their surfaces measured 5*0 by 7*5 centimeters. The second of these was placed in the path of one of the pencils to compensate for the passage of the other through the same thickness of glass. The whole of the optical portion of the apparatus was kept covered with a wooden cover to prevent air currents and rapid changes of temperature.
The adjustment was effected as follows: The mirrors having been adjusted by screws in the castings which held the
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mirrors, against which they were pressed by springs, till light from both pencils could be seen in the telescope, the lengths of the two paths were measured by a light wooden rod reaching diagonally from mirror to mirror, the distance being read from a small steel scale to tenths of millimeters. The difference in the lengths of the two paths was then annulled by moving the mirror 6,. This mirror had three adjustments; it had an adjustment in altitude and one in azimuth, like all the other mirrors,
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but finer; it also had an adjustment in the direction of the incident ray, sliding forward or backward, but keeping very accurately parallel to its former plane.- The three adjustments of this mirror could be made with the wooden cover in position.
The paths being now approximately equal, the two images of the source of light or of some well-defined object placed in front of the condensing lens, were made to coincide, the telescope was now adjusted for distinct vision of the expected interference bands, and sodium light was substituted for white light, when the interference bands appeared. These were now made as clear as possible by adjusting the mirror ef’ then white light was restored, the screw altering the length of path was very slowly moved (one turn of a screw of one hundred threads to the 5 inch altering the path
nearly 1000 wave-lengths) till the colored interfer-reappeared light ‘These were now given a convenient width and position, and the apparatus was ready for observation.
The observations were conducted as follows : Around the cast-iron trough were sixteen equidistant marks. The apparatus was revolved very slowly (one turn in six minutes) and after a few minutes the cross wire of the micrometer was set on the clearest of the interference fringes at the instant of passing one of the marks. The motion was so slow that this could be done readily and accurately. The reading of the screw-head on the micrometer was noted, and a very slight and gradual impulse was given to keep up the motion of the stone; on passing the second mark, the same process was repeated, and this was continued till the apparatus had completed six revolutions. It was found that by keeping the apparatus in slow uniform motion, the results were much more uniform and consistent than when the stone was brought to rest for every observation ; for the effects of strains could be noted for at least half a minute after the stone came to rest, and during this time efiects of change of temperature came into action.
The following tables give the means of the six readings ; the first, for observations made near noon, the second, those near six o’clock in the evening. The readings are divisions of the screw-heads. The width of the fringes varied from 40 to 60 divisions, the mean value being near 50, so that one division
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