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sulphide, we shall still obtain interference bands. If we surround each tube with a coil of copper wire, use polarized light, and close the circuit through one coil, there will be a rotation of the plane of polarization of the ray which passes through the magnetic field. If the rotation be not so much as to prevent, or too much affect, the interference of the parts of the divided train of waves, a change of velocity of light while in the magnetic field will produce a displacement of the interference bands. If we complete the circuit in the two coils alternately, we double the displacement which is sought to detect.
If any displacement is observed, it will be necessary to determine whether it may not be due to strain or displacement of parts of the apparatus. In the case of the plates ab, cd, f and g, this may be accomplished by watching the effect of closing alternately the circuits through the two coils while tubes i and h contain only air. But a disturbance which should affect the position or length of one of these tubes needs special means for its detection. The plates which close the ends of these tubes are accordingly placed as shown at k, /, m and 11, and are silvered except where they cover the tubes. When proper adjustments are made, the observer can see interference bands through the tube by placing the eye at c, interference bands at the covers of the nearer end of the tubes by placing the eye at c\ and interferences at the covers of the farther ends of the tubes by placing the eye at cn. Since the interferences produced in these three cases are due to rays which have passed through only glass and air, they can be so adjusted as to detect with ease and certainty a displacement which would be entirely insensible in the irregular and most unstable interference bands produced after a ray of light has passed through a liquid so optically unstable as carbon bisulphide. Our means of detecting the effect of mechanical disturbances were therefore greatly superior to those of detecting the change of velocity in question, and the statement that no displacement of interference bands was caused by mechanical disturbance due to the opening and closing of the current through the coils is open to no doubt.
The plates ab and cd were 9.3 by 2.0 cm.; / and g were 2.0 cm. by 2.0 cm.; k, /, m and n were 4.3 cm. by 2.0 cm. The tubes Ji
and / were 1.7 cm. in inside diameter, and were 38.1 cm. long. They lay in adjustable supports, and were not in contact with the coils which surrounded them. The core around which the wire of the coils was wound was rectangular in section. Around a free space of 4.2 cm. by 2.2 cm. was first a nonconducting layer of 0.6 cm. in thickness, then a space 0.6 cm. thick for the circulation of water; and, lastly, a coil of wire. Each coil was made of two wires laid side by side, so that when the same current was sent through the two wires in opposite directions, its effect was null. A commutator was so arranged as to send a constant current through one wire of each coil, while the same current was so sent through the other wires of the two coils as to neutralize the effect of the current in the first wire of one coil, and to double it in the other coil. The heating due to the passage of the current was therefore identical in the two coils. The wire of each coil weighed about 82 kilogrammes, and the length covered by the wire was 30 cm.
The flame of a Bunsen lamp was colored by the introduction of some sodium compound as the usual source of light, because the interference bands can, in homogeneous light, more easily be given forms which permit delicate discrimination ; a luminous gas flame and the electric arc were also employed. A Nicol prism was sometimes interposed, and the plane of polarization was placed in various positions. The optical parts of the apparatus were so related that the visible interference bands coincided with the surface of the more distant mirrors ; their displacement can then be detected by referring their position to lines ruled on the mirrors.
With this apparatus, one observer adjusted the interference bands in width and position ; when these were made suitable, the second observer used the commutator to produce a magnetic* field, first in one coil and then in the other. He announced the change from one to the other in such a way that the first observer did not know which coil was acting, so that the effect of any possible bias might be excluded.
With this apparatus, both observers suspected a slight displacement of the interference bands, and both satisfied themselves that this displacement was in the direction which indicated acceleration in the light passing through the magnetic field. The amount of