The octant has a mahogany frame and limb with a brass index arm, fittings, and a brass stop for the index arm. It also has inlaid ivory plates on the crossbar and on the back of the frame. There is no tangent screw and the clamping screw is on the back of the index arm. The octant has three socket shades, two red, and one green. Index-glass adjustment is made by a screw and on both horizon glasses by levers and clamping screws. The sight vane has two pinholes and a swivelling shutter, whereas the back sight vane has one pinhole. A brass-capped screwdriver, which is perhaps a replacement, is fitted in the crossbar. Decorative engravings are found on the lower half of the index arm. The octant has no box.

The instrument has an inlaid ivory scale from -5° to 95° by 20 arcminutes, measuring to 91°. The octant has an ivory vernier measuring to 1 arcminute, with zero at the centre.

This is the only instrument known bearing the name of John Sandwell, which may suggest he was its owner rather than the maker.