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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
tank is immersed another tank, which supplies a current of bisulphide having the same temperature as the water. This arrangement for preventing changes of temperature in the column of carbon bisulphide in the tubes of the apparatus greatly increases the steadiness of the interference bands to be scrutinized, and so increases the delicacy of the observations.
The illustration taken from a photograph shows the apparatus ready for use. At the extreme right are seen the commutator, amperemeter, and resistance coils, used in managing the electric current. The wooden stand at the extreme left carries the source of light and the condensing lens. The adjacent stone pier carries the coils; between them is seen the cubical block of stone which supports the diagonally placed mirrors ab and cd of Fig. 1. Apparently just above this block, but really some yards to the rear, is a double tank supplying water and carbon bisulphide to the apparatus. The reading telescope with which the observations were made is marked by the hanging cloth. On the left edge of the pier is seen an iron stand carrying a Nicol’s prism for polarizing the ray of light sent through the apparatus ; on the wooden stand to the right of the pier is seen the analyzer, by means of which the rotation produced was measured while the current was adjusted so as to secure the rotation desired.
When a current of twenty-seven amperes is sent through the coils of this apparatus, there is produced a rotation of the plane of polarization of light by one-half a circumference ; in the previous apparatus, we could utilize a rotation no larger than about fifty degrees of arc. The rotation utilized now is therefore three and one-half times as great as that in the former case, involving a corresponding increase in the change of velocity which it is hoped to detect.
With this apparatus, a current was sent through the coils at the call of the observer at the telescope. The current had been adjusted so that it should produce a rotation by half a circumference of the plane of polarization of the light employed at the time. This rotation caused the interference bands, which had been seen before the passage of the current, to shift their position by half a wavelength ; this was due to the reversal of phase of the ray which had