THE ASTROPHYSICAL JOURNAL

AN INTERNATIONAL REVIEW OF SPECTROSCOPY AND ASTRONOMICAL PHYSICS

volume lxviii DECEMBER 1928 number 5

CONFERENCE ON THE MICHELSON-MORLEY EXPERIMENT¹

HELD AT THE MOUNT WILSON OBSERVATORY PASADENA, CALIFORNIA

FEBRUARY 4 AND 5, 1927

The presence of Professor A. A. Michelson and Professor H. A. Lorentz in Pasadena in the early months of 1927 offered an exceptional opportunity for a conference on the theoretical and practical aspects of the Michelson-Morley experiment. Since Professor Michelson had planned, with the co-operation of the Mount Wilson Observatory, to repeat the experiment, such a conference was especially desirable. This was arranged largely on the initiative of Dr. Charles E. St. John. The experimental side was further presented by Dr. Roy S. Kennedy. This was supplemented by a mathematical treatment of the light-path by Professor E. R. Hedrick, as developed by himself and Professor L. Ingold, and by an account presented by Professor P. S. Epstein of the Trouton-Noble experiment, recently repeated at the California Institute of Technology by Chase, and of other recent experimental investigations. Illuminating discussion followed the presentation of the general reports. The shorthand notes were taken by Dr. Fritz Zwicky and Glenn H. Palmer, of the California Institute. These have been reviewed by the authors.

¹ Contributions from the Mount Wilson Observatory, Carnegie Institution of Washington, No. 373.

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A. A. MICHELSON

The addresses by Professors Michelson and Lorentz were followed by a detailed account of the results obtained by Professor D. C. Miller, who, fortunately, was also able to be present.

Reports

I. PROFESSOR A. A. MICHELSON (UNIVERSITY OF CHICAGO)

In 1880 I conceived for the first time the idea that it should be possible to measure optically the velocity w of the earth through the solar system. There had been earlier attempts to discover first-order effects, based on the idea of a system moving through a stationary ether. (First-order effects are proportional to w/c, where c = light-velocity.) Talking in terms of the beloved old ether (which is now abandoned, though I personally still cling a little to it), one might have expected that the aberration of light would be different for a telescope filled with air and with water, respectively. The experiments, however, showed, contrary to the then-established theory of light, that no such difference was present.

FresnePs theory was the first to account for this result. Fresnel assumed that matter was able to drag along the ether partially (entrainment of ether), giving it a velocity w^r, so that

w' — pw.

He was able to determine p (FresneFs coefficient) in terms of the refractive index

This coefficient is easily obtained from the negative result of the following experiment. Two light-beams travel along the path (Fig. 1; 0,1, 2, 3, 4, 5) in opposite directions and give rise to a set of interference fringes. 7 is a tube filled with water. Now if the whole system moves with the velocity w through the ether, a shift of fringes would be expected on moving the tube from position I to II.

l I 2.

Fig. i