Here's why:

* Coarse adjustment knob: This knob moves the stage up and down in large increments. It's used for initial focusing, especially when starting with low power objectives.

* Fine adjustment knob: This knob makes smaller adjustments to the stage, used for fine-tuning focus once the coarse adjustment has brought the specimen into view.

Remember: You should always start with the lowest power objective (usually 4x or 10x) and use the coarse adjustment to bring the specimen into focus. Then, you can switch to higher power objectives (40x or 100x) and use the fine adjustment knob for sharper focusing.