Portrait of Anton van Leeuwenhoek (1632 - 1723), a Dutch microscopist, using one of his microscopes. Although Van Leeuwenhoek had no formal scientific training, he became an enthusiastic microscopist by making over 500 single lens microscopes throughout his life. Van Leeuwenhoek made the first detailed descriptions of various microorganisms, such as bacteria and protozoa. He did not believe in the concept of spontaneous generation, and was ultimately proved correct a century and a half later.