Sorry but I mixed it up it was just the other way round If I understood it well the sodium light was needed for course adjustment as it has a larger coherence length In any case white light was used for the experiment As explained by Jerry on sci physoics relativity The arms were Pretty nearly equal though Although Michelson and Morley used sodium light for collimating the apparatus the actual experiment was performed using white light from an argand burner The colored fringes were much easier to visually monitor on the other hand the limited coherence length of white light meant that the path lengths needed to match within microns Note in Fig 4 the piece of glass c used to compensate for the difference in light paths Thus I will now reinsert white