The sextant consists of a triangular frame, an index arm which can be moved across the bottom arced limb, two sights and two reflecting mirrors. One mirror is moved with the arm, and the other (half of which is transparent) is fixed to one limb. One sight is fixed below this mirror and the other on the opposite limb. The arc of the bottom limb is 60degrees and a venier scale on the index arm allows for more accurate reading.
An altitude-measuring device. It is based on the principle of light reflection and angles of incidence as the octant, but it has the capability to record over 120 degrees of arc, while having an actual arc of 1/6 of a circle.