L. Page — A Century's Progress in Physics. 339 



a I (1 + \ p) . 



The other ray will reach the mirror c after the latter has 

 moved forward a distance 



i -fi > 



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

 length of roughly 2 I (1 + j3 2 ). The difference in path of 



the two rays is (3 2 1 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, 

 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, — p 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 



