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Every chapter on microbes starts with someone who first saw them. That person was not a professor. Antonie van Leeuwenhoek was a cloth merchant in Delft who ground his own lenses, wrote only in Dutch, and was doubted for years. This post covers what he saw, how others checked it, and a worked example of how small his "little animals" really were.
Who was Leeuwenhoek?
The UC Museum of Paleontology biography says he was born in Delft on 24 October 1632, apprenticed in a linen-draper's shop in 1648, and set up as a draper in Delft around 1654. He also worked as a surveyor, a wine assayer and a minor city official. He had no university degree and knew no language but Dutch.
At some point before 1668 he learned to grind lenses. The biography suggests he was inspired by Robert Hooke's illustrated book Micrographia. He made over 500 microscopes, of which fewer than ten survive. Each was a single tiny lens set in a brass plate, held close to the eye, and some magnified over 200 times, with clearer images than the compound microscopes of the time.
What did his 1676 letter report?
From 1673 Leeuwenhoek sent letters to the Royal Society of London. The famous one, dated 9 October 1676, was translated and published in Philosophical Transactions in 1677 under the title "concerning little animals by him observed in rain-well-sea- and snow water; as also in water wherein pepper had lain infused."
In a 2015 review for the journal's 350th anniversary, Nick Lane calls it "the first detailed description of protists and bacteria living in a range of environments". He also notes that the letter was cut to about half its length by the Society's secretary, Henry Oldenburg.
The letter reads like a diary, and it shows a careful experimenter. As Lane quotes it, Leeuwenhoek caught rain "in a new Porcelain dish, which had never been used before, but found no living creatures at all in it". Starting with a clean vessel was a control: when creatures appeared days later, he knew they had not come from a dirty dish. He also soaked peppercorns, crushed and whole, and watched the animalcules multiply.
Worked example: how small were his little animals?
Leeuwenhoek had no micrometre scale, so he compared sizes with a grain of coarse sand. Lane quotes him: "even if 100 of these very wee animals lay stretched out one against another, they could not reach the length of a grain of course sand". In a later clarification he set the size of such a grain so that 80 of them in a row make one inch.
One grain: 1 inch / 80 = 25.4 mm / 80 = 0.3175 mm.
One animalcule: less than 0.3175 mm / 100 = 0.003175 mm.
In micrometres: 0.003175 mm × 1,000 = about 3 µm.
That is bacterial size. NCERT Class 11 Biology gives bacteria as 3 to 5 µm, so his estimate is the right scale.
Lane also reports that Leeuwenhoek later admitted he had played down his numbers. He had written of more than 1,000,000 creatures in one drop of pepper-water, and said he might truthfully have put it at eight times as many: 8,000,000.
Common mistakes with sizes like these:
Losing a factor of 1,000. 1 mm = 1,000 µm, and 1 µm = 1,000 nm.
Confusing magnification and resolution. Enlarging an image does not show finer detail unless the lens resolves it.
Treating "smaller than" as "equal to". His statement gives an upper limit, not an exact size.
How did the Royal Society check his claims?
Not on trust. Lane describes how the Society asked the botanist Nehemiah Grew to reproduce the observations, and Grew failed. In 1677 Hooke tried, and on his third attempt with pepper-water he saw the animalcules and demonstrated them to the Fellows. Leeuwenhoek also sent eight written testimonies from respected local witnesses. He was elected a Fellow of the Royal Society in 1680.
He kept observing. A letter of 17 September 1683, published in Philosophical Transactions in 1684, described "animals in the scurf of the teeth": bacteria from dental plaque. NCERT Class 11 Biology credits him as the first to see and describe a live cell.
Why did his discoveries fade?
Leeuwenhoek never explained how he made or lit his lenses, and nobody matched his instruments for a long time. Lane writes that his work was doubted or dismissed in later centuries until twentieth-century studies confirmed his bacterial observations. In 1981 the microscopist Brian Ford found Leeuwenhoek's own specimens at the Royal Society and, using a surviving microscope, showed a resolution better than 1 µm. Leeuwenhoek died in 1723 at the age of 90.
What can a student take from Leeuwenhoek's way of working?
You do not need a title to do careful science. You need patience, good tools and honest notes.
Use controls. The clean dish made his results believable.
Share your method. His secrecy cost him: others could not repeat his work, and it was nearly forgotten.
If the scale of microbes, cells and molecules is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the person whose book started Leeuwenhoek looking, read our post on Robert Hooke's Micrographia, and for exam biology on microbes, see microbes in human welfare.
Try this now: a bacterium is 2 µm long. How many would fit across a 1 mm pinhead, and how many across Leeuwenhoek's 0.3175 mm grain of sand?
