Surveyors' square consisting of a hollow cylinder with eight vertical slits placed 45° apart from one another. The cylinder base has a central fitting to attach the instrument to a tripod (now missing). The upper part is closed by a disk with a graduated rim; there is a magnetic compass in the center, with a cover and an alidade with viewers. The upper part was used for surveying, while the cylinder with the eight slits was used to trace perpendicular alignments and 45° angles. Signed by Domenico Lusverg. Provenance: Medici collections.