The octagon has an ebony frame and limb with an index arm made from two-thirds wood, and one-third brass. The device has brass fittings, a brass stop for the index arm, and an inlaid ivory plate on the back of the frame. There is no tangent screw, and the clamping screw is missing from the back of the index arm. The octagon has three socket shades, two in red, and one in green. Index-glass adjustment is made by a screw and both horizon glasses by levers, wing nuts, and milled clamping screws. The sight vane has two pinholes and a swivelling shutter, whereas the back sight vane has one pinhole. There is an ivory pencil head, with the pencil missing, fitted in the crossbar. Floral decoration, including Masonic symbols, is found on the brass section of the index arm. One of the feet on the frame is an old replacement. The octagon has no box, though the Museum’s record mentions a stepped oak 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.