Surveyor's square with compass, 19th century

Antique brass octagonal surveyor's square with compass. This is a very nice scientific instrument, surveyor's tool.
 
 The surveyor's square is a cylinder or octagonal prism pierced with longitudinal slots and used to trace on the ground, using stakes, straight lines and perpendiculars. The opposite openings are formed either by a narrow slit or by a rectangular opening divided in two by a silk thread. The visual ray passing through this slit, and the silk thread covering a stake placed at any point, these three points are used to determine the position of a straight line.
 
 The square is marked "  SL  ", initials of the SOCIÉTÉ DES LUNETIERS which produced at the end of the 19th century glasses, compasses, various scientific and marine instruments, today become "  ESSILOR  ".
 
 Scientific, vintage and original collector's item in the spirit of cabinets of curiosities.
 
 In good condition, in its juice with traces of seniority and wear.
 Golden brass color.
 
 DIMENSIONS  :  H12.5xD4.8cm
 DATE:  XIXth
 ORIGIN:  France