Page:Collected Physical Papers.djvu/31

 II

ON A NEW ELECTRO-POLARISCOPE

In a paper read before the Asiatic Society of Bengal "On Polarisation of Electric Rays" (May, 1895), I gave an account of some experiments which showed that crystals which do not belong to the Regular System, produce double-refraction of the electric ray, and that the refracted beams are plane-polarised.

Among the numerous crystals examined, I found some exhibiting the so-called depolarising action in a very marked degree, even when the thickness of the crystal was less than the wave-length of the electric radiation. I found, for example, Nemalite, a ﬁbrous variety of Brucite, exhibiting this action with pieces which were comparatively thin. Different varieties of Satin Spar, Serpentine, Tourmaline and a few others were found to be very effective depolarisers.

From the various experiments to be presently described, I was led to suppose that these crystals transmit the ordinary and the extraordinary rays with unequal intensities. It would thus seem possible to quench one of the two rays by absorption, ordinary radiation after transmission through these crystals thus becoming plane-polarised. It should, however, be mentioned here, that crystals as a rule are far more transparent to electric radiation than to ordinary light, and as a consequence greater thickness of crystals would be required for the complete absorption of one of the two rays.

The apparatus with which the following experiments were carried out, consists of a Radiator emitting short electric waves, a cylindrical lens of sulphur for rendering