to detect the FitzGerald-Lorentz Erred. 



681 



At the ends, after tilling the spaces between the planks, 

 were bolted iron supports for our mirrors. The whole was 

 placed on a round tioat, which in turn rested in a basin or! 

 mercury. 



Our sixteen mirrors were each 4 inches in diameter. The 

 mirrors rested each on the points of three adjusting screw.-, 

 against which they were held by springs. On the bedplate, 

 at the intersection of the arms of the cross, were placed a 

 plane half-silvered mirror and a compensating plate ; these 

 had been, as is usual, cut from the same plane-parallel disk, 



Fig. 1 is a diagram, not to scale, of the optical arrangements. 



Fk. 1. 



Fiff. 2. 



Light from a source S reaches the mirror D. Part is trans- 

 mitted, reaching the mirror II. It is successively reflected 

 to 2. '.). 4. 5, 6 3 7. iiii' I 8. From 8 it returns by the same path 

 t<> I), where part is reflected to the observer at T. Another 

 part of the incident ray i- reflected along the other arm of 

 the cross, i- similarly passed to and fro, returned, and at last 

 transmitted to the observer. In the apparatus actually used. 

 mirror 5 lay above 3, rather than to one side of it ; fig. 2 shows 

 this arrangement. The whole path of the light along these 

 mirror- was enclosed and covered, to lessen the effect of air- 

 currents and other disturbances. An acetylene flame was 

 carried as a source of light. A telescope magnifying thirty-five 

 diami _ ve distinct vision of mirror 8, at whose surface 



the interference-fring<- are apparently localized. 



The mirror- being silvered and polished were put in place, 

 and the lengths of the two path- were measured with a split 

 rod and then made nearly equal. Establishing interferences 



