"::: 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. 
 http://www.aip.org/history/gap/PDF/michelson.pdf  

 Thus I will now reinsert ""white"". 
 

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