338 Ifichelson and Morley — Relative Motion of the 



their surfaces measured 5*0 by 7'5 centimeters. The second of 

 these was placed in the path of one of the pencils to compen- 

 sate 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 hav- 

 ing been adjusted by screws in the castings which held the 



x~*r 



/ / > 



//// 





\\ X ^\\ V v /' ////// 





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 e,. This mirror had three adjustments; it had an adjust- 

 ment in altitude and one in azimuth, like all the other mirrors, 



