A spectrograph in which dispersion of the incident light into a spectrum is achieved by the use of an echelle grating. The particular characteristic of an echelle grating is that the profile of the parallel grooves is step-like or zigzag, and their spacing is relatively wide. The light to be dispersed is directed on to the grating at right angles to the faces of the grooves. The direction of the light is thus at a large angle with the normal to the grating as a whole. This produces a set of many overlapping spectra with a high degree of resolution. A second, low-dispersion grating, or a prism, arranged perpendicular to the echelle is used to separate out the overlapping spectra.