(1) Serpentine.—This substance, which appears fibrous, transmits the ordinary and the extraordinary ray, with unequal intensity. A fairly thick piece completely absorbs vibrations parallel to the fibres, and transmits vibrations perpendicular to the fibres. Ordinary radiation, after transmission through a thick piece of serpentine, would be plane-polarized, the vibration taking place perpendicular to the fibres. A thick piece of serpentine thus acts as an efficient polarizer.
There are certain important points in connexion with selective conductivity and the phenomena of polarization by absorption exhibited by certain substances, which will be dealt with in a future paper.
(2) Nemalite.—This crystal exhibits this effect in a still more marked degree.
(3) Tourmaline also produces the depolarization effect. The difference in absorption of the ordinary and the extraordinary rays is, however, not so great as in the case of light.
(4) Beryl, Apatite, and Barytes are also very good crystals for exhibiting the depolarization effect.
Polarization Produced by other Substances.—I found many other natural substances producing polarization, the most interesting being vegetable fibres. Common jute (Corchorus capsularis) exhibits the property in a very marked degree. I cut fibres of this material about 3 cm. in length, and built with it a cell with all the fibres parallel. I subjected this cell to a strong pressure under a press. I thus obtained a compact cell 3 X 3 cm. in area, and about 5 cm. in thickness. This was mounted in a metallic case, with two openings about 2 X 2 cm. on opposite sides for the passage of the radiation.
This cell was found to quench vibrations parallel to the fibres, and transmit vibrations perpendicular to the fibres. Jute cells could thus be made to serve as polarizers or analysers.
Effect due to Strain.—Could be exhibited by stratified rocks, the plane of stratification being inclined at 45° to the horizon.
Effects similar to that produced by unannealed glass can be imitated by a block of unequally chilled paraffin.
The polarization-apparatus described above may also be used as a polarimeter, the rotation of the analyser being measured by the graduated disk.
