these little-known æther waves are now being gradually discovered. Confining our attention to the electric waves, we find that there are many important problems which may perhaps be better attacked with these comparatively slow vibrations; among which may be mentioned the determination of the indices of refraction of various substances which are opaque to visible light, but are transparent to the electric ray; the relation between the dielectric constant and the refractive index when the rates of oscillation are made comparable in the two determinations; the variation of the index with the frequencies of vibration. Then there are the phenomena of double refraction, polarization, and the magnetic rotation of the electric ray; the determination of the wave-length, and other problems of a similar nature.
The fascination of the subject drew me to its study, though the investigations were rendered exceedingly difficult in India from want of facility for making the necessary apparatus. I ultimately succeeded in constructing a few instruments with which I was able to obtain the values of the indices of refraction of various substances for electric waves, the wave-length of electric radiation, to demonstrate the phenomena of double refraction and polarization of the electric rays, and to find out certain substances which act as electric tourmalines. The simplified apparatus with which many of the properties of electromagnetic radiation may be studied is here exhibited. This is a duplicate made by Messrs. Elliott, Brothers, of the apparatus which I brought from India. I also take this opportunity of thanking Mr. Bolton, F.R.A.S., of the Mathematical Instrument Department, Calcutta, for the divided circle in my apparatus.
The following are the experiments which may be carried out with this apparatus:—
- A. Verification of the laws of reflexion.
- 1. Plane mirrors.
- 2. Curved mirrors.
- B. Phenomena of refraction.
- 1. Prisms.
- 2. Total reflexion.
- 3. Opacity caused by multiple refraction and reflexion.
- 4. Determination of the indices of refraction.
- C. Selective absorption.
- 1. Electrically-coloured media.
