I had about 8 swift microscopes like this. You could try a stronger solvent I read about using toluene or benzene to clean them, but I'm not comfortable using those in my sciecne classroom. I attempted acatone, but it didn't do much to clean them up. I ended up doing was ordering new 40x objectives for about $80 each. While that's about a third of the cost of the microscope, the microscopes I have are better quality than what I could reasonably purchase. The newer plastic microscopes in my price range didn’t seem to focus as easily as the older ones for students.