where, when the adjustments are completed, the interference fringes appear to be located.
The apparatus as described, consisting of the optical plane surfaces, the steel cross and the mercury tank and float, has been used by the writer in all experiments from 1904 to the present time, except that for the experiments of December, 1921, the steel cross was replaced with a base of concrete. In 1923, the small reading telescope was replaced by an astronomical telescope of 13 centimeters aperture, having a magnifying power of fifty diameters. The whole path of the light in the apparatus is enclosed; this cover was made of pine wood throughout for the experiments of 1904; in 1905 the cover had glass sides for all arms, thus making the apparatus wholly transparent in the horizontal plane; this arrangement, shown in Figs. 13 and 16, has been used to the present time.
Adjustment of the interferometer
When the mirrors are in position, the distances between them, about 425 centimeters, are compared by means of light wooden rods and the mirrors are adjusted so that the two light-paths, each consisting of eight different portions, are approximately equal. Sodium light from the common laboratory type of sodium lamp is used to establish interference; by observing the visibility maxima of the sodium interference system, the adjustment is made for the center of this system where the white-light fringes may be found. When the apparatus was first assembled on Mount Wilson, the time required for the approximate adjustment of the distances between mirrors with the wood rods was about one hour, for the centering of the mirrors fifteen minutes, for finding the fringes with sodium light thirty minutes, and for finding the fringes with white light forty-five minutes, or two hours and a half for the entire operation. Upon another occasion, the fringes for sodium light were found with ten minutes of searching and the white-light fringes in thirty-five minutes more. The mercury arc and other monochromatic sources have been tried for the preliminary adjustments but the sodium light is preferred because the middle portion of the interference system can be easily located, which corresponds to equal light-paths in the two arms of the
interferometer. White-light fringes were chosen for the observations because they consist of a small group of fringes having a central, sharply defined black fringe which forms a permanent zero reference mark for all readings. Previous to 1924, a small acetylene lamp of the kind used on bicycles, was the source of light, the lamp being carried on a bracket attached to the end of one arm of the interferometer, as shown in Figs. 10 and 13. Such a lamp produces a concentrated, brilliant and very steady light with the minimum production of heat and the lamp itself is very simple and of small weight and it burns for several hours with little attention. For the observations of 1924 and for part of the observations of April, 1925, the source was placed outside of the interferometer room, as is explained later, and a larger lamp of the kind used for automobile headlights, shown in Fig. 14, was used. In April, 1925, the small acetylene lamp was again adopted, now being placed on the top of the cover of the interferometer, over the central axis, as shown in Fig. 16, the light being introduced into the light-path by two mirrors on the end of one arm. This arrangement has been continued to the present. Monochromatic fringes have never been used in the ether-drift observations, though experimental trials have been made, as is described later.
The interference fringes appear to be formed on the surface of the most distant mirror, optically speaking, No. 8, of the series as described. Attached to the supporting frame of this mirror is a small arrow-head of brass, which projects into the field of view, almost in contact with the mirror, forming a fixed fiducial mark for determining the position of the fringe system. Before beginning observations the end mirror, No. 8, on the telescope arm is very carefully adjusted to secure vertical fringes of suitable width. There are two adjustments of the angle of this mirror which will give fringes of the same width but which produce opposite displacements of the fringes for the same change in one of the light-paths. Very careful attention is required always to secure that adjustment of this critical angle which causes the arrow-head pointer to appear to the right of the central black fringe when the light-path of the telescope arm of the interferometer increases in effective length; a
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
