17 assemblies of jointed bars, forming the linkage controlling the movement of the pencil-holder and the far-sight of the Perspective Instrument. In all cases the bars fitting to the case are of mahogany of flat rectangular section, their free ends guarded with brass and provided each with a cross hole; the horizontal bars are of mahogany, of flat rectangular section; and the other bars are of brass of flat rectangular section (but narrower than the wooden bars). Seventeen assemblies of jointed bars for controlling the perspective instrument 1790-1819