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determining the difference of time required by two interfering rays to traverse circuits in opposite directions, would require a path one kilometre square in a horizontal plane to give a displacement equal to .7 of a band for latitude 45°. This would necessitate a very high degree of refinement indeed over any previous attempts at interference. The method of determining the velocity of light which I devised us far back as 1889, and tried a number of years ago,^[1] and which consisted of an eclipsing system made up of a rotating double mirror and a grating, thus combining the principles of the toothed wheel and rotating mirror methods of Fizeau and Foucault, would be delicate enough to show a variation of one part in ten thousand, providing synchronism could be maintained during a short interval of time. With a suitable system of mirrors and observing telescopes, a single observer would be able to bring into his field of view both beams of light after their passage through the eclipsing systems. If now one of the rotating mirrors were either gaining or losing on the other, the observer would see, alternately, eclipses of one ray and the other, if their times of transit were different. If their times of transit were the same, then the two fields would maintain a constant relative intensity, each going through its maximum and minimum simultaneously as the relative phases of the eclipsing systems varied. The latter would correspond to the condition of a moving, the former to that of a quiescent æther. Thus the experiment would be possible, even if perfect synchronism were not attainable, but only sufficiently so to make the frequency of the successive maxima in the field of view less than a few times a second, or slow enough for the eye to resolve the fluctuations of intensity. If, by means of mirrors, a common source of light were employed, the half-shade principle in the field of view could be used which would be very sensitive in showing any difference in intensity (even if rapidly fluctuating) between the two portions of the field due to any slight difference in the time of propagation of the two rays with and against the æther drift. If we take the conditions in the experiment referred to,^[2] namely, an aperture of 2.5 cm. and a distance of .02 cm. between the lines of the reflecting grating, with a radius of 1 m. and 250 revolutions per second, 10,000,000 eclipses per second could be obtained; and, if we carried this to the limit in speed and resolving power, four times as many

1. Vice-Pres. Address, Sect. B, Amer. Assoc. Adv. Sci., Pittsburg Meeting, 1902; Science, July 18, 1902.
2. This apparatus was the phototachometer, modified for the purpose, which Newcomb used in determining the velocity of light.