A shift of one fringe in the Michelson-Morley experiment corresponds to a change in the round-trip travel time along one arm of the interferometer by one period of vibration of light (about 2 ✕ 10−15 s) when the apparatus is rotated by 90°. t = 2 tacross = 2L c2−u2 = 2L c 1− u2/c2 , what velocity through the ether would be deduced from a shift of one fringe? (Take the length of the interferometer arm to be 12 m.)