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The first named difficulties were entirely overcome by mounting the apparatus on a massive stone floating on mercury ; and the second by increasing, by repeated reflection, the path of the light to about ten times its former value.
The apparatus is represented in perspective in fig. 3, in plan in fig. 4, and in vertical section in fig. 5. The stone a (fig. 5)is about 1*5 meter square and 0*3 meter thick. It rests on an annular wooden float bb, 1*5 meter outside diameter, 0*7 meter inside diameter, and 0*25 meter thick. The float rests on mercury contained in the cast-iron trough cc, 1*5 centimeter thick, and of such dimensions as to leave a clearance of about one centimeter around the float. A pin d, guided by arms gggg, fits into a socket e attached to the float. Th$ pin may be pushed into the socket or be withdrawn, by a lever pivoted at / This pin keeps the float concentric with the trough, but does not bear any part of the weight of the stone.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 center 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 partr reflected to d,\ the two pencils followed the paths indicated in the figure, bdedbf and bdfitdpf respectively, and were observed by the telescope /. Both / and a revolved witfi the stone. The mirrors were of speculum metal carefully worked to optically plane surfaces five centimeters in diameter, and the glasses b and c were plane-parallel and of the same thickness, 1*25 centimeter;
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,