I was setting up a Michelson Interferometer the other day & was trying to adjust it so the movable & fixed paths are indentical in length. I could not really tell when they were identical so I asked a colleague. He told me that as the path lengths become equal, the spacing between the rings in the interference pattern get larger. I don't believe this can be true. The rings come from the spherical mirror that changes the (approx.) planewave beam into a varying phase wavefront so you get a bullseye pattern instead of a single light or dark spot. The spacing is determined by the curvature of the lens. Does anyone agree with this? Also, is there an easy way to tell when the two path lengths are equal? I found it's not reliable to judge by the "darkest dark" or "brightest bright" in the bullseye.