As long as there is only loose dirt on the microscope optics, it is advisable to remove it with compressed air. This method is particularly gentle and prevents damage to the optics. Small bellows are often sufficient to generate the required stream of air.
When removing stubborn dirt, never use abrasive materials such as paper or microfiber cloths, as they can damage the lens surfaces and lead to scratches. Apart from this, stubborn dirt should never be removed dry. Another important point to remember is that only pure solvents, distilled water and clean tools may be used. Always apply cleaning liquids with certified cotton swabs or pads used for medical purposes to make sure they are clean enough and do not contain any foreign objects. Solvents containing ammonium should also be avoided. These can attack the anti-reflection coatings of the microscope optics and cause permanent damage. Acetone should not be used for cleaning either, as it may attack plastic components in the microscope.
There is an extremely simple and gentle way of removing water-soluble dirt which is worth trying first. Blow on the impurity so that the water vapor in your breath is used for cleaning. Then remove the dirt by rubbing absorbent cotton in a circular movement from the middle of the dirt outwards. This is the most effective way of removing the dirt without distributing it unnecessarily over the lens.