a larger number of breaks would have been necessary even for one hour's work.
Lens for rendering the beam parallel.—In my first apparatus, with the help of an ordinary glass lens and suitable diaphragms, the beam was made approximately parallel. This was more or less a guess-work, as the index of glass for the electric ray has not yet been determined. I have, however, succeeded in determining the electric index for Sulphur, which is very near 1.734. With the knowledge of this index, a cylindrical lens of Sulphur has been constructed, whose focal distance is known with accuracy. The source of radiation, the spark gap, is a line, and the lens is adjusted till its focal line and the spark gap coincide. In this way, a parallel beam of electric radiation is obtained.
Polariser and Analyser.—The success of the experiment depends greatly on the care with which the Polariser and the Analyser are constructed. Fine copper wire, 0.2 mm. in diameter, was carefully wound in parallel lines round two thin sheets of mica; there were about 25 lines in one centimetre. The mica pieces were then immersed in melted paraffin, and the wires thus fixed in situ. By cutting round, two circular pieces containing the gratings were obtained. One of these acted as a Polariser and the other as an Analyser.
Receiver.—The receiving circuit consists of a spiral-spring coherer in series with a modified Daniell cell and an aperiodic galvanometer of D'Arsonval type.
In a square piece of ebonite a shallow rectangular depression is cut out, and a single layer of steel spiral springs 2 mm. in diameter and 1 cm. in length is laid
