Cleaning optical surfaces

You can clean a seriously stained or corroded surface by gentle polishing with a fine wet rag and a dab of rouge or cerirouge. This is a last resort of course, on a surface which would otherwise be discarded. Apart from this you should never, never touch a dirty optical surface with any solid other than a fine sable-hair artist's brush dipped in a suitable solvent such as analytical quality iso-propyl alcohol. The sweat on your fingers (which helps in fingerprinting) is acid enough to etch glass. There is never any reason to allow a cloth to touch an optical-quality glass surface, as it will almost certainly have abrasive silica particles embedded in it.

Camera lenses

Camera lenses are best kept with a permanent anti-ultra-violet filter in place. If it is necessary to clear dust, a blower-brush is the proper device. If the glass surface has stains or dried water-spots, then the solvent of choice is iso-propyl alcohol. Diethyl ether can also be used in a well-ventilated place and ethyl alcohol too, although it is not such a good solvent for lipids. Diethyl ether evaporates quickly and the consequent cooling of the surface may cause water-drop condensation in high humidity surroundings. Ethyl alcohol will dissolve water.