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20 sides bulged out, and the other two bulged in. On interposing this piece between the crossed Polariser and Analyser, it exhibited double~refraction.

Effect due to compression

It appeared that stratified rocks, which, from the nature of their formation, were subjected to great pressure, would serve well for the purpose of this experiment. My anticipations were verified, as the following experiments will show:—

I took a cube of Sandstone, about 5 cm. on each side, and held it between the Polariser and the Analyser, with the plane of stratiﬁcation inclined at 45°. The galvanometer at once responded.

I then rotated the piece of rock, and adjusted it with its ''plane of stratification parallel to the vibration plane of the Polariser. There was now no depolarisation effect'', the galvanometer remained unaffected. On further rotation in the same direction, the depolarisation effect re-appeared, to disappear again when the plane of stratification was made parallel to the vibration plane of the Analyser.

Experiments with many other rocks, gave similar result.

(The Electrician, Dec. 1895.)