Abstract: Refractometer for measuring the refractive index of a medium (2), using the limiting angle method. A source (4) illuminates a source slit (5) which has as its image with respect to an optical system (S) one of the parallel and regularly spaced slits (5') of a grille (6) fixed perpendicular to the optical axis each of the slits (5') corresponding to an angle of incidence of the optical beam on a prism (3). Turning of a rhombohedron causes the image (5') of the source slit (5) on the grille (6) to be successively on each of the slits (5') of the grille (6). The pupil (L.sub.1), located in the plane of the source, is conjugate with the exit face (8) of the prism (3) and remains stationary when the image (5') of the source slit (5) is moved over the grille (6).