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metre thick, and of such dimensions as to leave a clearance of about one centimetre around the float. A pin d, guided by arms g g g g, fits into a socket e attached to the float. The pin may be pushed into the socket or be withdrawn, by a lever pivoted at f. This pin keeps the float concentric with the trough, but does not bear any part of the weight of the stone. The annular iron trough rests on a bed of cement on a low brick pier built in the form of a hollow octagon.
At each corner of the stone were placed four mirrors dd ee, fig. 4. Near the centre of the stone was a plane parallel glass b. These were so disposed that light from an argand burner a, passing through a lens, fell on b so as to be in part reflected to d/; the two pencils followed the paths indicated in the figure, bdedbf and bd/e/d/bf respectively, and were observed by the telescope f. Both f and a revolved with the stone. The mirrors were of speculum metal carefully worked to optically plane surfaces five centimetres in diameter, and the glasses b and c were plane parallel of the same thickness, 1. 25 centimetre; their surfaces measured 5.0 by 7.5 centimetres. 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
of the Earth and the Luminiferous Æther. 455
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 millimetres. The difference in the lengths of the two paths was then annulled by moving the mirror e/. This mirror had three adjustments: it had an adjustment in altitude and one in azimuth, like all the other mirrors, 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