[Vol. LXIH, No. 1635
aperture of the eyepiece of the telescope, about a quarter of an inch in diameter; the observer makes sixteen readings of the position of the interference fringes in each turn, at times indicated by an electrical clicker; these operations must be continued without a break through a set of observations, which usually lasts for about fifteen or twenty minutes, and this is repeated continuously during the several hours of the working period.
When observations are in progress the interferometer to which the observing telescope is attached is caused to rotate on the mercury float so that the telescope points successively to all points of the compass, that is, it points to all azimuths. A relative motion of the earth and the ether should cause a periodic displacement of the interference fringes, the fringes moving first to one side and then to the other as referred to a fiducial point in the field of view, with two complete periods in each rotation of the instrument. Beginning when the telescope points north, the position of the fringes is noted at sixteen equidistant points around the horizon. The azimuth of the line of sight when the displacement is a maximum having been noted at two different times of day, it is a simple operation to calculate the right ascension and declination, or the “apex” of the presumed “absolute” motion of the earth in space. The determination of the direction of the earth’s motion is dependent only upon the direction in which the telescope points when the observed displacement of the fringes is a maximum; it is in no way dependent upon the amount of this displacement nor upon the adjustment of the fringes to any particular zero position. As the readings are taken at intervals of about three seconds, the position of the maximum is dependent upon observations covering an interval of less than ten seconds. A whole period of the displacement extends over only about twenty-five seconds. Thus the observations for the direction of the absolute motion are largely independent of ordinary temperature disturbances. The observation is a differential one and can be made with considerable certainty under all conditions. A set of readings usually consists of twenty turns of the interferometer made in about fifteen minutes’ time; this gives forty determinations of the periodic effect. The forty values are simply averaged to give one “observation.” Any temperature effect, or other disturbing cause, which is not regularly periodic in each twenty seconds over an interval of fifteen minutes would largely be cancelled out in the process of averaging. The periodic effect remaining in the final average must be real.
The position of the fringe system is noted in units of a tenth of a fringe width. The actual velocity of the earth’s motion is determined by the amplitude of the periodic displacement, which is proportional
to the square of the relative velocity of the earth and the ether and to the length of the light path in the interferometer. A relative motion of thirty kilometers per second, equal to the velocity of the earth in its orbit, would produce a displacement of the fringes from one extreme to the other, of 1.1 fringes. Disturbances due to temperature or other causes lasting for a few seconds or for a few minutes might affect the actual amount of the observed displacement and thus give less certain values for the velocity of relative motion, while at the same time the position of maximum displacement is not disturbed. Thus it is to be expected that the observations for the velocity of motion will not be as precise as the observations for the direction of motion. The two things, magnitude and azimuth of observed relative motion, are quite independent of each other.
It is desirable to have observations equally distributed over the twenty-four hours of the day; since one set requires about fifteen minutes of time, ninety-six sets, properly distributed, will suffice. The making of such a series usually occupies a period of ten days. The observations are finally reduced to one group and the mean date is considered the date of the epoch. The observations made at Mount ^Wilson in 1925 correspond to the three epochs, April 1, August 1 and September 15, and are more than twice as numerous as all • the other ether-drift observations made since 1881. The total number of observations made at Cleveland represent about 1,000 turns of the interferometer, while all the observations made at Mount Wilson previous to 1925 correspond to 1,200 turns. The 1925 observations consist of 4,400 turns of the interferometer, in which over 100,000 readings were made. A group of eight readings gives a value for the magnitude and direction of the ether-drift function, so that 12,500 single measures of the drift were obtained. This required that the observer should walk, in the dark, in a small circle, for a total distance of 100 miles, while making the readings. Throughout these observations the conditions were exceptionally good. At times there was a fog which rendered the temperature very uniform. Four precision thermometers were hung on the outside walls of the house; often the extreme variation of temperature was not more than one tenth of a degree, and usually it was less than four tenths of a degree. Such variations did not at all affect the periodic displacement of the fringes. It may be added that while the readings are being taken, neither the observer nor the recorder can form the .slightest opinion as to whether any periodicity is present, much less as to the amount or direction of any periodic effect.
The hundred thousand readings are added in groups of twenty, are averaged and then are plotted in curves. These curves are subjected to mechanical harmonic
April 30, 1926]
analysis for the purpose of determining the azimuth and magnitude of the drift. In this work all the original observations have been used, without any omissions and without the assignment of weights; furthermore, there are no corrections of any kind to be applied to the observed values. The results of the analyses are finally charted in such a way as to show the variation in the azimuth of the drift throughout the day of twenty-four hours for each epoch, and the variation in magnitude is similarly charted. The observations of 1925 thus provide six curves, three showing the variation in azimuth for the different epochs and three showing the variation in magnitude. The curves are shown in Figs. 1 and 2. The dots,
connected by the light lines, represent single observations, each being the average of the readings from twenty turns of the interferometer during an interval of about fifteen minutes. The heavy line represents an arbitrary average of the single observations for the one epoch. In Fig. 1 the base line represents the twenty-four hours of the local civil day; a position on this line corresponds to a direction of motion to the north, while a point above the line indicates an easterly azimuth and one below the line, a westerly azimuth. In Fig. 2 the base again represents the hours of the civil day, while the magnitude of the ether drift, that is, the velocity of relative motion, throughout the day, is charted in kilometers per sec
ond. It is at once evident that there is something real in the observations; each curve has a definite and a characteristic form; certainly, the results are not zero, neither are they due to accidental errors of observation. The azimuth of the observed effect, Fig.
1, varies in a periodic manner throughout the twenty-four hours of the day, the average being about 45° west of north, with the time of greatest westerly deviation varying with the time of year. Fig. 2 shows that the magnitude of the effect also varies periodically, with its maximum of about ten kilometers per second occurring at different times of day at different times of year.
It has been impossible to specify any effects of temperature, radiant heat, magnetism, gravitation or any other cause, which can produce the systematic variations indicated for the different epochs. The presumption was then made that the effects may be due to the motion of the earth and of the whole solar system through the ether, that is, to a real ether drift. Various graphic and numerical solutions were