Lens cleaning:

Here is a tip for cleaning objective lenses on microscopes: Cotton buds are too big to reach inside the protective recess but toothpicks (the strong double-ended round ones) are ideal.
Dip in alcohol or white spirit to dissolve the build-up and then the fine tip can be used gently like a brush. Finally, wrap one in a piece of lens tissue and use to polish the lens. The wood doesn’t seem to scratch and the lenses come out very clean.
It is not necessarily a recommended method for expensive lenses.