Antoni van Leeuwenhoek (1632–1723), a fabric merchant from Delft, the Netherlands, used tiny glass spheres to study various microscopic objects at high magnification with surprisingly good resolution. A contemporary of Sir Isaac Newton, Christiaan Huygens, and Robert Hooke, he is said to have made over 400 microscopes and bequeathed 26 of them to the Royal Society of London. (A handful of these microscopes are extant in various European museums.) Using his single-lens microscope, van Leeuwenhoek observed what he called animalcules – or micro-organisms, to use the modern terminology – and made the first drawing of a bacterium in 1683. He kept detailed records of what he saw and wrote about his findings to the Royal Society of London and the Paris Academy of Science. His contributions have made him the father of scientific microscopy.

Van Leeuwenhoek was an amateur in science and lacked formal training. He seems to have been inspired to take up microscopy by Robert Hooke's illustrated book, Micrographia, which depicted Hooke's own observations with the microscope. In basic design, van Leeuwenhoek's instruments were simply powerful magnifying glasses, not compound microscopes of the type used today. An entire instrument was only 3– 4 inches (8–10 cm) long, and had to be held up close to the eye; its use required good lighting and great patience. Van Leeuwenhoek devised tiny, double-convex lenses to be mounted between brass plates.