The discovery of new species is difficult but possible. You must be already quite advanced and very specialized in a particular field. Sometimes amateur microscopists cooperate with researchers. The problem is that for a proper characterization microscopy alone is rarely sufficient. You need to do do extensive biochemical or molecular biological tests, which go far beyond mere microscopic observation. In that sense, amateur microscopy is somewhat different from amateur astronomy. If you discover a new comet or supernova, then characterization that this is something new is relatively easy. You have to measure out the exact position of the new object on a photograph and then check with a database. In Biology the issue is not as straight forward. You first have to dig through a whole bunch of literature and know the type of organisms that you work with quite well. Many microscopic species might look similar to each other but are still different. There is no way to distinguish them by microscopic observation alone.