 A historical case that has been extensively discussed is the case of the hapless mesosome. For more than 20 years (ca. 1960–1983), this was thought to be a membrane-bound structure inside bacterial cells, not unlike the nucleus or the mitochondria of the cells of higher organisms. However, it could only be seen in electron microscopy (bacterial cells being too small for light microscopy). Because electron microscopy requires a vacuum, biological samples require a lot of preparation. Cells must be chemically fixed, which was done mostly by using Osmium tetroxide at the time. Furthermore, the samples must be protected from the effects of freezing water. For this purpose, cryoprotectants such as glycol must be used. Finally, the material must be cut into extremely thin slices, as an electron beam does not have much penetrating power. Electron microscopists used two different techniques: The first involved embedding the cells in a raisin before cutting with very sharp knives (so-called microtomes). The other technique used fast freezing followed by fracturing the cells along their membranes. At first, mesosomes were seen in electron micrographs under a variety of conditions, but then there were increasing findings were they failed to show. Thus, suspicion started to rise that mesosomes were really a microscopic artifact. Ultimately, this is what microbiologists and electron microscopists ended up concluding by the mid-1980s, after more than two decades of research on these structures.