Many astronomical spectrographs are inefficient in the sense that light from the telescope image is both rejected at the entrance slit and wasted by the low quantum efficiency of the photographic plate. There are many problems for which the second of these losses may be reduced by the use of a photoelectric detector. It is shown that in such cases the addition of a Fabry–Perot interferometer to a conventional monochromator will allow the use of a much wider entrance slit for a given resolution. The theory of this Fabry–Perot monochromator is considered and leads to the conclusion that the interferometer is best used in an on-axis arrangement. Such a system has been developed and tested experimentally both in the laboratory and with a 120 cm telescope. Comparisons have been made between a prism monochromator used to study short regions of the spectrum of stars and nebulae, both alone and with an interferometer in an on-axis and an off-axis arrangement. The results show that the on-axis arrangement gave a gain of light of about thirteen, and was limited by the goodness of the surfaces of the optical flats used. The system described can be attached to any existing monochromator.