Why should you never use the coarse adjustment knob on high power?

 Once you get the item generally focused on the coarse adjustment, you should leave it alone. If you try to move it again, the chance is you'll run into the slide and either knock the specimen or even scratch the microscope's lens. Always use the fine adjustment, and nothing else, once you have set the coarse adjustment to the appropriate level. Hope this helps!

In the air and in the lower fre ah water