454 Messrs. Michelson and Morley on the Relative Motion 



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 pin may be pushed into the socket or be withdrawn, by a 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/. 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. 



Fig. 3. 



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 re- 

 flected to d { ; the two pencils followed the paths indicated in the 

 figure, b d edbf and b d, ^^/respectively, and were observed 

 by the telescope/. Both / 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 e were plane parallel of the same thickness, 

 1-25 centimetre ; their surfaces measured 5*0 by 7*5 centi- 

 metres. 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 



