This is the fine focus mechanism from a Leitz Microscope built around 1939 (estimate based on the serial number).

The knob turns a cam via a worm gear. The head of the microscope (including the objective, the eyepiece, and the coarse focus mechanism) rides on top of the cam. The cam is the thing that looks like an off center wheel. The top of the wheel is about 2mm thicker at the starting position vs the ending position.

The raised screw pushes a slider that blocks further rotation of the knob when the focus reaches its highest or lowest position. Without this mechanism, the direction of focus would reverse after passing the thickest or thinnest part of the cam, which could make it very confusing to operate the knob.

After I put everything back together, I measured ten turns of the knob as about 1.75mm of vertical motion, so each 1/50th mark on the knob corresponds to about 3.5 microns. That's not exact, but I'm pretty sure I measured it accurately enough to rule out the possibility of it being a nice round number like 4 microns or .0001 inches.

I found a nice description of a very similar mechanism on an older microscope in this page: [ Ссылка ]

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