An experimental azimuth circle designed by Capt.Chetwynd in 1906. Brass circle with prisms mounted on fore side. Consists of a prism rotating about its axis and a second one also rotating on its axis, fitted on a hinged bracket above the first prism. Both prisms fitted with two shades. Illumination for compass card by two small electric lamps. Three roller bearings are fitted on which circle can rotate.