This instrument is designed to measure one of the errors that may occur when taking measurements with a sextant (the apparent dip or raising of the horizon line). It uses a prism system to line up two images of the horizon and so produce a correction that can be applied to the sextant readings.

The instrument was designed in Germany by Dr. Pullfrich and was made by Carl Zeiss, the leading German manufacturer of optical instruments. This particular instrument came into the possession of Commodore R. P. Galer while he was serving on HMS 'Glory' in 1919, following the seizure of a Russian battleship at Archangel.

The instrument consists of a clack-lacquered metal and chrome casing containing prisms and lenses. Through the telescope the observer sees the reflected horizon to the left and to the right, at an angle of 180 degrees. By turning the micrometer with a white (+) and red (-) scale the reflected horizons are aligned. The dip or rising is then read from the micrometer, whereby white is for rising and red for dip. The instrument is contained in a fitted leather carrying case.