Should be used after Course Specimen Focus Knob to "fine tune" the Focus.
If your eyes become tired or you develop a headache while using a microscope, chances are you are not using the fine focus properly, but are trying to focus things with your own eyes. USE THE FINE FOCUS TO AVOID THESE PROBLEMS!
Rotating this Knob Translates the Specimen Stage a Slight Amount in the Z direction.
Experienced Microscopists ALWAYS have One Hand on the Fine Specimen Focus Knob while observing specimens. This allows them to "optically section" the specimen AND avoid eye strain.