special preparation. Pieces of glass or ebonite, too, are often found sufficiently strained to exhibit double refraction of the electric ray.
Phenomena of Double Absorption
In search of a crystal polariser, I naturally turned to tourmaline, but was disappointed to find it utterly unsuitable as a polariser. There is a difference in transparency in directions parallel and perpendicular to the length, but even a considerable thickness of the crystal did not completely absorb one of the two rays. Because tourmaline polarises visible light by selective absorption, it does not follow that all kinds of radiation would be so polarised.
It was a long time before I could discover crystals which acted as electric tourmalines. In the meanwhile I found many natural substances which produced polarisation by selective unilateral absorption. For example, I found locks of human hair to polarise the electric ray. I have here two bundles of hair; I interpose one at 45°, and you observe the depolarisation effect. The darker specimen seems to be the more efficient. Turning to other substances more easily accessible, I found vegetable fibres to be good polarisers. Among these may be mentioned the fibres of aloes (Agave), rhea (Boehmeria nivea), pine-apple (Ananas sativus), plantain (Musa paradisiaca). Common jute (Corchorus capsularis) exhibits the property of polarisation in a very marked degree. I cut fibres of this material about 3 cm. in length, and built with them 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 cm. by 3 cm. in area, and 5 cm. in thickness.
