This
is standard procedure. It's easier to focus and locate the specimen
under low power using the coarse adjustment; you get a larger field of
view. Also it's a safety procedure. The high power objective has a
relatively long length and there is a possibility of breaking the glass
coverslip and/or slide if the coarse adjustment is used. Most
microscopes are "bifocal", that is, in switching from low to high power,
the image is nearly in focus with minor adjustment with the fine
adjustment.
