Antonie van Leeuwenhoek writes a letter to the Royal Society describing "animalcules", later known as protozoa.

Antonie Philips van Leeuwenhoek was a Dutch microbiologist and microscopist in the Golden Age of Dutch art, science and technology. A largely self-taught man in science, he is commonly known as "the Father of Microbiology", and one of the first microscopists and microbiologists. Van Leeuwenhoek is best known for his pioneering work in microscopy and for his contributions toward the establishment of microbiology as a scientific discipline.

Anthonie van Leeuwenhoek (1632-1723). Natuurkundige te Delft Rijksmuseum SK-A-957
Anthonie van Leeuwenhoek (1632-1723). Natuurkundige te Delft Rijksmuseum SK-A-957 (Source: Wikimedia Commons)
Antony van Leeuwenhoek and his
Antony van Leeuwenhoek and his "Little animals"; being some account of the father of protozoology and bacteriology and his multifarious discoveries in these disciplines; (1932) (19122164704) (Source: Wikimedia Commons)
The Geographer
The Geographer (Source: Wikimedia Commons)
Leeuwenhoek Eschenholz
Leeuwenhoek Eschenholz (Source: Wikimedia Commons)

Watch: Leeuwenhoek: The First Master of Microscopes

From the Journey to the Microcosmos channel.

Sources and further reading: Wikipedia — Antonie van Leeuwenhoek, Animalcule, Protozoa (CC BY-SA). Images: Wikimedia Commons.