Conference on the Michelson-Morley experiment held at the Mount Wilson observatory Pasadena, California February 4 and 5, 1927

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that a should not be much less than 0.025 X, which was the value finally used. Substituting these values in the last equation, we get

ôx = 5Xio“s\

as the least detectable change in position of one of the mirrors. This corresponds to a change of optical length of path

81= 20x= io-4 X .

To take full advantage of the possibilities of the arrangement would have required perfect mirrors and an intenser and, therefore, hotter source of light than would have been desirable near the sensitive apparatus, as well as lengthening the interval between observations, thus allowing greater chance for any steady temperature shift to show itself. No attempt was made in the experiment, therefore, to go below values of hi equal to 2 X io“3 X; such variations were detectable without the least uncertainty.

With this apparatus the velocity of 10 km/sec. found by Professor Miller would produce a shift corresponding to 8X10-3 wavelengths of green light, which is four times the least detectable value.

The experiment was performed in a constant-temperature room in the Norman Bridge Laboratory at various times of day, but oftenest at the time when Miller’s conclusions require the greatest effect. The sensitiveness of the eye was tested for each trial by the placing or removal of a small weight on the slab before and after rotating it. There being no fluctuations in the field of view, it was unnecessary to take the average of a number of readings. As has been shown, a shift as small as one-fourth that corresponding to Miller’s would have been perceived. The result was perfectly definite. There was no sign of a shift depending on the orientation.

Because an ether drift might conceivably depend on altitude, the experiment was repeated on Mount Wilson, in the 100-inch telescope building. Here again the effect was null.

[Note added April, 1928.—Illingworth at the California Institute of Technology has continued the work with Kennedy’s apparatus, using improved optical surfaces and a method of averaging. He concludes1 that no ether drift as great as 1 km/sec. exists.]

1 Physical Review, 30, 692, 1927.



[Because of lack of time Professor Hedrick presented only a summary of the following contribution, prepared by himself and Professor Ingold of the University of Missouri.]


The celebrated experiment by Michelson to determine the relative motion of the earth and the luminiferous ether was first made in 1881.1 Objection to the mathematical theory was raised by H. A. Lorentz in 1886,2 and in 1887 the theory was modified by Michelson and Morley to meet this objection.3 It is the theory accompanying the account of their 1887 experiment that is usually given and that is now generally accepted.

Until about 1898 it does not appear that any further serious objections were raised against the theory. From that time on, however, numerous papers4 dealing with the matter have appeared, which, in many instances, contain objections to one feature or another of the theory. The differences of opinion appear to arise mainly from different conceptions regarding the mechanism of interference phenomena.

In view of the wide diversity of opinion on the subject, it has seemed worth while to work out the theory anew, on the basis of some reasonable hypothesis that has been employed in dealing with other phases of interference phenomena.

Some portions of the present investigation appear to be closely related to part of the work of Righi as reported by Stein,5 and

1 American Journal of Science, 22, 120, 1881.

2 Archives Néerlandaises, 31, 2me livre, 1886.

s Philosophical Magazine (5), 24, 449, 1887.

4 We mention the following: Sutherland, ibid. (5), 46, 23, 1898; Hicks, ibid. (6), 3, 9, 1902; Sutherland, Nature, 63, 205, 1900; Luroth, Ber. d. Bayr. Ak. d. W., 7, 1909; Kohl, Annalen der Physick, 28, 259, 1909; Budde, Physikalische Zeitschrift, 12, 979, 1911, and 13, 825, 1912; Righi, Sessions of the Royal Institute of Bologna, 1919 and 1920.

For replies to some of these articles consult the following: Lodge, Philosophical Magazine (5), 46, 1898; Morley and Miller, ibid. (6), 9, 669, 1905; Laue, Annalen d. Physik, 33, 186, 1910, and Physikalische Zeitschrift, 13, 501, 1912; Debye, Beiblätter zu den Annalen der Physik, 34, 1910.

s “Michelson’s Experiment and Its Interpretation according to Righi,” Memorie della Societa Astronomica Italiana, 1, 283, 1920.