of the Earth and the Luminiferous JEther, 455 



mirrors, against which they were pressed by springs, till light 

 from both pencils could be seen in the telescope, the lengths 



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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 r This mirror had three adjust- 

 ments : it had an adjustment in altitude and one in azimuth, 

 like all the other mirrors, but finer ; it also had an adjust- 

 ment in the direction of the incident ray, sliding forward or 

 backward, but keeping very accurately parallel to its former 

 plane. The three adjustments of tins 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 



