One of the advantages of the Marshall microscope was this Hevelius screw focusing mechanism. It consists of a top piece that could be fixed to the support pillar via a jam screw (A), and a sliding piece that connected to the microscope body. This afforded a great advantage over the more typical friction focus of contemporary instruments.
Focus knob
Numbers engraved on the top of the support pillar noted the initial position of the jam screw for each objective used.