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the pages—I interposed it, with its edge at 45°, between the crossed polariser and analyser of an electro-polariscope. The extinguished field of radiation was immediately restored. I then arranged both the polariser and the analyser vertical and parallel, and interposed the book with its edge parallel to the direction of electric vibration. The radiation was found completely absorbed by the book, and there was not the slightest action on the receiver. On holding the book with its edge at right angles to the electric vibration, the electric ray was found copiously transmitted. An ordinary book would thus serve as a perfect polariser of the electric ray. The vibrations parallel to the pages are completely absorbed, and those at right angles transmitted in a perfectly polarised condition.

(Proc. Roy. Soc., Jan. 1897.)