WHAT THEORETICALLY COULD CAUSE A FRINGE SHIFT?
A priori, there could only be four possible causes for an interference fringe shift in Michelsonís experiments: 1) the theoretical absolute displacement of the mirrors from the stationary ether and the light ray; 2) a change in the transmission velocity of light waves; 3) a change in the wave length and/or in the frequency of receipt of light waves; or 4) a change in the distance/time interval of propagation for one light pencil relative to the other.
1. out-of-phase light waves
2. different relative out-of-phase
 In any event, a fringe shift caused by a different color of light (different wavelength) would be independent of (and irrelevant to) any fringe shift caused by the solar orbital motion of the Earth through space, so the lightís color was irrelevant to Michelsonís experiments for two reasons.
 This conclusion will become relevant when we compare the Doppler effect of light to the null results in Michelsonís experiments in Chapter 12A.