A spot in the view can sit on the eyepiece, the objective, the condenser or the slide. Find it first, then clean only that piece.
Turn the eyepiece in its tube while you look through it. A spot that turns with it is on the eyepiece.
Move the slide. A spot that travels with the specimen is on the slide.
Run the condenser up and down. A spot that sharpens and blurs as it moves is on the condenser.
Swing a different objective into place. If the spot disappears, it was on the first objective.
Use the air blower on every glass surface before anything touches it. Grit dragged across a lens by paper can scratch its coating.
A hand blower puts nothing on the glass but air, and it works on the eyepiece top lens, the objective front lenses and the condenser.
Fold a sheet of lens paper and put one drop of isopropyl alcohol on the paper. Wet the paper, never the lens, so no fluid runs into the lens mount.
Wipe in a slow spiral from the center of the lens out to the edge, with no pressure. Use a fresh part of the paper for each pass and stop when the paper comes away clean.
Eyepieces collect skin oil from eyelashes and fingers, so they need this more often than any other lens.
Each of these is within reach on most desks, and each one can mark a lens.
Fog or dirt on the inside of an objective or eyepiece cannot be reached from outside. Stop there and contact the microscope's manufacturer.
Switch the light off and put a dust cover on after each use. Store the microscope somewhere dry and out of direct sun.
Wipe the oil off the 100X objective before you put the microscope away. Dried oil is far harder to remove than fresh oil.
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A booklet of 50 lens cleaning tissue sheets.
A silicone air blower, a lens pen with a brush and a carbon tip, and two washable microfiber cloths.