Coarse focus vernier scale
The coarse focus mechanism is rack & pinion. The fine focus uses a fine-pitch screw thread mechanism actuating a lever.
A: Fine focus screw thread moves brass block "B" via an internal lever (see diagram, right).
B: Coarse focus is rack and pinion
Patent No. 215878, Improvements in Microscopes, May 27, 1879. Covers the independently swinging substage assembly and mirror and the fine adjustment mechanism and a stage centering mechanism. (Information from the American Artifacts website).