Antonie van Leeuwenhoek

Chapter 1 · Counting Threads

A tradesman's reason for grinding glass

1632 – 1670 · Delft · Amsterdam

Antonie van Leeuwenhoek was born in Delft in October 1632, the son of a basket-maker. Johannes Vermeer was born in the same town in the same year; Leeuwenhoek was later appointed executor of Vermeer's estate, and whether the two men knew each other well is a question art historians still ask.

His father died when he was five. He was apprenticed to a linen draper in Amsterdam at sixteen, and by 1654 was back in Delft running his own drapery. He also held a series of minor civic posts — chamberlain to the sheriffs of Delft, and later surveyor and wine-gauger — which gave him a steady income and a great deal of time.

He had no Latin and no Greek. This mattered enormously: the entire learned literature of Europe was closed to him, and he could not read a single scientific book. He wrote only in Dutch, in the ordinary idiom of a Delft tradesman.

His interest in lenses began commercially. Drapers used small magnifying glasses to count the threads per inch in cloth and judge its quality. Leeuwenhoek learned to make his own, and then kept going long past any commercial reason for doing so.

By the 1670s he had made lenses better than any in Europe, and he was using them on everything he could find.

Why this matters

Leeuwenhoek's exclusion from the learned world meant he inherited no expectations about what he ought to see — which may be why he reported what was actually there.

You have the draper and his magnifying glass. What would you ask him?

Ask Leeuwenhoek

• “Why does a draper need a magnifying glass at all?”
• “What did having no Latin keep you out of?”
• “Did you know Vermeer?”
• “What did your civic offices give you besides money?”
• “When did looking stop being about cloth?”

Chapter 2 · A Bead of Glass in a Brass Plate

Why his instruments beat everybody's

1660s – 1723 · Delft

The learned world used *compound* microscopes: two or more lenses in a tube, of the kind Hooke drew in *Micrographia*. They looked impressive and magnified perhaps thirty or fifty times before the image degraded into coloured fringes and blur.

Leeuwenhoek did something different and far less glamorous. He made a *single* tiny lens — a bead of glass, sometimes barely a millimetre across — clamped between two brass plates with a pinhole, mounted the specimen on a spike, and held the whole thing up to his eye almost touching his eyeball.

It is uncomfortable, it gives a tiny field of view, and it works. A single very small lens of short focal length can reach two or three hundred times magnification with far less of the distortion that ruins a compound instrument. He made over five hundred of them.

He never sold one. He never took a pupil. And he never disclosed his best method of making the lenses — how he produced the smallest and clearest of them is still not fully settled, though blowing and drawing glass threads, and possibly crushing and melting fragments, are the leading candidates.

That secrecy is the central fact of his career. Nobody could match his results for over a century, because nobody could match his instruments. He was giving the world observations it had no way to check.

Why this matters

Leeuwenhoek's single-lens design outperformed the compound microscopes of his day by an order of magnitude — and his secrecy about it stalled the field for a century.

You have the bead of glass and the secret. What is your question?

Ask Leeuwenhoek

• “Why does one small lens beat several large ones?”
• “Why would you not tell anyone how you made them?”
• “What is it actually like to use one of your microscopes?”
• “Five hundred lenses — why so many?”
• “How were people supposed to check your findings?”

Chapter 3 · A World in a Drop of Water

1674, and the first sight of micro-organisms

1674 – 1683 · Delft · Berkelse Mere

In 1674 Leeuwenhoek took water from Berkelse Mere, a lake near Delft, expecting to look at the cloudiness in it. He found the water full of creatures moving about — swimming, turning, some of them a thousand times smaller than anything then known to be alive. He called them *animalcules*.

He then went looking. He found them in rainwater that had stood in a pot. He found them in an infusion of crushed pepper. And in 1683 he scraped the white matter from between his own teeth, mixed it with clean rainwater, and found it swarming — rods, spirals and spheres, in enormous numbers. He noted that after drinking very hot coffee the ones in his mouth stopped moving.

Those were bacteria. He was the first human being to see them, and the drawings he sent to London are recognisable to a modern microbiologist.

The Royal Society did not believe him. A Dutch draper with no education was reporting creatures nobody else could find, using instruments nobody else had. They set Robert Hooke — the best microscopist in England — to repeat the work. Hooke tried, failed, improved his method, tried again, and confirmed it. Leeuwenhoek was elected a Fellow in 1680. He never attended a meeting and could not have understood one if he had.

“I saw very plainly that these were little eels, or worms, lying huddled together and wriggling.”

— Antonie van Leeuwenhoek, letter to the Royal Society, 1676

Why this matters

Leeuwenhoek saw bacteria in the 1670s, and nothing followed from it for nearly two hundred years — the clearest case in science of an observation arriving before any framework could use it.

You have the drop of lake water and everything in it. What would you ask?

Ask Leeuwenhoek

• “What did you think the animalcules actually were?”
• “Why scrape your own teeth and look at it?”
• “You had them under your lens for fifty years — did you never connect them to illness?”
• “What did it feel like not to be believed?”
• “How did Hooke finally see what you saw?”

Chapter 4 · Corpuscles, Capillaries and Sperm

Fifty years of looking at everything

1674 – 1700 · Delft

The animalcules are the famous discovery. They are not the only one.

He described red blood corpuscles, measured them, and noted that they are flexible enough to squeeze through the finest vessels. In a tadpole's tail, and in the leg of a frog, he watched blood flowing from the small arteries into the small veins through connecting vessels — independently confirming Malpighi's capillaries, and completing Harvey's circulation with his own eyes.

He described the banded striations of skeletal muscle. He described yeast cells in fermenting beer. He described the structure of wood, of the lens of the eye, and of insect mouthparts.

In 1677 he described spermatozoa in semen — his own, obtained, as he was careful to tell the Royal Society, as a natural consequence of marriage and not by any sinful contrivance. He counted them and estimated their number. This was genuinely important and genuinely awkward, and the Society published it.

He also used what he saw to argue against spontaneous generation. It was still widely held that weevils arose from wheat and fleas from dust. Leeuwenhoek followed the life cycles under the lens and showed eggs, larvae and adults in sequence, in insect after insect. Every creature he examined came from another of its kind.

He was working entirely alone, on whatever came to hand, with no research programme at all.

Why this matters

Leeuwenhoek's tracking of insect life cycles under the lens was serious evidence against spontaneous generation more than a century before Pasteur settled it.

You have fifty years of looking at whatever was in front of him. What is your question?

Ask Leeuwenhoek

• “How did you show that weevils do not arise from wheat?”
• “What did you see in a tadpole's tail?”
• “How did you decide what to look at next?”
• “Why were you so careful about how you described the semen work?”
• “Did you ever look at something and decide not to report it?”

Chapter 5 · No Pupil, No Theory, No Successor

What one man's eyes cannot do

1700 – 1723 and after · Delft

Leeuwenhoek kept writing to London until the end. His last two letters, describing the disease that was killing him — a rare condition of uncontrolled diaphragm contraction, now sometimes called Leeuwenhoek's disease — were dictated and sent by his daughter Maria after his death on 26 August 1723. He was ninety.

He left twenty-six microscopes to the Royal Society, but not the method. He had taken no pupil, founded no school, held no post, and belonged to no tradition. He distrusted theory and said so; he thought people who explained things they had not seen were untrustworthy.

The consequence is stark. Almost nothing followed from his work for a century. The microscopes he left were dispersed and lost — only about a dozen survive anywhere. His technique died with him, and microscopy did not exceed his magnifications until achromatic lenses were developed in the 1830s.

He had bacteria under his lens in 1676. Pasteur and Koch established germ theory in the 1860s and 1870s. Nearly two hundred years separate the seeing from the understanding, and the reason is not that his work was lost — it was published, in the Royal Society's *Philosophical Transactions*, available to everyone.

It is that an observation is not an explanation. Leeuwenhoek is the standing proof that seeing something first is not the same as knowing what it is, and that a discovery with no theory and no successors can simply sit there.

Why this matters

Leeuwenhoek's isolation — no pupils, no theory, no shared method — meant that two centuries passed between seeing bacteria and understanding them.

You have the letters, the lost lenses and the long silence after. What would you ask?

Ask Leeuwenhoek

• “Why take no pupil in fifty years?”
• “What is wrong with theory?”
• “Would you tell me the lens method now?”
• “Does it matter that nothing followed from your work for a century?”
• “What was the strangest thing you ever put under a lens?”

What Leeuwenhoek changed

Leeuwenhoek was the first human being to see bacteria, protists, spermatozoa and red blood corpuscles, and he independently confirmed the capillaries that completed Harvey's circulation. He also demonstrates the limit of pure observation: because he had no theory, no pupils and no shared method, almost none of it went anywhere for nearly two hundred years, until germ theory finally supplied a framework his findings could fit into.

A debate that continues

How Leeuwenhoek actually made his finest lenses is still not settled, and the extent of his acquaintance with his fellow Delft citizen Vermeer remains a matter of speculation.

Keep exploring — ask Leeuwenhoek

• “What would you look at first with a modern microscope?”
• “Can an observation be worth anything without a theory?”
• “Who should have carried on your work?”

Related lives

• Robert Hooke — Who Named the Cell and Weighed the Spring
• Marcello Malpighi — Founder of Microscopic Anatomy
• Martinus Beijerinck — Who Found The Thing That Was Not A Bacterium
• William Harvey — Discoverer of the Circulation

Related themes

Cells and microscopy · Micro-organisms · Observation and explanation