- D. Phenomena of interference.
- 1. Determination of the wave-length by curved gratings.
- 2. Bi-prism experiments.
- E. Double refraction and polarization.
- 1. Polarizing gratings.
- 2. „ crystals.
- 3. Double refraction produced by crystals.
- 4. „ „ other substances.
- 5. „ „ by strain.
- 6. Circular polarization.
- 7. Magnetic rotation.
- 8. Electro-polariscope and polarimeter.
In the list of experiments above-mentioned, the determination of the wave-length by curved gratings has been carried out with a larger apparatus (vide Proc. Roy. Soc. vol. lX., "On the Determination of the Wave-length of Electric Radiation"). Experiments with circular polarization and magnetic rotation and with the bi-prism are still in progress. All the others have been repeated with the apparatus to be described below.
The complete apparatus consists of:—(1) A radiating apparatus emitting electric waves of short length; (2) A receiver used as a detector of electric radiation; and (3) Various accessories for the study of the different phenomena.
I used various methods for the production of oscillatory discharge. One method was to imbed a row of metallic beads, with small spark spaces, in solid paraffin, the end beads being in connexion with the electric generator. Another method was to have the two sparking-balls immersed in kerosene; this is effective, but troublesome. The simplest method, however, is Prof. Lodge's arrangement of two side balls and an interposed sphere.
Electric oscillation is produced by sparking between two beads of platinum and an interposed sphere of the same metal. The discharge ceases to be oscillatory when the ball is roughened, and a platinum ball resists, to a great extent, the disintegrating action of the sparks. Two jointed electrodes carry the two beads at their ends. The distance between the beads and the interposed sphere can thus be adjusted. This is a matter of importance, as the receiver does not properly respond if the spark length is too large. It is more convenient to use short electric waves, and these are
