A far ir spectrometer has been built which uses a double pass optical system to increase the spectral dispersion. Because of the energy limitations in this spectral range, this means of increasing the dispersion is an efficient way to improve the resolving power of a grating monochromator. The same optical system is also used to advantage in a lamellar grating interferometer where the increased dispersion improves the performance at higher frequencies. Consideration is also given to instrument errors and signal noise which can affect, and in some cases limit, the present performance of a far ir Fourier interferometer.