Don't touch the emulsion with your fingers at all, there are several different emulsion cleaners on the market designed to do this very thing, Omega Satter makes one that we used to use in the photo shop I worked at, unfortunately non of them are 100% effective we found in actual use, but they will clean some of the dirt up that may make it easier to scan or view them..but again, don't touch the emulsion with your fingers, even with solution on your finger tips you still run the risk of depositing oils on the slide, we used to use the special q tips designed for cleaning tape heads.