Fine focus is controlled by the thumbwheel (arrow) at the base of the main support pillar. When this wheel is turned a central support shaft raises or lowers the objective/microscope assembly.
Coarse focus is set by manually setting the position of the central shaft according to the objective used. Note that the position changes according to whether the microscope is used as a Single (L) or Double magnifier (R). The lateral thumbscrew fixes the coarse position in place.
Close examination of the support pillar shows it to be comprised of three concentric shafts: The outer octagonal support pilar (A), the round central coarse adjust shaft (B), and the round, inner fine focus shaft (C).