Antonie van Leeuwenhoek

Based on Wikipedia: Antonie van Leeuwenhoek

In 1676, a cloth merchant in Delft peered through a tiny glass bead and saw something that would change humanity's understanding of life itself. Swimming in a drop of pond water were creatures no human had ever seen before—thousands of them, darting and spinning, completely invisible to the naked eye. Antonie van Leeuwenhoek called them "animalcules," little animals. He had just discovered the microbial world.

What makes this moment even more remarkable is who made the discovery. Leeuwenhoek wasn't a university professor or a trained natural philosopher. He was a draper—someone who sold linen, yarn, and ribbons to seamstresses and tailors. He had no formal scientific education, never learned Latin (the language of science at the time), and worked entirely alone. Yet he would become the first person to see bacteria, the first to observe sperm cells, and the first to watch red blood cells squeezing single file through the tiniest blood vessels. The Royal Society of London, the most prestigious scientific organization in the world, would eventually call him to membership, even though he never attended a single meeting and wrote all his letters in colloquial Dutch.

A Draper's Curiosity

Leeuwenhoek was born in Delft on October 24, 1632, during what historians call the Dutch Golden Age—a period when the small republic was producing some of the world's greatest painters, scientists, and merchants. His father made baskets and died when Antonie was five. His mother remarried a painter, but the stepfather also died when Antonie was around ten. The boy was shuffled off to live with an uncle who practiced law.

At sixteen, he became an apprentice bookkeeper at a linen shop in Amsterdam. Six years later, he returned to Delft, married a woman named Barbara de Mey, and opened his own draper's shop. Of his five children with Barbara, only one daughter survived infancy—a heartbreakingly common outcome in seventeenth-century Europe.

Leeuwenhoek was good at business and even better at making connections. By 1660, he had secured a comfortable position as chamberlain at the city hall, responsible for maintaining the building, heating the rooms, and—importantly—keeping silent about the discussions that took place there. It was steady, respectable work that left him time for other pursuits. Later he added the jobs of land surveyor and official "wine-gauger," the person responsible for measuring and taxing wine imports.

He lived steps away from the famous painter Johannes Vermeer, who was baptized just four days before Leeuwenhoek. Some art historians have speculated that Leeuwenhoek appears in Vermeer's paintings "The Astronomer" and "The Geographer," though the evidence is disputed. What we know for certain is that when Vermeer died bankrupt in 1675, Leeuwenhoek was appointed executor of his estate—a sign of how respected he had become in their small city of 24,000 people.

The Secret of the Lenses

The story of how Leeuwenhoek became a microscopist begins with thread.

As a cloth merchant, he needed to examine the quality of fabric and yarn. The magnifying glasses available at the time weren't good enough for his exacting eye. So he began making his own lenses.

Here's where Leeuwenhoek's genius—and paranoia—come into play. For centuries after his death, no one could figure out how he made lenses so powerful. He deliberately misled people, allowing visitors to believe he ground his lenses by hand like a spectacle maker. He kept his best microscopes hidden, showing guests only inferior instruments.

The secret was finally cracked in 1957, when a scientist named C.L. Stong tried a different approach: instead of grinding glass, he melted it. Take a thin rod of glass, heat the middle in a flame, and pull the ends apart. You get two long whiskers of glass. Now put the tip of one whisker back in the flame. The glass melts into a tiny, nearly perfect sphere. That sphere is your lens.

A 2021 study using neutron imaging—a technique that can see inside objects without destroying them—confirmed that at least one of Leeuwenhoek's surviving microscopes contains exactly this type of spherical lens with a short glass stem attached. The method, ironically, was first described by Robert Hooke, another early microscopist who would later express amazement at what Leeuwenhoek could see. Hooke apparently never realized the potential of his own technique.

Leeuwenhoek's microscopes were tiny—the largest about five centimeters, or two inches, long. They looked nothing like modern microscopes. Each was a small metal plate, made of silver or copper, with the tiny glass bead lens mounted in a hole. On the back was a pin where you would stick your sample, and three screws that let you move the sample up, down, left, right, forward, and back. You held the whole device up to your eye, almost touching your face, and looked through the lens at whatever was impaled on the pin.

He made over 500 lenses in his lifetime and at least 25 complete microscopes, though only nine survive today. The best surviving ones can magnify up to 275 times. Based on what Leeuwenhoek described seeing, scientists believe some of his lost instruments could magnify up to 500 times—a remarkable achievement for hand-made glass beads.

Little Animals Everywhere

In 1673, a Dutch physician named Reinier de Graaf wrote to the Royal Society of London with exciting news: he knew a man in Delft whose microscopes "far surpass those which we have hitherto seen." The Society published Leeuwenhoek's first letter, which described mold, bees, and lice.

But the real breakthrough came in 1674. Looking at pond water, Leeuwenhoek saw things moving. Lots of things. He assumed that if they were moving on their own, they must be alive. He called them "dierkens" or "diertjes"—little animals in Dutch. His correspondent Henry Oldenburg, secretary of the Royal Society, translated this as "animalcules."

Leeuwenhoek had discovered single-celled organisms. What he called animalcules we now divide into many categories: protists like amoebas and paramecia, bacteria, and more. Some of what he saw were actually multicellular organisms too small to see without magnification. But the key insight was that there existed an entire world of living things completely invisible to the naked eye.

He found them everywhere. Pond water teemed with them. Rainwater collected them. And his own mouth was full of them.

"Although I am now fifty years old," he wrote to the Royal Society, "my teeth are well preserved, because I am in the habit of rubbing them with salt every morning."

Even so, when he scraped the white matter from between his teeth and looked at it under his microscope, he found more animalcules than there were people in the Dutch Republic. In 1683, he drew what we now recognize as bacteria—the first person ever to do so. Among the shapes he sketched were what appear to be Selenomonads, large bacteria that still inhabit human mouths today.

The Skeptics

When Leeuwenhoek sent his observations of single-celled organisms to the Royal Society in 1676, the response was not immediate acceptance. The idea that invisible creatures swarmed through every drop of water and every human body was, frankly, hard to believe.

The Royal Society did something remarkable: they organized a fact-checking mission. They sent a delegation to Delft that included Alexander Petrie (minister of the English Reformed Church in Delft), two Lutheran ministers, a Scottish knight named Robert Gordon, and four other witnesses. Their job was to look through Leeuwenhoek's microscopes and determine whether the cloth merchant was actually seeing what he claimed to see.

He was. In 1677, the Royal Society fully acknowledged his observations. Three years later, they elected him a Fellow—one of the highest honors in science. Leeuwenhoek was, by his own account, "taken aback." He never traveled to London for the induction ceremony and never attended a single Society meeting. But he had his portrait painted with the fellowship certificate prominently displayed, signed by King James II of England.

The Famous Visitors

Word spread. Over the following decades, some of Europe's most notable figures made pilgrimages to Delft to peer through the draper's microscopes.

The list reads like a who's who of the late seventeenth century: Christiaan Huygens, the physicist who developed the wave theory of light; Gottfried Wilhelm Leibniz, the philosopher and mathematician who invented calculus independently of Newton; John Locke, the political philosopher whose ideas would later influence the American founders; William III of Orange and his wife Mary II, who would become king and queen of England; and Tsar Peter the Great of Russia, who was traveling Europe incognito to learn about Western technology. Peter visited Leeuwenhoek's boat moored in a canal, and Leeuwenhoek gave him an "eel-viewer"—a device for watching blood circulate through the transparent blood vessels of a tiny eel.

To the disappointment of many visitors, Leeuwenhoek refused to show his best microscopes. He kept his cutting-edge instruments secret and showed guests only lower-quality lenses. He was secretive about his methods, suspicious of assistants, and fiercely protective of his discoveries. Robert Hooke, who had done important early work in microscopy, complained that the entire field had come to rest on one man's shoulders.

Blood, Sperm, and Muscles

Leeuwenhoek didn't just discover microbes. He systematically explored the microscopic structure of larger organisms, including himself.

He pricked his own finger to get blood samples. He placed pieces of his own skin under the lens. He described the structure of muscle fibers and discovered their characteristic banded pattern in 1682—a finding that would eventually help scientists understand how muscles contract.

In 1674, he became one of the first people to clearly identify red blood cells. Others had seen them before—Marcello Malpighi and Jan Swammerdam—but Leeuwenhoek was the first to understand what he was looking at. He watched individual cells squeezing through capillaries, the tiniest blood vessels, in the tails of tadpoles.

"If we now plainly perceive that the passage of the blood from the arteries into the veins of the tadpole is not performed in any other than those vessels, which are so minute as only to admit the passage of a single globule at a time, we may conclude that the same is performed in like manner in our own bodies and in those of other animals."

This was a breakthrough in understanding circulation. William Harvey had demonstrated decades earlier that blood circulated through the body, but no one had seen exactly how blood got from arteries to veins. The answer was capillaries, and Leeuwenhoek watched it happen.

In 1677, he turned his microscope on semen and became the first to describe spermatozoa. His interpretation was wrong—he believed that the complete miniature human was contained within the sperm cell, a theory called "preformationism"—but his observations were accurate. He could see the sperm cells' tails propelling them forward.

He even investigated what makes horseradish burn your tongue, using vinegar to test his theories about irritation.

A Different Kind of Scientist

Leeuwenhoek never wrote a scientific paper. He never wrote a book. He only wrote letters—about 190 of them to the Royal Society alone, plus many others to various correspondents. The letters were rambling, digressive, and written in colloquial Dutch rather than scholarly Latin. Henry Oldenburg, the Royal Society's secretary, learned Dutch specifically to translate them.

This unconventional approach reflected Leeuwenhoek's outsider status. He had no formal training in natural philosophy, no university education, no mentor in the scientific method. He was a cloth merchant who happened to make the best lenses in the world and had endless curiosity about what he could see through them.

His faith informed his work. Like many Dutch Reformed Calvinists of his era, he saw his discoveries as revelations of divine craftsmanship. The intricate structures of tiny creatures were, to him, proof of God's creative power. His contemporary Jan Swammerdam felt similarly—both men approached their microscopes with something like reverence.

The Legacy

Leeuwenhoek died on August 26, 1723, at the age of ninety—an extraordinary lifespan for the seventeenth century. He had been making observations until near the end.

His discoveries laid the foundation for microbiology, the study of microscopic life. Before Leeuwenhoek, no one knew that bacteria existed. No one understood that a drop of water could contain more living creatures than a city contained people. No one had watched blood cells navigate through vessels barely wide enough to let them pass.

The germ theory of disease—the understanding that many illnesses are caused by microscopic organisms—wouldn't emerge for another 150 years. Louis Pasteur and Robert Koch would eventually prove that specific bacteria caused specific diseases. But they could only do so because Leeuwenhoek had first shown that such tiny living things existed at all.

Today he is remembered as "the Father of Microbiology," though he might have found the title odd. He was, after all, just a draper who wanted to see his thread more clearly. The fact that this led him to discover an invisible world of living creatures says something about the power of curiosity combined with the right tools—and the willingness to trust your own eyes, even when they show you something no one has ever seen before.