The dioptra, or alidade, consists of a brass ruler at both ends of which there is a hinged flange, each cut by a narrow slit and a wide slit. Both flanges may function as eyepiece or objective; when the eye is near a thin slit, the opposing wider fissure, with a vertical wire fixed in the middle, serves for sighting. Grid divisions are engraved on the ruler for scale reproduction of the field survey.

Device on display.