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

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which we then performed the experiment. It was proposed to look for a certain effect in order to check a certain theory. We had definite pictures in our minds as to what should happen. We calculated the magnitude and azimuth of the effect from the theory and discussed our experimental results in relation to these specific expectations. In every case we found that the result was negative as to these expectations. But it was never numerically zero, not even in the original Michelson and Morley experiment. It was zero in so far as the motion of the earth in its orbit is concerned. The remaining effect, however, was large enough to be measured. Experiments were performed to prove that it was not due to magnetic deformation of the frame, nor to temperature disturbances, since the effect was systematic. It was suggested that the ether might be entrained differently inside and outside of a masonry building.

In the autumn of 1905, Morley and Miller removed the interferometer from the laboratory basement to a site on Euclid Heights, Cleveland, free from obstruction by buildings, and having an altitude of about three hundred feet above Lake Erie and about eight hundred and seventy feet above sea-level. The house was purposely of very light construction, and was transparent (glass windows) in the direction of expected drift. Five sets of observations were made in 1905-1906, which give a definite positive effect of about one-tenth of the then-expected drift. Professor Morley retired from active work in 1906, and it devolved upon the present speaker to continue the experiments. It seemed desirable that further observations should be carried out at a much higher altitude, but numerous causes prevented the resumption of observations.

The deflection of the light from the stars by the sun, as predicted by the theory of relativity, was put to test at the solar eclipse of 1919. The results were widely accepted as confirming the theory. This revived the writer’s interest in the ether-drift experiments, the interpretation of which had never been acceptable to him. Through the kindness of President Merriam and Directors Hale and Adams, a site was provided at the Mount Wilson Observatory on the top of Mount Wilson, at an elevation of about six thousand feet. The ether-drift interferometer was set up here in February, 1921, and observations were carried on during the succeeding five years.


Observations were begun in March, 1921, using the apparatus and methods employed by Morley and Miller in 1904, 1905, and 1906, with certain modifications and developments in details. The very first observation gave a positive effect such as would be produced by a real ether drift, corresponding to a relative motion of the earth and ether of about 10 km/sec. But-before announcing such a result it seemed necessary to study every possible cause which might produce a displacement of fringes similar to that caused by ether drift; among the causes suggested were magnétostriction and radiant heat. In order to test the latter, the metal parts of the interferometer were completely covered with cork about one inch thick, and fifty sets of observations were made showing a periodic displacement of the fringes, as in the first observations, thus showing that radiant heat is not the cause of the observed effect.

In the summer of 1921 the steel frame of the interferometer was dismounted and a base of one piece of concrete, reinforced with brass, was cast in place on the mercury float. All the metal parts were made of aluminum or brass; thus the entire apparatus was free from magnetic effects and the possible effects due to heat were much reduced. In December, 1921, forty-two sets of observations were made with the non-magnetic interferometer. These show a positive effect as of an ether drift, which is entirely consistent with the observations of April, 1921. Many variations of incidental conditions were tried at this epoch. Observations were made with rotations of the interferometer clockwise and counter-clockwise, with a rapid rotation and a very slow rotation, with the interferometer extremely out of level, due to the loading of the float on one side. Many variations of procedure in observing and recording were tried. The results of the observations were not affected by any of these changes.

The entire apparatus was returned to the laboratory in Cleveland. During the years 1922 and 1923 many trials were made under various conditions which could be controlled and with many modifications of the arrangements of parts in the apparatus. An arrangement of prisms and mirrors was made so that the source of light could be placed outside of the observing-room, and a further complication of mirrors was tried for observing the fringes from a stationary telescope. Methods of photographic registration by