Page:Collected Physical Papers.djvu/35

Rh There are two principal positions of the Polariser and the Analyser:—


 * (1) Parallel position.—When both the gratings are horizontal.


 * (2) Crossed position.—When the polarising and analysing gratings are at right angles to each other.

In the ﬁrst position, the radiation being transmitted through both the gratings, falls on the sensitive surface, and the galvanometer responds. The field is then said to be bright. In the second position the radiation is extinguished by the crossed gratings, the galvanometer remains unaffected, and the field is said to be dark. But on interposing certain crystals with their principal planes inclined at 45° to the horizon, the field is partially restored, and the galvanometer spot exhibits large deflection. This is the so-called depolarising action of double-refracting substances.

Experiment with Serpentine

I obtained a piece of fibrous Serpentine of greenish colour from the Geological Department of India; its thickneess was about 2 cm. I interposed it with its fibres inclined at 45°, between the crossed Polariser and Analyser. There was at once a restoration of the field.

The Polariser and the Analyser were now made parallel, and the piece of Serpentine placed with its fibres vertical or parallel to the vibration of the electric ray; the galvanometer now ceased to respond, proving the complete absorption of the ray by the Serpentine.

The piece of Serpentine was now held with the fibres horizontal, and the radiation was found to be copiously transmitted.