This surveying compass, also known as a dioptric prismatic compass, was added to the collection of scientific instruments and teaching aids in 2020 together with around 320 surveying instruments from the Institute of Geodesy and Photogrammetry.
Dioptric prismatic compass
The dioptric prismatic compass is a special version of a compass that does not require a visible compass needle. Instead, the entire dial, referred to as a “dry dial”, aligns itself to the north magnetically by means of a gimbal. A locking device with which the dry dial can be fixed or released is attached at the side.
Users take measurements by holding the prismatic compass in their hand. The sighting device with sighting wire is unfolded for use. The compass also contains a hinged eyepiece prism, a “cylinder magnifier”, which is used for reading the graduated circle printed on the dial. This is divided into 360° and printed back-to-front on the dry dial, but can be read correctly through the prism. 0° is in the south, 90° in the west, 180° in the north and 270° in the east. If you hold the compass with the free-floating dial in front of your eye, aim at a direction using the sighting device, then release the locking device to fix the dial, you can read off the actual directional value directly without the need for conversion. The measuring accuracy that could be achieved using this compass was enough for it to be used to create simple maps and plans.1
Johann Georg Oeri, manufacturer
The dry dial bears the inscription “Oeri in Zürich”. This is the maker’s mark of Johann Georg Oeri. Born in Zurich in 1780, Oeri completed an apprenticeship as a silver turner, then trained as a mechanic in Paris under Jean Nicolas Fortin. Fortin was a well-known manufacturer of optical instruments, scales and barometers.
Oeri returned to Zurich from Paris in 1807 and founded his mechanical workshop at number 34, Trittligasse the following year. Oeri soon became a well-known mechanic and instrument maker and was commissioned to make physical and mathematical apparatuses, for example, by Zurich physicist and astronomer Johann Kaspar Horner. Jakob Goldschmid was taken on as an apprentice by Oeri at his workshop in 1832, later taking over the workshop following Oeri’s death in 1852. Goldschmid also continued the workshop successfully and became known for his precise instruments, especially aneroid barometers.2 The collection of scientific instruments and teaching aids includes some instruments constructed by Goldschmid, such as the orthogonal planimeter after Kaspar Wetli, which is also available as a 3D digital model.
While little is known about Johann Georg Oeri, his brother Hans Jakob Oeri is well-known as an artist who produced historical and wall paintings. One of his oil paintings titled “Das Pariser Atelier” [The Paris Studio] shows, from left to right, mechanic and instrument maker Johann Georg Oeri, another brother Hans Oeri, who worked as a tinsmith, as a self-portrait Hans Jakob Oeri and, at the right in the picture, David Sulzer, also a painter. This picture is on display at Winterthur Art Museum.
The animation of this digital copy makes it easy to understand how the principle of a dry dial works. Due to the high resolution, even the (back-to-front) dial is easy to read, which is hardly possible in the original object. However, 3D digitalisation not only allows for a better understanding of the instruments’ functions, but also serves to preserve the object. The digitalised object is valuable due to its rarity. Only three other known dioptric prismatic compasses by Oeri exist, one of which is also in the collection of scientific instruments and teaching aids. In addition to digital preservation, the 3D model also makes this rare object more accessible. Thus, it no longer has to be viewed on site.
Photogrammetry and modelling
The animated structured were produced by hand.