What is fringe shift?

techysafi

I was reading this http://mysite.verizon.net/cephalobus...ndics_1965.pdf This paper shows some images and says no shift has been observed so they come to conclusion that light does not travel at the velocity of (c plus/minus v) rather its always c regardless the motion of the source. Now I don't really understand what the picture of would be if some fringe shift were present?

But to make it simple for you to explain I'm giving some clue what I have understood so far. Take a look Did I get it correctly?