Earth mid the Luminiferous Ether. 



337 



The first named difficulties were entirely overcome by mount- 

 ing 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, # 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 centi- 

 meter around the float. A pin d, guided by arms gggg, 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 the socket or be withdrawn, by a lever pivoted at f. This pin 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 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 part reflected 

 to d,\ the two pencils followed the paths indicated in the figure, 

 bdedbfa,nd bd,e t dpf respectively, and were observed by the tele- 

 scope /. Both / and a revolved with 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 ; 



