[SOLVED] About observing the mewcury spectrum with a Fabry-Perot interferometer Hello! I've been trying to observe the interference fringe pattern from a mercury discharge tube, but the pattern has not already shown up. I simply don't understand what I'm doing wrong. The first thing I did was to align both mirrors with a laser. With that source I saw the fringes, but after replacing the source the pattern gets lost. I've tried putting convergent lenses between the interferometer and the source but it doesn't help at all.