The adjustment knob used with high power on a microscope is called the fine focus knob.

Here's why:

* High Power Magnification: When you switch to high power, the objective lens is much closer to the specimen. This requires a very precise adjustment to get the specimen in focus.

* Fine Focus Knob: The fine focus knob makes very small adjustments to the stage's height, allowing you to achieve the sharpest image possible at high magnification.

It's important to remember that you should always start with the coarse focus knob at low power to get the specimen roughly in focus before switching to high power and using the fine focus knob for fine adjustments.