This instrument consists of a diameter cut away and two folding sight vanes. The diameter has a crossbar at centre which holds a raised section to accommodate the compass spigot. The back sight vane is a wide slit with a V notch at the top. It has two spring clips. The fore sight is an open arch and thread,(missing). There is a raised point at the top of the vane. The thread also runs along the diameter. Both vanes can be locked upright by small levers.