This Smith & Beck microscope has a coarse focus (A) and fine focus (B) mechanism.
Coarse focus moves the microscope body tube via a rack & pinion mechanism.
Fine focus is actuated via this "short-arm" mechanism. The microscope body is mounted to a sleeve assembled over the main support pillar. The axial position of the sleeve (and therefore the height of the microscope objective over the sample) can be finely controlled by turning the fine-threaded adjustment screw (A) against the short lever arm. Since the arm fulcrum is distal to the pillar, pushing the are up (L) raises the pillar. Pushing the arm down (R) lowers the pillar. In this way, the objective focus can be finely adjusted.
Fine focus knob.