The sextant has a black-lacquered circular brass plate, with the upper segment cut away, and a wooden handle. A spring-loaded pin locks when the observation is made. Attached to the sextant is a fixed bracket with a 53mm sight-tube at both ends, and a spirit level centred between the two. The object end of the sight-tube has cross wires and two shades in red and green.

The horizontal line of reference is established by an arc-shaped pendulum, pivoted at the centre of the instrument, and hanging freely during the sighting. The pendulum is then locked by a trigger mechanism before the sextant is read.

The instrument has a brass scale to 105° on either side of 0° by 1 arcminute, measuring to 92°. The sextant has two verniers, one for altitude measuring to 1 arcminute, with zero at the centre, the other for zenith distance measuring to 1 arcminute, with zero at the right.

The sextant is contained in a square fitted wooden box.

Hall Colby of New York patented ‘Colby’s altimeter or self-adjusting quadrant or sextant’ (no. 1200 of 1854 and no. 1736 of 1855) and in the USA (no. 100,119 of 1870).