Fresnel's Rhomb

Device for obtaining circularly polarized light. It consists of a rhomb of St. Gobain gla having acute angles of 54° 30′. A beam of plane polarized light incident perpendicularly, or nearly so, upon one of the faces, its plane making an angle of 45° with that of reflection, suffers two total reflections, and emerges parallel to its original direction, but circularly polarized. This is due to the fact that at each reflection one of the rays into which the incident light may be supposed to be resolved suffers a change of phase of of an undulation. The total retardation then is of a wave Colony 1877-80.