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or the retardation to be looked for in the field employed is one part in one hundred thousand times one hundred million.
Professor Rowland has shown in his discussion of eqs. (2) that they involve an increase in the velocity of light instead of a decrease such as follows from eqs. (4) ; and in the further discussion of the matter he has assumed that he is at liberty to write the equation 0 = mz/v, whereas in his equations 6 = mt is the only value of 6 which is admissible, since z and t are independent. It would seem, therefore, that so much of his discussion as depends upon this assumed value of d may not be valid.
Minneapolis, October, 1898.
Experiments on the Velocity of Light in a Magnetic Field.1
By Edward W. Morley and Dayton C. Miller.
At the Toronto meeting of the American Association for the Advancement of Science in 1889, Professor Henry T. Eddy (see Part I), discussed the partial differential equations of the motion of plane polarized light in a magnetic field. These equations contain terms “ expressing the transverse electromotive force due to the Hall effect. The particular solution of these equations which Rowland proposes contains a periodic factor dependent upon the time/’ In Professor Eddy’s paper, “ a different particular solution, containing a periodic factor dependent upon the space which the ray traverses in the field, is discussed at length and compared with the solution proposed by Rowland.” It is shown that the velocity of the ray would be increased if there is a periodic factor dependent upon the time, but would be decreased by a factor dependent upon the space.
In the discussion which followed the reading of Professor Eddy’s paper, one of the present writers suggested a form of apparatus which would detect the suspected change of velocity, provided it amounted to one part in fifty millions. The whole matter was of
1 Report of an experiment made with the aid of a grant from the research fund of the American Association for the Advancement of Science.
such interest that the section of the Association before which the paper was read, obtained, from the research funds of the Association, a grant of money with which to construct the apparatus and make the experiment. The apparatus was finished in the summer of 1890, and Professor Eddy came to Cleveland to take part in the experiment.
The optical part of the apparatus consists of the interferential re fractometer used by Michelson in his experiments to determine
whether light moves with the same velocity in different directions in the solar system. A train of waves of light from the source s, (Fig. 1) is divided at the second surface of aby which is coated with silver of such thickness that the division is nearly equal. If the mirrors, / and gy are properly adjusted, the two parts reunite at ab} and interference bands can be seen by the eye at e, if the distances off and g from ab are properly related. If, now, any force acts intermittently to change the velocity of one part of the ray while the paths are separated it may be detected by a motion of the interference bands, provided the change is not too small.
If we put two tubes, h and z, in the paths of the divided ray, make these of very nearly the same length, close them with plane parallel glasses of equal thickness, and fill the tubes with carbon bi