The octant is a reflection instrument designed by John Hadley c. 1731. It is used to measure the altitude of the Sun or a celestial body above the horizon at sea. A mobile arm carrying a mirror and pivoting on a graduated arc provides a reflected image of the celestial body overlapping the image of the horizon, which is observed directly. This model is signed Hughes, but the maker's exact identity cannot be determined because there were many craftsmen of that name in that period. Provenance: Lorraine collections.