Blotting paper soaked in water or copper sulphate solution does produce a certain amount of absorption; but even with these a certain amount of reflection is found to take place.
By proper screening, the disturbance due to stray radiations may, however, be got rid of. The radiating apparatus, with the exception of a tubular opening, is completely enclosed in a metallic box. The radiator tube extends right up to the refracting cylinder. The leading wires from the coherer are enclosed in a double coating of tin-foil.
Method of Experiment
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 had 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
