Antonie van Leeuwenhoek was the first to observe living microorganisms: "animalcules" in pond water, bacteria in dental plaque, and protozoa in various samples. He achieved this not with the compound microscope Hooke had popularized, but with simple single-lens instruments he made himself and whose secrets he jealously guarded. His magnifications — up to 270x — exceeded everything available at the time. The letters he sent to the Royal Society between 1674 and 1683 opened a window onto an unknown biological world and founded microbiology as an observational discipline.