Focus is done in two steps. First, the position of the sample holder (B) is set along the support shaft (A) according to which objective lens (1–7) and whether the instrument is used as a single-lens magnifier ("S") or a compound ("D", Double) microscope. Arrow points to the stage locating pins.
Focusing the Watkins microscope
Fine focus was accomplished by turning a thumbscrew (arrow) at the base of the support shaft. This moved a rod connected to the sample stage. By turning the thumbscrew small changes in focus are possible (e.g., position "A" vs. "B").
The fine focus control rod traverses through a hole in the trapezoidal support shaft. It is flanked by two locating pins to keep the objective mount from rotating during focusing.