Antonie van Leeuwenhoek writes a letter to the Royal Society describing "animalcules": the first known description of protozoa.
Antonie Philips van Leeuwenhoek ( AHN-tə-nee vahn LAY-vən-hook, -huuk; Dutch: [ɑnˈtoːni vɑn ˈleːuə(n)ˌɦuk] (listen); 24 October 1632 – 26 August 1723) was a Dutch businessman and scientist in the Golden Age of Dutch 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.
Raised in Delft, Dutch Republic, van Leeuwenhoek worked as a draper in his youth and founded his own shop in 1654. He became well recognized in municipal politics and developed an interest in lensmaking. In the 1670s, he started to explore microbial life with his microscope. This was one of the notable achievements of the Golden Age of Dutch exploration and discovery (c. 1590s–1720s).
Using single-lensed microscopes of his own design and make, van Leeuwenhoek was the first to observe and to experiment with microbes, which he originally referred to as dierkens, diertgens or diertjes (Dutch for "small animals" [translated into English as animalcules, from Latin animalculum = "tiny animal"]). He was the first to relatively determine their size. Most of the "animalcules" are now referred to as unicellular organisms, although he observed multicellular organisms in pond water. He was also the first to document microscopic observations of muscle fibers, bacteria, spermatozoa, red blood cells, crystals in gouty tophi, and among the first to see blood flow in capillaries. Although van Leeuwenhoek did not write any books, he described his discoveries in letters to the Royal Society, which published many of his letters, and to persons in several European countries.