beam of light is literally split in two by a halfsilvered mirror, and the two beams of light may be made to travel paths at right angles to each other. At the end of the desired path, each beam is reflected back upon itself and the two come together where they first separated. If the two right-angled paths are optically equal, the reunited beams of light will blend with the waves in concordance. If, however, the paths of the light in the interferometer differ either in actual length or in the optical properties of the medium through which the light passes, differences of phase will result which may be observed as interference fringes. Observation of these fringes enables one to detect exceedingly small changes in the relative velocities of the light in the two paths of the interferometer, the measurements being made in terms of the wave-length of the light.
Michelson at once applied his interferometer to detect the relative motion of the earth and the ether as proposed by Maxwell. Alexander Graham Bell provided for the construction of the new instrument, Fig. 1, which was made by
Fig. 1. Michelsons ether-drift interferometer of 1881.
Schmidt & Haensch of Berlin. The half-silvered mirror was placed over the central axis, and two arms at right angles, each 120 centimeters long, carried the end mirrors. The apparatus could be set with the telescope arm pointing in different azimuths and it should be possible to detect the effect of the orbital motion of the earth when the light travels in the direction of this motion and at right angles to it.
The first trials of the ether-drift experiment were made at the Physikalisches Institut of the
Nature 65, 343 (1902); E. W. Morley and D. C. Miller, Phil. Mag. [6] 9, 669 (1905); A. Righi, Comptes Rendus 168, 837 (1919); 170, 497, 1550 (1920); 171, 22 (1920). E. R. Hedrick, Astrophys. J. 68, 374 (1928).
University in Berlin; but the disturbances produced by street traffic made it impossible to see the fringes except in the middle of the night. The experiment was transferred to the Observatory in Potsdam, the interferometer being mounted in a hollow place in the lower part of the brick pier which supported the big telescope. The report of the experiment, published in 18813 (with a correction explained in the paper of 1887),4 states that, considering only the motion of the earth in its orbit, the displacement of the interference fringes to be expected would be 0.04 of the fringe width; the displacements actually observed varied from 0.004 to 0.015 of a fringe width and were considered to be merely errors of experiment. The conclusion was that the hypothesis of a stationary ether was not confirmed.
The Michelson-Morley Experiments, Cleveland, 1887
While he was still in Europe, in 1881, Michelson was appointed to the Professorship of Physics in the newly organized Case School of Applied Science in Cleveland and thus became acquainted with the late Professor Edward W. Morley, Professor of Chemistry in Western Reserve University, these two institutions being located side by side. Professor Morley proposed several important developments in the interferometer and in the method of using it, so that it became adequate to measure the then expected effect in the ether-drift experiment. Having secured an appropriation from the Bache Fund of the National Academy of Sciences, a new interferometer was constructed, embodying these improvements; the optical parts were made by the late John A. Brashear of Pittsburgh. In order to avoid disturbances of vibration and distortion, the optical parts were mounted on a solid block of sandstone, Fig. 2, which was floated on mercury contained in a circular tank of cast iron. This support by floatation made it possible to turn the interferometer to different azimuths while observations were in progress. The practicable limit for the size of the stone base was 150
3 A. A. Michelson, Am. J. Sci. [3] 22, 120 (1881).
4 A. A. Michelson and E. W. Morley, Am. J. Sci. [3] 34, 333 (1887); Phil. Mag. [5] 24, 449 (1887); J. de Physique [2] 7, 444 (1888).
Fig. 2. The Michelson-Morley interferometer of 1887.
centimeters square and 30 centimeters thick. In order to obtain the necessary sensitivity, the effective light path was increased by reflecting the light back and forth so that it traversed the diagonal of the square stone block eight times, giving the effect of an interferometer with an arm about 1100 centimeters in length. The expected displacement of the fringes due to a velocity equal to that of the earth in its orbit was 0.4 of a fringe width.
Michelson and Morley performed the historic experiment in the northwest room of the basement of the Main Building of Adelbert College in Cleveland in 1887; their entire series of observations was of six hours duration, one hour at noon on each day of July 8, 9 and 11, and one hour in the evening of July 8, 9 and 12 and consisted of thirty-six turns of the interferometer, readings being made at each of sixteen equidistant points in each turn. The method of observation was arranged to detect the preconceived effect of the motion of the earth toward a known point in space with a given velocity, and hence no general series of observations was made. The brief series of observations was sufficient to show clearly that the effect did not have the anticipated magnitude. However, and this fact must be emphasized, the indicated effect was not zero; the sensitivity of the apparatus was such that the conclusion, published4 in 1887, stated that the observed relative motion of the earth and ether did not exceed one-fourth of the earths orbital velocity. This is quite different from a null effect now so frequently imputed to this experiment by writers on Relativity. It also seems necessary to call
attention to another historical fact: Michelson and Morley made only the one series of observations, in July, 1887, and never repeated the ether-drift experiment at any other time, notwithstanding many printed statements to the contrary.
In the original account of their experiment, Michelson and Morley give the actual readings for the position of the interference fringes in the six sets of observations. The upper one of the two long curves in Fig. 3, shows the average
Fig. 3. Fringe displacements of the original Michelson-Morley experiments of 1887.
of the three sets of readings taken at noon, and the lower long curve is the average for the three sets taken in the evening. These curves show the fringe displacements for a full turn of the interferometer, while the ether-drift effect being sought is periodic in each half turn. To find the latter effect, the second half of the long curve is superimposed on the first half by addition, which
