Biography: As a young man Anton van Leeuwenhoek worked in a drapery store, where he used magnifying glasses to count thread densities. Perhaps inspired by Robert Hooke's Micrographia (an account of Hooke's microscopic investigations in botany, chemistry, and other branches of science, published in 1665), he began building microscopes. He examined hair, blood, insects, and other things around him, keeping detailed records and drawings of his observations. Although compound microscopes with more than one lens had been invented at the end of the fourteenth century, they were able to magnify objects only 20 to 30 times. Van Leeuwenhoek's single-lens microscopes were basically powerful magnifying glasses, but his superior lens-grinding skills and acute eyesight enabled him to magnify objects up to 200 times. Van Leeuwenhoek made each microscope for a specific investigation, and he had his specimens permanently mounted so he could study them as long as he wanted. His discoveries include protozoans (1674), blood cells (1674), bacteria (1676), spermatozoa (1677), and the structure of nerves (1717). By the time of his death at the age of ninety, van Leeuwenhoek had constructed more than 400 microscopes.