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.
Blow off grit first, then wipe with lens paper.
Find the spot before you clean
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.
When half the view is dark
A dark crescent across one side of the view comes from something sitting part way into the light path. McCrone's troubleshooting article names the usual suspects, and none needs cleaning.
The objective is not clicked in. Turn the nosepiece until the objective clicks into place.
The light path selector is part way. On a trinocular head, the knob that sends light to the camera port can be bumped. Push or pull it fully to a setting.
The condenser's top lens has swung partly out. It can be knocked when the stage is moved or removed.
Blow the dust off first
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.
Then wipe the smudges
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 check them often.
Keep these away from the glass
Each of these is within reach on most desks, and each one can mark a lens.
Cosmetic cotton swabs and facial tissue. The cotton can carry additives, and tissue can hold tiny wood fibers.
Acetone. It damages the plastic parts on a microscope.
Paper towels and shirt tails. Both leave lint and can scratch.
A screwdriver. Objectives and condensers are aligned at the factory. Clean their outside faces only.
When to stop
Fog or dirt on the inside of an objective or eyepiece cannot be reached from outside. Stop there and contact the microscope's manufacturer.
Keep it clean between sessions
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.
Cleaning supplies on Amazon
Affiliate links. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you.
