A CENTURY'S PBOGEESS IN PHYSICS 373 



ft I 

 1 -ft' 



and on returning find m at m'. Hence its path has a 

 length of roughly 2 I (1 + /? 2 ). The difference in path of 

 the two rays is 2 Z and consequently they should be a 

 little out of phase on meeting at d. By rotating the 

 apparatus clockwise through 90 the directions of the 

 two rays relative to the earth's motion are interchanged, 



FIG. 4. 



a 



m 



and the interference fringes would be expected to shift 

 an amount corresponding to a difference in path of 2 2 1. 

 This quantity is of course small, 2 is about one one- 

 hundred millionth, but so sensitive are the methods of 

 interferometry that Michelson felt confident that he 

 would be able to detect the earth's motion through the 

 ether. The apparatus consisted of a table which could 

 be rotated about a vertical axis in much the same way 

 as a spectrometer table, and provided with arms a meter 

 long to carry the mirrors b and c. With this length of 

 arm the interference fringes from sodium light should 

 shift by an amount corresponding to four hundredths of 

 a wave length when the table is rotated through a right 

 angle. When the experiment was first performed the 

 apparatus was placed on a stone pier in the Physical Insti- 



