Focus uses a Hevelius screw mechanism. Jamscrew (B) fixes the base of the mechanism to the square support pillar. Turning thumbscrew "A" causes the microscope body support to slide up or down along the pillar. Passemant added an interesting device (seen above thumbscrew "B"). It may keep the body support orthogonal to the support pillar, or act as a spacer to maintain separation between the jamscrew mechanism and body support.
