Page:Collected Physical Papers.djvu/36

16 In other words, Serpentine transmits vibrations perpendicular to the fibres, but absorbs vibrations parallel to the fibres. Ordinary radiation would thus, after transmission through Serpentine, be plane-polarised, the vibration taking place perpendicular to the fibres. To ensure complete polarisation, the piece should be fairly thick.

An efficient Polariser or Analyser can thus be made of substances like Serpentine, provided that the thickness is sufficiently great.

Satin Spar, Tourmaline and Nemalite

I also found different crystals exhibiting unequal transparency to polarised radiation in different directions. Satin-spar exhibits it, the electric vibration being more easily transmitted across the fibres. I next tried some experiments with a piece of black Tourmaline about 2 cm. in thickness. With this thickness, it was not possible to obtain complete extinction, unless the intensity of the incident radiation was considerably diminished. I at first arranged the Polariser and the Analyser parallel, and the Tourmaline was successively held vertical and horizontal. The Receiver responded with unequal intensities, the response being more energetic when the length of Tourmaline was parallel to the electric vibration. With fibrous varieties of crystals, I found the vibration, as a rule, more easily transmitted perpendicular to the length of fibres.

I now held the Tourmaline horizontal, and by varying the aperture at the end of the radiating tube, diminished the amount of radiation, so that at a certain point there was no response in the Receiver. On now