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Art. LI.—The relative Motion of the Earth and the Ether; by Albert A. Michelson.
To account for the phenomenon of aberration Fresnel supposes the luminiferous ether at rest, the earth moving through this medium without communicating any perceptible part of its motion. On this theory it has been shown* that it should be possible to detect a difference of the velocity of light in two directions at right angles. As no such difference was observed, it would seem to follow that Fresnel’s hypothesis is incorrect.
Another theory is that of Stokes, in which the aberration is accounted for if the relative velocity of the earth and the ether have a potential. This requirement, however, is inconsistent with the results of the experiment just cited, which indicates that at the earth’s surface the relative motion is zero.
In the hope of detecting a relative motion corresponding to a difference of level, the following experiment was undertaken.
I take this opportunity of gratefully acknowledging the faithful and efficient services rendered in the execution of this work by Professor S. W. Stratton and Mr. C. R. Mann.
Light from the source s, a calcium light or an electric arc lamp, separated into two pencils at a plane-parallel glass plate,
o, lightly silvered. The two pencils were reflected by double mirrors along the paths oabcoe, and ocbaoe, respectively. The two paths being equal, interference fringes could be observed with the aid of the telescope at e. Fig. 2 shows details of the corner at c: pq are plane-parallel glass disks, cemented to the
*This Journal, November, 1887.
ends of the iron pipes; mn, plane glass plates silvered on front surface, and provided with adjustments in two planes; omnb, the path of the pencil of light. The apparatus was set up in the vertical east and west plane, the light traversing the entire circuit of the Ryerson Laboratory, a path about 200 feet long and 50 feet high.
It was found that under ordinary conditions the temperature disturbances in this length of air made it impossible to measure the position of the fringes; and the difficulty was only slightly remedied by enclosing the whole path of the light in a wooden box. By making this enclosure an iron pipe and exhausting the air to within a hundredth of an atmosphere, it was found possible to measure the position of the central bright fringe to within something like a twentieth of the fringe-width.
A difficulty is encountered in the selection of a fiducial mark. The double image of the source does not remain on the cross hairs of the observing telescope for any great length of time, notwithstanding the precaution of the double reflections at the corners, but by using this double image itself as the fiducial mark, any possible errors due to daily temperature changes, etc., are eliminated. This double image and the interference fringes are not in focus at the same time, but by sacrificing a very