Antoni van Leeuwenhoek (1632-1723), Dutch maker of microscopes, who made pioneering discoveries concerning protozoa, red blood cells, capillary systems, and the life cycles of insects.

Born in Delft, Holland, Leeuwenhoek received little or no scientific education. While a haberdasher and a chamberlain for the sheriffs of Delft, he devised, as a hobby, his single, tiny, double-convex lenses mounted between brass plates and held close to the eye. Through them he was able to peer at objects mounted on pinheads, magnifying them up to 300 times (a power that far exceeded that of early compound microscopes). In 1668 he confirmed and developed the discovery by Italian anatomist Marcello Malpighi of capillary systems, demonstrating how the red corpuscles circulated through the capillaries of a rabbit's ear and the web of a frog's foot. In 1674 he gave the first accurate description of red blood corpuscles. He then observed what he called animalcules—known today as protozoa and bacteria—in pond water, rainwater, in human saliva, and in 1677 he described the spermatozoa of both insects and humans.

Leeuwenhoek opposed the prevalent theory of spontaneous generation and demonstrated that granary weevils, fleas, and mussels are not created from wheat grains and sand but develop from tiny eggs. He described the life cycle of ants, showing how the larvae and pupae originate from eggs. Leeuwenhoek also observed plant and muscle tissue, and described three types of bacteria: bacilli, cocci, and spirilla. He kept the craft of making his lenses a secret, however, so that not until the improvement of the compound microscope in the 19th century was the next observation of bacteria made.

In recognition of his discoveries, he was made a fellow of the Royal Society of England and was visited by such notables as Queen Anne of England and Peter the Great, tsar of Russia.