When viewing a slide with the high power objective on a microscope, the fine adjustment knob should be used to sharpen the focus.
The fine adjustment knob allows for precise adjustments to the focus, making it ideal for high power viewing where details are smaller and less forgiving to larger adjustments. In contrast, the coarse adjustment knob is used for bringing the slide into general focus when using lower power objectives.
Using the fine adjustment knob at high power helps prevent damage to the slide or the objective lens, as any sudden or large movements could cause them to collide. Therefore, always rely on the fine adjustment knob when fine-tuning your focus under high magnification.