This part is used to bring the specimen into sharp focus. This part causes a very small up and down motion when used.

Fine focus.

Explanation:

If you're referencing a microscope, then it would be fine focus. Coarse focus is a basic focus used to properly see a specimen, while fine focus enhance clarity and precision. To use both, you have to gently twist a knob until you can see the specimen at a desired clarity.

Here's a reference image if you need it, it's labeled all the parts of a microscope.