The original Michelson-Morley experiment was made to determine whether or not motion relative to the classical ether could be detected. The results appeared to indicate that the ether was carried along with the apparatus. Repetitions of this experiment by Professor D. C. Miller seemed to bring out relative motion, from 5 to 10 kilometers per second, varying with the sidereal time. The object of the experiments of Michelson, Pease, and Pearson was to test Miller's results by a purely

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... method. Observations were made in the large grinding room of the optical shop of the Mount Wilson Observatory in Pasadena, at times corresponding to Miller's maximum and minimum effects, and also, as a check, at times midway between maximum and minimum. The first interferometer, 1926-1927, was of ordinary structural steel in the form of a cross, each arm having a rectangular box section, the light beams passing inside. The frame was mounted horizontally, at first on a ball-bearing pedestal, and then on a wooden float in a tank of mercury. The observer, riding upon the apparatus, made estimates to the tenth of a fringe. Large temperature drifts were found, besides periodic errors due to the central pivot. A similar frame of low-expansion steel was next built, but owing to the light cross-section of the metal was not rigid enough. To dampen the vibrations from the adjoining shop of the Observatory, attempts were made to mount the apparatus on pneumatic cushions, on ball bearings, and in other ways, but these were unsuccessful. The length of path in these instruments was 55 feet. They gave no evidence of an ether drift.