Just as Galileo used his telescope to open the human horizon to the planets and stars of space, so van Leeuwenhoek used his microscope to open human awareness to the microscopic world that was invisibly small and that no one had even dreamed existed.
He discovered protozoa, bacteria, blood cells, sperm, and capillaries. His work founded the science of microbiology and opened tissue studies and plant studies to the microscopic world. He completed human understanding of the circulatory system.
Anton van Leeuwenhoek was born in 1632 in Delft, Holland. With no advanced schooling, he was apprenticed as a cloth merchant and assumed that buying and selling cloth would be his career.
But van Leeuwenhoek was curious about the world and interested in mathematics. Completely self-taught, he learned enough math to moonlight as a surveyor and read what he could about the natural world around him. He never learned any language other than Dutch, so he was never able to read any of the scientific papers and research (all written in Latin or French).
Microscopes existed in Holland by 1620. Christian Huygens and Robert Hooke were the first two scientists to make scientific use of microscopes. Both designed and built two-lens microscopes (two ground glass lenses inside a thin metal barrel).
In 1657 van Leeuwenhoek looked through his first microscope and was fascinated. He tried a two-lens microscope, but was disappointed by its distortion and low resolution. When he built his first microscope, he used a highly curved single lens to gain greater magnification.
By 1673 van Leeuwenhoek had built a 270-power microscope that was able to see objects only one-one-millionth of a meter in length. Van Leeuwenhoek remained very secretive about his work and never allowed others to see his microscopes or setup.
Van Leeuwenhoek started his microscopic studies with objects he could mount on the point of a pin, a bee’s mouth parts, fleas, human hairs, etc. He described and drew what he saw in precise detail. By 1674 he had developed the ability to focus on a flat dish and turned his attention to liquids, water drops, blood cells, etc.
Those 1674 studies were where he made his great discovery. He discovered a host of microscopic protozoa (bacteria) in every water drop. He had discovered microscopic life, invisible to the human eye.
Van Leeuwenhoek expanded his search for these unseeably small creatures and found them everywhere: on human eyelashes, on fleas, in dust, and on skin. He drew and described these tiny creatures with excellent, precise drawings.
Each drawing often took days to complete. As an amateur, Van Leeuwenhoek had to work at his science in the evenings and early morning hours when not at work. Embarrassed by his lack of language skills and by his poor spelling (even in Dutch), van Leeuwenhoek felt hesitant to publish any articles about his wondrous findings.
Beginning in 1676, he agreed to send letters and drawings to the Royal Society of London. They had them translated into English. That extensive collection of letters (written and collected over many decades) formed the first and best map of the microscopic world. What van Leeuwenhoek observed shattered many scientific beliefs of the day and put him decades, if not centuries, ahead of other researchers.
He was the first to claim that bacteria cause infection and disease. (No one else believed it until Pasteur proved it in 1856.) Van Leeuwenhoek saw that vinegar killed bacteria and said that it would clean wounds. Again, it was two centuries before his belief became standard medical practice.
It was also 200 years before anyone built a better microscope. But with his marvelous microscope, van Leeuwenhoek discovered the critically important microscopic world.
In 1999 scientists discovered the largest bacterium ever. The organism can grow to as large as .75 mm across, about the size of the period at the end of this sentence. The newfound bacterium is 100 times larger than the previous record holder. For comparison, if the newly discovered bacterium was the size of a blue whale, the average bacterium would be the size of a newborn mouse.