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78 in the article referred to for measuring the velocity of light can be readily adapted to the problem before us. The first depends on the Faraday "effect," the second on the Kerr electrostatic "effect". The second method, in conjunction with a half-shade elliptical polarizer which I devised several years ago, has given preliminary results indicating a superiority over those described above, quite beyond my expectations, in the arrangement as originally planned for determining the velocity of light a number of years ago.



In fig. 1, k, k'  are two condensers containing, say, the dielectric, carbon disulphide or nitrobenzol, giving the Kerr "effect," and placed with their azimuths so as to give a "crossed" system. p, c, a and p', c', a'  are the polarizer, elliptical half-shade compensator, and analyser of the two respective optical systems placed in juxtaposition. g is the spark-gap and in a half-silvered mirror system for sending identical beams in opposite directions through the optical systems p' k k' c' a'  and p k' k c a.

A and A' represent the azimuths of the various elements as seen from the one side or the other. Thus at $$A, p_{1}$$ and $$a_{1}$$ are the azimuths of the polarizer and analyser, k and k'  the traces of the