Prismatic alidade of the Schmalcalder type fitted for mounting a Colongue deflector on a French marine compass. Consists of a diameter which is cut away and has two hinged sights. The diameter has a crossbar at the expanded centre which attached to the compass spigot and accommodates the deflector.The back sight is fitted with a prism and two shades. The fore sight has a frame with sighting thread and carries a spirit level and reflector.