When using the high power objective on a microscope, you should use the fine adjustment knob. This knob allows for precise focusing by making very small adjustments to the position of the objective lens. The fine adjustment knob is essential when working with high magnification because even slight movements can significantly affect the focus.
Using the coarse adjustment knob at high power can cause the lens to come too close to the slide, potentially damaging both the slide and the lens. Therefore, always switch to the fine adjustment knob when using the high power objective to ensure clear and safe viewing.