Reality Check for the following methods: You will notice that all of the manufacturers have different methods of cleaning microscope optics. The solvents they recommend are somewhere between difficult and impossible for most students and amateurs to obtain. I’ve used 70% – 90% Isopropyl Alcohol successfully for many years. Use a reversed eyepiece or 10x magnifier to examine the surface of the lens first to see where the dirt or contamination is, if there is any. Use an air blower first (NOT CANNED AIR) to blow off any dust or dirt. Then distilled  or deionized (DI) water can be used initially. Or you may simply exhale onto the lens surface to provide moisture.

Only use small amounts of whatever solvent you are using. Don’t flood objectives or eyepieces with solvent. Just wet the Q-tip or lens tissue with a little bit. If using lens tissue, twist it up into a little wad and use that. Do NOT use lens tissue flat with your fingertip as you run a high risk of pushing and dirt into the lens surface or coating and scratching it. While newly manufactured objectives should be pretty well sealed, it is possible given enough time or a failed seal, for the solvent to leak into the objective probably ruining it. Sometimes oil immersion lenses will leak oil into the inside. It doesn’t happen often, but when it does the objective is ruined.

70% Isopropyl Alcohol will likely leave a thin film residue on the lens that can be seen with the reversed eyepiece and reflected light. It may be cleaned off easily with moisture or DI water. 90% Isopropyl Alcohol is preferred as it leaves little to no residue. 

If the material on the lens is water soluble, as are body fluids or leftover pond water, and water alone does not remove it, a couple of drops of dish detergent may be added to 25-50 ml of DI water and the mixture used. Then afterward remove any remaining soap with clean DI water. Cotton swabs or Q-Tips may be used, and small cotton balls for eyepieces. I avoid the use of lens tissue.

The key to all this is to BE GENTLE. The coatings on microscope lenses are very fragile, and the optical glass is relatively soft and easily scratched as well. Be careful. Use two hands when removing or reattaching objectives. The angled screw threads are difficult to start, and it’s easy to drop the objective if you are not careful.