SOME time ago I was told (I think by Prof. R. W. Wood) that the late Prof. Morley had expressed himself as anxious about the immense superstructure that had been built on the null result obtained by Prof. Michelson and himself during a comparatively few very careful experiments. He was anxious that the experiment should be repeated under different conditions and the result confirmed. We now know that such repetition has been undertaken, at first in connexion with Prof. Morley, by Prof. D. C. Miller of Cleveland, for whose zeal, enthusiasm, and enterprise we must feel high admiration. Not six or a dozen repetitions, but thousands of them, have now been made, on the tops of mountains, on plains, with frames of different materials, and with a sufficient length of light path to give a result of 1 part in 1000 million. The undoubted result of these repetitions has been to confirm the null result of Michelson and Morley, so far as regards the orbital motion of the earth, and thereby establish one of the foundations on which the theory of relativity was at first based, up to what at that date had seemed to be the probable errors of experiment.