Question

Why should you not look down the microscope all the time as you try to focus the specimen?

Answer 1

Reason: While you look down the microscope, it racks the lens away from the slide.

And stops when something comes into focus.

This is what you are meant to be seeing, but it could be dust on the bottom of the slide or the top of the cover slip.

The dust can also be on the condenser lenses on the light bulb.  

The best way to focus is to use only the fine focusing adjustment.

Answer 2

Looking in from sideways (not looking down the microscope yet!), bring the objective lens close to the example (say 3 mm.) Using the coarse focussing alteration.

Looking down the microscope constantly, rack the lens far from the slide, and stop when something comes into focus.

Hope the answer helps you.