In the far infrared spectroscopy, the echelette gratings are widely used not only as a dispersive medium but also as filters for short wavelength spectra. There are difficulties, however, that many kinds of gratings must be provided for the measurement of wide spectral region, because the optimum of the wavelength for a single grating is quite limited. These difficulties will be removed by making the depth of the grating variable. For the far infrared radiation, such mechanisms can be constructed and operated with sufficient accuracy
