The microscope coarse adjust is via rack & pinion, with the rack attached to a triangular support pillar.

Fine focus (A) is via the "long arm" mechanism.

When inserted (below) a spring presses against the objective mount to add tension to the internal lever (depicted as a blue line below).
As the fine focus thumbwheel presses down, the lever pivots up, moving the objective away from the sample. As the thumbwheel screws up, the internal spring presses the lever end down. This moves the objective toward the sample.