The dual mirror goniograph is a nautical instrument for the simultaneous taking of two bearings between three objects or marks. The instrument was constructed in 1877 at the Hydrographic Institute, Pula at the Department for Nautical Instruments. It was used to enable navigators to determine the position of a ship when in sight of land, independently from the magnetic compass. The instrument was mostly used by the officers of the Austro – Hungarian Navy providing them with the means to effeiciently and accurately determine the position of a ship. Of all the means available at the time, it was the preffered instrument at the end of the 19th and the beginning of the 20th century. The dual reflection of a beam of light and a system of connected rulers and mirrors, create bearings when the two mirror images of the observed known objects become vertical together with directly observed object. It is then, that the optical straight lines which are running through the observed objects and the three edge rulers, intersect at the point inside the instrument revieling the position of the ship. Due to it’s shape (resembling a station- pointer), the instrument is placed on the navigation chart after the bearings are taken, so the edges of the ruler adjoin the observed objects. The point of the intersection of the edge rulers provides a ship's position. The instrument was used not only in navigation but also in land surveying, due to its practical and accurate measuring abilities. The description and the usage of the instrument, it’s optical principle and the rectification of the instrument’s inaccuracies are all included in this work.