1. An interferometer for use in infrared spectroscopy comprising, a beamsplitter having a partially reflective coating on an optical, infrared transmitting substrate for receiving an infrared input beam and dividing it into two beams, one a transmitted beam and the other a reflected beam, path defining optical means defining two separate optical paths, one for each of said two beams from said beamsplitter, such that said paths have a first intersection without any optical path length difference, a refractive window capable of the reception and transmission therethrough of infrared beams without the inducement of significant reflections, said refractive window being oriented and adapted for rotation about an axis substantially at said first intersection of the paths of said two beams to produce a difference between the optical path lengths of said two beams while obtaining four spectra per revolution, means for rotating said refractive window, and receiving and redirecting optical means for receiving each of said two beams from said refractive window and redirecting each of said two beams toward said refractive window for their retransmission back along said two optical paths defined by said path defining optical means to said beamsplitter to generate optical fringes.