reading for such a position is recorded as plus. When the pointer appears to the left of the central fringe the reading is minus, corresponding to a shortening of the telescope arm. The adjustment commonly employed is such that from six to ten fringes appear in the field of view and so that the central black fringe is never more than two fringe-widths from the pointer. Fig. 7 shows
Fig. 7. The interference fringes as seen in the interferometer.
the field of view with adjustments for narrow fringes and for wide fringes, the latter corresponding to the conditions of actual observation.
Method of using the interferometer
The method of using the interferometer for the detection of an ether-drift presumes that the telescope arm of the instrument will be placed in the line of motion of the earth with respect to the ether as projected on the plane of the interferometer, while the other arm is at right angles to this motion. The interference fringes will indicate a certain reading with respect to the pointer in the field of view. The apparatus is then turned through an angle of azimuth of 90 so that the effect of the earths motion on the apparent velocity of light is transferred from the telescope arm to the other arm, with the result that the interference system will be displaced by an amount depending upon the square of the ratio of the velocity of the earths absolute motion to the velocity of light. However, the direction of the earths absolute motion is unknown and it is not possible to place the interferometer certainly in the desired positions. The interferometer is therefore caused to rotate slowly on the mercury float so that the telescope points successively to all azimuths. A relative motion of the earth and ether would then cause a
periodic displacement of the interference fringes, the fringes moving first to one side and then to the other as referred to the pointer in the field of view, with two complete periods in each rotation of the instrument.
Uniform temperature conditions are important as regards the dimensions of the apparatus and the refraction of the air in the light path. Usually the apparatus is kept in motion for an hour or more before readings are begun; sometimes a fan is used to secure uniform temperature distribution and the opening of the windows on all sides is common. However, when observations are made in daylight, the windows must be covered with curtains or dark paper. The apparatus is set in rotation by a pull of a few ounces on a fine string attached to the wooden float; a moderate pull on this string, even up to its breaking point, does not produce any appreciable distortion in the steel interferometer which rests on the float. The interferometer turns so easily and has such inertia that when once started it will continue to turn for an hour and a half or more without any further pull or push. It rotates with such freedom that it is literally floating without acceleration or distortion.
The purpose of the observation is to determine the amount of the displacement of the fringes and the direction in which the telescope points when this displacement is a maximum. The observer has to walk around a circle about twenty feet in diameter, keeping his eye at the moving eyepiece of the telescope attached to the interferometer which is turning on its axis steadily, at the rate of about one turn in fifty seconds; the observer must not touch the interferometer in any way and yet he must never lose sight of the interference fringes, which are seen only through the small aperture of the eyepiece of the telescope, about a quarter of an inch in diameter. The string attached to the float, mentioned above, may be used as a sensitive guide to assist the observer in maintaining the proper circular path. A delicate metal brush attached to the wood float touches in succession sixteen equidistant contact pieces on the mercury tank, closing an electric circuit which operates a small sounder and indicates the instants at which readings are to be made.
It is entirely practicable to make the readings
of the positions of the interference fringes corresponding to the sixteen equidistant azimuths of one complete turn of the interferometer when the instrument is turning at the rate of one turn in about fifty seconds. A set of readings which corresponds to a single observation, represented by one point on the charts of the original observations, usually consists of twenty turns, involving three hundred and twenty readings, made in about eighteen minutes. The time midway between the beginning and ending of the set of readings is taken as the time of the observation. The twenty turns are ordinarily observed in continuous succession; however, if a single reading at any one azimuth is lost, due to vibration of the support or other cause, the entire turn is cancelled. The adjustments are maintained so that the central fringe of the field of view, Fig. 7, is never more than two fringe-widths from the fiducial point. Often the temperature drift is such that the fringes shift more than this before a set of twenty turns is completed. When this occurs, the fringe system is restored to its central position simply by placing a small weight of two or three hundred grams on the end of the arm or by removing a weight from the arm. This is done without stopping the uniform turning of the apparatus and usually without interrupting the readings; if a reading is lost, the entire turn is cancelled and the observations are continued until twenty complete turns of readings have been secured. Only rarely is it necessary to readjust the fringes by means of the screws against which the end mirror rests. On some occasions the temperature conditions are so steady that no adjustment of fringes is required through several sets, which may cover an interval of an hour or more. Such sets of observations are repeated continuously during the several hours of the working period.
It is considered very important that the interferometer should not be enclosed in a metallic casing, nor even in an opaque covering; also that it should not be placed inside a room with heavy walls such as are required for a constant-temperature room. The apparatus should be, as nearly as possible, in the open so that there is no possibility of entrainment of the ether in massive materials surrounding it. The instrument is very sensitive to changes of
temperature and to vibration of the support and the quantity to be measured is extremely small. When the apparatus is used with the least possible covering, it is subjected to greater temperature disturbances than when fully protected, which results in greater dispersion among the separate readings; it is therefore necessary to accumulate a larger number of readings as rapidly as possible under these conditions and to arrange and combine them in such a manner that the systematic ether-drift effect will be fully preserved while the temperature changes which proceed more slowly will be eliminated in the final average. As the readings are taken at intervals of about three seconds, the position of the maximum displacement is dependent upon readings covering an interval of less than ten seconds. A complete period of the displacement takes place in about twenty-five seconds. Any temperature effect or other disturbing cause which is not strictly periodic in every twenty-five seconds over an interval of fifteen minutes would largely be cancelled out in the process of averaging, while a real periodic effect persists. Thus the observations for the direction of the absolute motion are largely independent of ordinary temperature variations. The observation is a differential one and can be made with considerable certainty under all conditions. 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 give less certain values for the velocity of the ether-drift while, at the same time, the position of maximum displacement is not altered.
Previous to 1925, the time of actual readings had been confined to one or two hours at a predetermined time of day; the time required for preparation and preliminary adjustments would perhaps be two hours more. The procedure adopted in 1925 makes it necessary to have observations equally distributed over the twenty-four hours of the day to determine the curve of diurnal variation. Allowing a few minutes for reading thermometers and for making readjustments of fringes and also a few minutes for relaxation, two sets of readings may be made in each hour through a working day, or night, of eight hours. The accumulation of a hundred sets
