I've been reading up on Fourier Transform Infrared Spectroscopy and the Michelson Interferometer. My main sources are "Principles of Instrumental Analysis" by Skoog etc and Fourier - Transform Infrared Spectrometry by Griffiths and Haseth. I believe I understand the theoretical principles behind the Michelson Interferometer. We may obtain molecular vibrational data by exposing molecules to infrared radiation. Experimentally, FT-IRs use some sort of interferometer to obtain this vibrational data. I get that the basic Michelson Interferometer is comprised of: a fixed mirror, a beam splitter, a movable mirror, a source and a detector. I understand that the variation in the intensity of the beams passing to the detector and returning to the source as a function of the path difference yields the desired spectral information. My question: When you analyze some organic compound, what part of the Michelson Interferometer does it become? When you hit your sample with the radiation, does it take place of the Fixed Mirror? I guess that's what I don't understand. I haven't studied chemistry of physics in awhile so any clarification would be highly appreciated.