Pyefinch focusing mechanism: The Hevelius Fine Adjustment
Johannes Hevelius (1611-1687) modified the Hooke microscope by adding a fine-pitch thumbscrew for fine focus. "Hence, I have called in aid our focussing screw, by the help of which, I felt sure that rightly applied, even if the screw be quite slender and of small price, I should be able to focus the microscope with as great perfection as the matter shall demand, not to one line or another only, but two, three or even four whole inches, and this easily and gently with one or another finger, always retaining the line of direction; so that no one—even a blundering operator—could ever be able easily to go wrong." ... "You will see, therefore, that this focussing screw confers no small advantage on microscopes, and it is not beyond possibility that it may serve for other instruments in the future..."

from Machina Coelestis 1673, J. Hevelius .

The focusing device is a Hevelius screw. The top mount can be fixed in position. Turning the screw thread slides the stage up or down relative to the objective.
Numbered positions on the body support shaft place the stage in an appropriate position for each numbered objective. Once positioned you lock the top screw. Fine focus then slides the stage up or down relative to the objective.