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the acceleration thus suspected was not more than one part in one hundred millions. But the displacement was so slight that they could not be confident of its existence.
In the examination of the effect of a magnetic field on a ray passing through it, the appearance on making and breaking the circuit suggested the possibility of a change in the density of carbon bisulphide while in the field. There was produced a momentary displacement of the interference bands, which was unmistakable, being many times as great as the displacement which it was suspected continued as long as the current passed. It was easy to imagine that the force which quickly brought the molecules of the liquid into the new positions in which they produce rotation of the plane of polarization, might carry them beyond the position of equilibrium, to which they would quickly return. To examine this phenomenon, a thick brass box filled with carbon bisulphide was placedin each coil, and the two boxes were connected with each other and with a glass tube of small diameter. After all parts of the apparatus had come to a uniform temperature, the level of the liquid was brought to a convenient part of the glass tube, and a current of twenty-seven amperes was sent through the two coils, in such direction as to produce a magnetic field in both coils. Each time the circuit was completed, there was a momentary rise of the liquid by 0.06 mm., and an instantaneous fall to 0.03 mm. On breaking the circuit, there was a momentary fall to —0.03 mm., and an instantaneous rise to 0.00 mm. This displacement could be well measured three or four times by means of a reading micrometer with an amplification of about sixty diameters, the probable error of a reading being less than 0.005 mm. After three or four measurements, the heat developed in the coils made it necessary to wait many hours for equilibrium of temperature to be established again. There were 380 cubic centimeters of carbon bisulphide in the magnetic field ; the area of the capillary tube was 0.55 square millimeters; the lasting change of density or the change in the volume of the containing envelope was therefore one part in twenty-three million. The intensity of the field was 1,650 centimeter-gramme-second units.
This matter was afterward investigated with another brass box
made to fit loosely between the poles of a dynamo-electric machine, whose coils had been so connected that the box was placed between a north and south magnetic pole. The material of this box was free from magnetic admixture, and was supported so as not to be in contact with any part of the dynamo. In this envelope, we have not yet been able to detect any such change of density or volume as was suspected before. The cause of the momentary displacement of the interference bands which was so obvious will therefore need further examination.
At the meeting of the American Association for the Advancement of Science in Indianapolis, in 1890, Professors Morley and Eddy made a report of the result of their experiment The matter seemed of such interest to the section that it obtained a further grant with which to continue the experiment with more powerful apparatus. Owing to other occupation, the construction of this apparatus has been delayed till the present year. With the assistance of Professor Dayton C. Miller, now associated with Professors Eddy and Morley, a new apparatus has been set up with which the acceleration or retardation produced would be three times as great as with the former apparatus. Further provision has been made for securing much greater thermal and optical stability of the carbon bisulphide used in the apparatus. As a result of these two modifications, our power of detecting small changes of velocity of light in a magnetic field is probably five times as great as in the case of the former apparatus.
The second apparatus differs from the former in two respects. The optical parts remain as before; the only difference in their use depends on the fact that the coils are now twice as long as before, so that the mirrors f and g are farther from the mirror ab. The tubes i and h are therefore 30 cm. longer and the column of carbon bisulphide in the magnetic field is twice as long as at first. Further, these tubes are so connected that a current of bisulphide can be passed through them at pleasure ; and around them are concentric tubes through which passes a current of water, intended to prevent the heating of the bisulphide by the passage of the electric current through the coils. A tank, placed in the room with the apparatus, supplies water at constant temperature, for this purpose ; in this