Here's why:

* High power magnifies greatly. This means the distance between the objective lens and the slide is very small.

* The coarse adjustment knob moves the stage up and down significantly. Using it on high power risks smashing the objective lens into the slide.

* The fine adjustment knob is used on high power. It makes tiny adjustments to focus, ensuring a clear image without damaging the objective lens.

Remember: Always start with the lowest power objective and focus using the coarse adjustment knob. Then, switch to higher powers and use the fine adjustment knob to focus.