threads 0.635 millimeter apart, and a turn of the screw through 16° causes a change of 100 wavelengths in the light path. These screws are turned by means of capstan pins in order to secure sensitive adjustment. Usually the final adjustment of the central fringe to the fiducial point is secured by means of small weights placed on the end of the arm of the cross, causing a change of length by flexure.
Tests have been made at various times to determine the rigidity of the steel cross; these show that the four arms are about equally rigid and that a weight of 282 grams placed on the end of one arm produces an elongation in the multiple light-path sufficient to displace the fringe system one fringe-width, which is less than one hundred-millionth part of the light path. Similar tests made on the concrete base used for the interferometer in December, 1924, showed that 30 grams on the end of the arm produce a displacement of one fringe-width; the concrete base was therefore nearly ten times as sensitive to distortion as is the steel.
A change in temperature of the apparatus as a whole causes a slight change in the relative lengths of the arms. The white-light fringes having been adjusted to the center of the field of view, a change in temperature causes the fringes to be displaced out of view; however, the change is quite reversible and a return to the first temperature brings the fringes again into view. It has occurred repeatedly that at the close of a day’s work the fringes would be in the field of view and upon returning the next day, after the drop and rise in temperature of the night, the fringes would be in the field without any readjustment. The temperature influence on the apparatus is so consistent that a scale of temperature is provided for the capstan pin of the adjusting screws. A change of 10° in temperature requires a change of about 18° in the turning of the screw, corresponding to a displacement of 112 wave-lengths in the double light-path.
The sodium light is used in making the adjustments when the apparatus is first assembled at the beginning of a series of observations. After the white-light fringes have been found these are rarely lost and it is not necessary to resort to the monochromatic light again during the entire period of observations, unless the
apparatus is disassembled for some cause. The white-light fringes have been kept in adjustment during a period of two weeks or more. Upon the completion of observations at Mount Wilson in September, 1925, the mirrors and other optical parts were removed and packed for safekeeping. When observations were resumed in February, 1926, the mirrors were repolished and all parts were reassembled; the fringes in white-light were found in less than one minute without the use of the sodium light.
Since 1927, the interferometer has been mounted on the campus of Case School of Applied Science, about 330 feet from Euclid Avenue; the passage of street cars and the motor traffic of the city thoroughfare do not interfere with the making of observations. However, it is interesting to note that the sound of the imperfectly muffled exhaust of a motor-truck or a motorcycle, which may be a thousand feet or more distant, will cause the fringes to disappear completely without the slightest tremor. When observations were being made on the Fourth of July, 1904, the discharge of large fire crackers twelve hundred feet distant, produced the same effect. This is due not to mechanical vibration, but to the passage of the sound waves through the air in the light path of the interferometer. On several occasions in the observations made at Mount Wilson, there were minute but very distinct seismic disturbances which for a few seconds completely obliterated the fringes. After one such "earthquake" or micro-seismism, it was necessary to readjust the end mirror through a distance of twenty wave-lengths. A man chopping a stump of wood, several hundred feet away, disturbed the fringes, as also did workmen on a highway three miles distant; the passing of an airplane overhead caused the disappearance of the fringes.
Observations by Morley and Miller in 1904
The interferometer with the steel-girder base was first used by Morley and Miller in a continuance of the test of the Lorentz-FitzGerald contraction hypothesis. For this purpose the mirrors were so mounted that the distances between them could be made to depend upon the lengths of rods of pine wood. On two ends of the cross, S and T, Fig. 6, are two upright frames of
Fig. 10. The Morley-Miller ether-drift interferometer arranged for tests of the Lorentz-FitzGerald
hypothesis, 1904.
cast iron, fastened by bolts; each frame carries four mirrors. Against the corners of each of these frames rest four pine rods, about 2 centimeters in diameter and 425 centimeters long. Each rod is supported throughout its length by a brass tube,
2.5 centimeters in diameter, and each pair of tubes is joined together in a vertical truss, as shown in Fig. 10. Against the farther ends of the wood rods, rest the frames which hold the other sets of mirrors. Each of the latter frames is freely suspended by two thin steel ribbons and is held firmly against the pine rods and through these against one of the two fixed mirror-holders; contact is maintained by means of adjustable spiral springs. Thus the distances between the opposite systems of mirrors depend upon the pine rods only, while the whole optical system is adequately supported by the steel cross.
The first observations with this apparatus were made in July, 1904, and consisted of 260 turns of the interferometer arranged in two series. The procedure was based upon the effect to be expected from the combination of the diurnal and annual motions of the earth, together with the presumed motion of the solar system towards the constellation Hercules. On the dates chosen for the observations there were two times of the day when the resultant of these motions, about 33.5 kilometers per second, would lie in the plane of the interferometer, 11:30 o’clock, a.m., and 9:00 o’clock, p.m. The calculated azimuths of the motion would be different for
these two times but the velocities would be the same and the observations at these two times were, therefore, combined in such a way that the presumed azimuth for the morning observations coincided with that for the evening. The observations for the two times of day gave effects having positive magnitudes but having nearly opposite phases; when these were combined, the half sum was nearly zero. This small result was opposed to the theory then under consideration and it seemed impossible to reconcile the observations with the known orbital motion of the earth. The report of these experiments, published in the Philosophical Magazine,9 in May, 1905, concludes with this statement: “If pine is affected at all as has been suggested, it is affected to the same amount as is sandstone. Some have thought that this experiment only proves that the ether in a certain basement room is carried along with it. We desire, therefore, to place the apparatus on a hill to see if an effect can be there detected.” The two curves for the ether-drift obtained from the morning and the evening observations of July, 1904, are shown in Fig. 11, being superimposed, as explained above; the lower curve represents the mean displacement thus obtained, which is the result given in the published account of these experiments.
In accordance with the results set forth later in this report, this procedure of 1904 was incorrect,
9 E. W. Morley and D. C. Miller, Phil. Mag. [6] 9, 680 (1905); Proc. Am. Acad. Sci. 41, 321 (1905).
