Page:Collected Physical Papers.djvu/117

Rh This was mounted in a metallic case, with two openings 2 cm. by 2 cm. on opposite sides for the passage of radiation. This cell absorbs vibrations parallel to the length of the fibres, and transmits those perpendicular to the length. Two such cells could thus be used, one as a polariser and the other as an analyser.

Turning to crystals I found a large number of them exhibiting selective absorption in one direction. Of these nemalite and chrysotile exhibit this property to a remarkable extent. Nemalite is a fibrous variety of brucite, chrysotile being a variety of serpentine. The direction of absorption in these cases is parallel to the length, the direction of transmission being perpendicular to the length. I have here a piece of chrysotile, only one inch in thickness. I adjust the polariser and the analyser parallel, and interpose the chrysotile with its length parallel to the electric vibration. You observe that the radiation is completely absorbed, none being transmitted. I now hold the piece with its length perpendicular to the electric vibration; the radiation is now copiously transmitted. Chrysotile is thus seen to act as a perfect electric tourmaline.

In a polarising grating, the electric vibrations perpendicular to the bars of the grating are alone transmitted, the vibrations parallel to the grating being absorbed or reflected. In a grating we have a structure which is not isotropic, for the electric conductivity parallel to the bars is very great, whereas the conductivity across the bars (owing to the interruptions due to spaces) is practically zero. We may, therefore, expect electric Rh