The microscope has different coarse (B) and fine (A) focus mechanisms. Fine focus knob (A) rotates the central fine-pitch screw. This, in turn, moves the entire coarse focus assembly (B) along the triangular support pillars.
Coarse focus is rack & pinion. Turning the knob slides the center support bar (A) up or down. It, in turn, is supported by the lateral, triangular support pillars.
Lateral triangular support pillars.
Side view showing the microscope body support structure (B) and fine focus crown gear.