Fig. 13. Interferometer with base of concrete, 1921.
entirely consistent with the observations of April, 1921.
Many variations of incidental conditions were tried at this epoch. Observations were made with the centering pin tight in its socket and then loose; with rotation of the interferometer clockwise and counterclockwise; with a rapid rotation of one turn in 40 seconds and a slow rotation of one turn in 85 seconds; with a heavy weight added first to the telescope arm of the main frame and then to the lamp arm; with the float extremely out of level because loaded first in one quadrant and then in the next quadrant; with the recording assistant walking round in different quadrants and standing in different portions of the house, near to and far from the apparatus.
The results of the observations were not affected by any of these changes.
It was demonstrated that the use of the concrete base did not change the effect observed with the steel base either in magnitude or azimuth. The concrete base was less affected than the steel by change of dimensions due to changes of temperature; but this slight advantage was counterbalanced by the fact that it accommodated itself more slowly to a change of temperature. In spite of the fact that the concrete was considerably heavier than the steel parts which it displaced, it was much less rigid. Tests showed that a weight of 30 grams placed on the end of the arm of the interferometer would produce a displacement of the fringes of one fringe width, while nearly ten times as much weight is required to produce the same effect with the steel base. The concrete base was abandoned and the original steel base has been used in all subsequent observations.
Laboratory Tests of the Interferometer, Cleveland, 1922-1924
The entire apparatus was returned to the laboratory at Cleveland; during the years 1922 and 1923 many trials were made under various conditions which could be controlled and with many modifications in the details of the apparatus. An arrangement of mirrors and prisms was made so that the source of light could be placed outside of the observing room, Fig. 14, the
Fig. 14. The interferometer in the laboratory, 1923.
light entering the rotating interferometer along the axis of rotation. A further arrangement of mirrors, rather complicated in practice, was tried, for observing the fringes from a stationary telescope; the necessity for frequent adjustment of the fringes in the field of view made this method impracticable. Experiments were made with devices for the photographic registration of the positions of the fringes, both from the fixed observing station and by means of a motion picture camera carried on the interferometer. Even with an arc light as the source, there was not sufficient illumination to produce a satisfactory photographic record without slowing the rotation of the apparatus more than is consistent with our method of procedure, and the necessity for frequent adjustment of the fringes also made this method unsuitable. After abandoning the photographic method, an astronomical telescope having an objective of 13 centimeters aperture and a focal length of 190 centimeters was mounted on the interferometer. The object-glass is attached to the steel base near the halfsilvered diagonal glass and the eyepiece is supported on the end of the arm, there being no tube for the telescope. With a magnification of fifty diameters, the fringes are observable on a large scale and with ample illumination, so that direct reading with the eye was very satisfactory; this arrangement has been used in all subsequent observations.
Trials were made with various sources of light; with electric arc and incandescent lamps, the mercury arc, acetylene lamp and also with sunlight. The interchange between sunlight and laboratory sources in no way altered the results. The final choice for the stationary source placed outside of the interferometer room (or house, on the mountain) was a large acetylene lamp of the kind commonly used for automobile headlights. This arrangement was used in Cleveland in 1924 and at Mount Wilson in September, 1924, and in April, 1925. The use of a stationary light source with the light brought to the interferometer in the axis of rotation required very careful adjustment of the several mirrors involved in order to avoid a periodic displacement of fringes due to non-axial alignment. Careful trials were made which showed that it was better to place the source on the interferometer outside of the
cover and near the axis; thus the relation of source to the instrument remained constant. When this method was adopted, the small acetylene lamp, such as was used in the earlier experiments, was employed. This method of illumination has been used exclusively since April 9, 1925.
An extended series of experiments was made to determine the influence of inequality of temperature in the interferometer room and of radiant heat falling on the interferometer. Several electric heaters were used, of the type having a heated coil near the focus of a concave reflector. Inequalities in the temperature of the room caused a slow but steady drifting of the fringe system to one side but caused no periodic displacement. Even when two of the heaters, placed at a distance of three feet from the interferometer as it rotated, were adjusted to throw the heat directly on the uncovered steel frame, there was no periodic effect that was measurable. When the heaters were directed to the air in the light-path which had a covering of glass, a periodic effect could be obtained only when the glass was partly covered with opaque material in a very nonsymmetrical manner, as when one arm of the interferometer was completely protected by a covering of corrugated paper-board while the other arms were unprotected. These experiments proved that under the conditions of actual observation, the periodic displacements could not possibly be produced by temperature effects.
The Mount Wilson Experiments, 1924
Upon the conclusion of the experiments just described, in July, 1924, the interferometer was taken again to Mount Wilson. In 1921 the apparatus had been located on the very edge of a deep canyon; it was feared that the air currents up and down the face of the canyon might produce a disturbance and also that the unsymmetrical distribution of the rock of the mountain itself might be undesirable. In August, 1924, a new site was chosen on a very slightly rounded knoll, removed from the canyons. The interferometer house, Fig. 15, was erected with its orientation, as regards the ridge of the roof and the location of the door, changed by 90° from
