Question

why should the fine focus knob typically be used with the 10x and 40x objective lenses?
the slide should be close to in focus after focusing at the 4x objective. focusing with the coarse focus knob would result in too large a change in focus.
focusing using the coarse adjustment knob with the 10x or 40x objective may crack the stage.
the coarse focus knob is nonfunctional with the 10x and 40x objective lenses.

Explanation:

When using a microscope, one typically starts focusing at the lowest - power objective (4X). After that, when switching to 10X and 40X objectives, the slide is already close to being in focus. The coarse focus knob moves the stage up and down by a relatively large amount, which can over - or under - focus the specimen easily. The fine focus knob makes small, precise adjustments to the focus, which is ideal for higher - power objectives. The coarse focus knob doesn't crack the stage and is functional with all objectives, just not suitable for fine - tuning at higher magnifications.

Answer:

The slide should be close to in focus after focusing at the 4X objective. Focusing with the coarse focus knob would result in too large a change in focus.