A method of measurement is described in which a Babinet compensator and a spectrometer are used. The compensator is not orientated at 45° to the vibration direction of the polarizer, as is usual, but at a smaller angle. The contrast of the fringe system is maintained by correct setting of the analyser. The theory of the method is given and mention is made of the accuracy. This is greatest for phase differences less than 45°. Calibration curves are drawn and photographs given to illustrate the method as applied to the measurement of the phase difference introduced by a mica sheet.