An initial theory of the multiple interferometers for spectroscopy was given by Houston and with some embellishments, by K. W. Meissner. In order to obtain high contrast, multi-pass operation of a single interferometer is universally employed. The fundamental advantage of multi-pass operation is of course that, with precisely parallel plates, the successive passes are automatically in register. In designing a multi-pass system and selecting the most suitable plate reflectivity the prime parameter is usually the required contrast or extinction. D. S. Cannell et al and D. Beysens took advantage of the great stability of the confocal interferometer and employed a double-pass arrangement. The multichannel advantage and potential for achieving good mechanical stability with static interferometers is advantageous, particularly so with increasing use of image plates. In the combined use of a static and scanning interferometer the acceptance cone of the static device must be made adequately large.