In October, 1676, the Royal Society received an astonishing letter which began, “In the year of 1675 I difcover’d living creatures in Rain water….” What was so astonishing was not that someone had seen small “living creatures,” but that the author stated matter-of-factly that there were “ten thousand times [more than those] reprefented by Monf. Swamerdam [a well-known Dutch scientist and microscopist].” Furthermore, these were the claims, not of a renowned scientist challenging the work of a peer, but of a man whose occupation was selling ribbons and buttons!1Clifford Dobell, Antony van Leeuwenhoek and His ‘Little Animals’ (New York: Russell & Russell, Inc., 1958) facing p. 113 The members of the Society could be excused for not believing their correspondent. After all, what we take for granted—bacteria, cells, protozoa—was completely unknown. Typical of the skepticism was Royal Society member Christiaan Huygens’s letter: “I should greatly like to know how much credence our Mr. Leeuwenhoek’s observations obtain among you…after vainly trying to see some of the things which he sees, I much misdoubt me whether they be not illusions of his sight….”2Dobell, p. 172 The Royal Society refused to accept Mr. Leeuwenhoek’s claims until one of their own members could duplicate his experiments. At first they failed. The Society could not see any “little animals.” The future of microbiology hung in the balance.

At the center of this tempest was the Dutch Calvinist Antoni van Leeuwenhoek. He was born in Delft, Holland, on October 24, 1632, during the middle of the Thirty Years’ War and baptized in the Reformed Church. At age sixteen he was sent to Amsterdam to learn the cloth trade, working as the shop’s accountant and cashier. Significantly, he also would have started using a magnifying glass in his work, examining the quality of the cloth. Upon returning to Delft in 1654, he married and set up his own shop selling cloth, ribbons and buttons. He seemed set for a typical, ordinary merchant’s life except for one exceptional gift…he had an extraordinarily inquisitive, disciplined mind. This God-given gift would soon lead him to other pursuits and world-wide fame.

Rainier de Graaf

In 1677, or 1668, he sailed to England. No one knows why he took the trip, but perhaps he went on business. If so, his fellow cloth tradesmen would no doubt have been talking about the recent bestseller, Micrographia, by Robert Hooke. For in the book Hooke relates his observations of things he had seen while using a microscope: mold, cork, and…cloth! He had learned more about the structure and design of cloth than any merchant. No doubt the cloth merchants would have been a buzz about such new knowledge and shown Antoni the book, perhaps planting the seed to pursue microscopy. For several years later, in 1673, we suddenly learn about his own investigations using a microscope in a letter sent to the Royal Society by the famed physician, Rainier de Graaf.

Antoni Leeuwenhoek had met Mynheer de Graaf at, of all places, a meeting where doctors were dissecting dead bodies. (Why a cloth trader suddenly began frequenting such meetings is a mystery. Was he interested in anatomy? Did he contemplate a career change? Did a friend compel him, or did he want to train his newly made microscopes upon the dead? No one knows.) Antoni had shown his physician friend his microscopes and some of his observations. Rainier de Graaf was so impressed that he was compelled to write to the Royal Society, “I am writing to tell you that a certain most ingenious person here, name Leeuwenhoeck, has devised microscopes which far surpass those which we have hitherto seen….”3Dobell, p. 41 He also enclosed Leeuwenhoek’s first letter to the Royal Society.

The Royal Society was only officially eleven years old at the time of de Graaf’s letter. Although started as an informal “chat” group in 1645, it was put on a firm foundation by Royal Charter in 1662 by King Charles II. At the time, universities did not have science departments. So, one of the few means of fostering education and information about scientific endeavors was through private associations. The Royal Society was one such association. Their principle means of communicating members’ activities was through their publication “Philosophical Transactions of the Royal Society.” This is where Antoni’s first letter appeared.

Van Leeuwenhoek Letter
Leeuwenhoek’s letters to the Royal Society Courtesy of the Delft University Library

Dated April 28, 1673, his letter described three things that he had seen using his hand-made microscopes: mold, bees, and lice. This was the first letter of hundreds that Leeuwenhoek would write to the Society over the next fifty years—letters rich with scientific observations and the circumstances and settings of those experiments. But, they were not scientific papers, per se. In his first reply to the Society on August 15, 1673, he admitted his lack of training: “…I have no style, or pen, wherewith to express my thoughts properly; secondly, because I have not been brought up to languages or arts, but only to business; and in the third place, because I do not gladly suffer contradiction or censure from others….”4Dobell, p. 41, 42 Instead of scientific papers, his letters read like a journal. He jumped from subject to subject, recording his straightforward, clear observations. His letters also included drawings that he had asked artists in Delft to make for him—parts of the bees and lice and other things that he would discover.

And what discoveries he would make over the ensuing fifty years! As one biographer wrote: “It can be seen…that Leeuwenhoek undoubtedly deserves the name of ‘the father of microbiology’; he acquired great merit as a histologist, and he did extremely valuable work as a botanist. He also advanced the study of both pure and applied entomology, and discovered not only spermatozoa, but also parthenogenesis, and the asexual propagation by means of budding; in addition to all this, he also set up a new theory of generation, and discovered numerous organisms.”5A. Schierbeek, Editor, Measuring the Invisible World (London and New York: Abelard-Schuman, 1959), p. 195

One year later, Leeuwenhoek wrote the letter that would throw his credibility into question. In 1674, he had visited Berkelse Lake. The lake, although clear during winter, would fill with clouds during the summer. Locals thought the dew caused the cloud formations in the water. Leeuwenhoek doubted this theory, so he tested the waters. Training his microscopic lens upon the lake water collected in a phial (and later the rain water he mentions in his letter) he discovered what he called his “little animals.” Today, we know them as protozoa, but in his day they were a complete revelation. No one had a clue that such creatures existed, let alone that you might be drinking them! He described his creatures so well that today’s scientists can often tell what kind of protozoa he saw. He went on to observe water from different sources: well, snow, sea. The best place to find the “animalcules,” he wrote, was in water laced with black pepper.

One of Antoni van Leeuwenhoek’s microscopes Courtesy of Brian J. Ford

He was delighted by his “little animals.” He wrote that “among all the marvels that I have discovered in nature, [these are] the most marvelous of all.”6Ibid., p. 66 Later on he would declare: “From all these observations, we discern most plainly the incomprehensible perfection, the exact order, and the inscrutable providential care with which the most wise Creator and Lord of the Universe had formed the bodies of these animalcules, which are so minute as to escape our sight….”7Ibid., p. 171

No doubt the Royal Society would have agreed with his assessments…if they could only see them! Since the arrival of his letter, the Society had failed to reproduce his experiment with the pepper water. They were handicapped, though, be-cause they did not have Leeuwenhoek’s remarkable microscopes. No one knows just when or how he learned to make his lenses. In his first letter to the society he mentions in passing that “they were recently invented.” Unlike a modern microscope, Leeuwenhoek’s microscopes were simple affairs. There were no eyepiece, diaphragm, compound lenses, etc. Rather, they were small, only several inches long. The lens was sandwiched between two metal plates, usually made of brass or silver. In front of the lens, a pin was stuck on a platform. On the end of the pin one would attach the specimen, then, using screws to adjust both the vertical and horizontal position, place the specimen closer or farther away from the lens. His microscopes were really just powerful magnifying glasses. During his lifetime he made nearly 250 microscopes of which nine survive. The best of his microscopes enlarges by 266 times.

While the Society continued its efforts to see Leeuwenhoek’s “little animals,” Antoni embarked on two efforts to convince the Society of the veracity of his claims and thereby preserve his reputation. First, he collected affidavits from others stating that they had seen his “little animals.” He sent eight letters from various witnesses that included public notaries, lawyers, doctors and ministers…but no scientists! Second, he thoroughly explained his method of observation.

It was Antoni van Leeuwenhoek’s exacting, methodical approach to his observations, and his meticulous recording and measuring of what he saw, that set him apart from other scientists of his day. Since he was observing things that no one else had ever seen, he invented his own system of measurement. For example, he would compare things to a grain of sand (1/30th of an inch) or the eye of a louse. He would divide his subject into sections, estimate the number of creatures in each sector, then multiply by the number of sectors to arrive at an estimate of the number of creatures in each sample. His explanation of his measuring method went on for up to four and five pages in several letters to the Society. But how had he developed such precision? Where did this ability to measure come from? We see a glimpse of his attention to detail in keeping the books for the cloth shop, and more clues are found in his side jobs.

Packets of dried algal “paper” specimens
Packets of dried algal “paper” specimens that Leeuwenhoek sent to London Courtesy of Brian J. Ford. Please visit his website at:

Besides his work as a haberdasher, Antoni had been awarded some plum posts in local government. In a letter in 1669, he mentions that he had been appointed surveyor by the City Council. He was also appointed the wine-gauger in 1679, holding the position for the rest of his life. The wine-gauger had to assay all wines and spirits entering the town, and to calibrate the vessels in which they were contained.8Dobell, p. 37 Measuring seemed to come easily to him and, as we have seen, he used it to great effect in his work as a microscopist.

In November of 1677, the Royal Society turned to Robert Hooke to duplicate Antoni’s results with the pepper-water. At the insistence of the Society, better microscopes were found and put to use. Finally…success. Hooke demonstrated, without doubt, that Leeuwenhoek’s “little animals” did exist and, just as importantly, in the size and number that he had described. The breakthrough caused a sensation. The name of “Leeuwenhoek” was on every tongue. Even the King, patron of the society, came to view the experiment and declare his marvel. The Royal Society would go on to make him a full member a few short years later.

Naturally, Leeuwenhoek was gratified. With justified stubbornness, he knew he was right: “…my efforts are ever striving towards no other end than, as far as in me lieth, to set the Truth before my eyes, to embrace it, and to lay out to good account the small Talent that I’ve received: in order to draw the World away from its Old-Heathenish superstition, to go over to the Truth, and to cleave unto it.”9Ibid., p.74 That did not mean all doubts melted away. He had convinced the Royal Society and, through its publication, the rest of the scientific community throughout Europe, but there were still his neighbors. For the common folk, it was all too much. Many thought that he was telling fairy tales—that he was “seeing more with his imagination than with his magnifying glass.”10Ibid., p. 64 Leeuwenhoek would retort: “I…can as plainly see them…as with the naked eye we behold…gnats sporting in the open air….”11Samuel Hoole, trans., The Select Works of Anthony van Leeuwenhoek, Vol. 1 (first printed 1798; reprint New York: Arno Press, 1977), p. v. But such doubts were minor distractions. Frankly, he seems to be having too much fun to be concerned about what others thought. He was consumed with making more and more powerful microscopes and using his lenses to discover one new thing after another.

Antoni van Leeuwenhoek portrait, 1686, by Jan Verkolje

He was the first to look at blood cells. He pricked himself and smeared the blood on a glass plate. As always, he tried to accurately measure what he called “globules” (red blood cells). He said that one “globule” was twenty-five thousand times smaller in volume than a grain of sand. Modern measurements show that he was almost exactly right.

One of his most important contributions was regarding the generation of life. At the time, people thought that worms, insects and other “lower” forms of life did not need parents—that they could spring up from decaying matter. Leeuwenhoek disagreed. “A flea or a louse can no more come…from… dirt than a horse from a dunghill.”12Schierbeek, p. 196 He believed that God had made all living things at creation, and all living things were descended from the ones created during the six days of creation. “From these observations we may be fully assured…that God, the all-wise Creator of the Universe, does not create any new species of things on this earth, but that, at the beginning, He so ordained and constituted all things that, his Creation being perfect, the seeds of plants…contain in themselves…the future plant…And this I take to be a certain truth, which prevails not only in plants, but in all living creatures whatsoever.”13Ibid., p. 183 It was also believed that the male did not contribute anything vital in procreation, except for maybe some nebulous “force” or life-giving “spirit”. He proved this wrong. He was the first to discover spermatozoa, examining semen from thirty different animals.

By now, Leeuwenhoek was squarely set in middle age. His first wife had died in 1693, and he had remarried a Calvinist preacher’s daughter. The only child to survive infancy from either marriage was a daughter named Maria. She would grow up to serve and care for her father, never marrying. Besides his wine-gauger job, he had also been appointed “Chamberlain of the Council-Chamber of the Worshipful Sheriffs of Delft.” His municipal positions were probably sinecures. There is also some evidence that he inherited money when his first wife had died.14Ibid., p 20 The income allowed him to quit his drapery business (there is no mention of his shop after his early adult years) and to apply most of his available hours to his scientific pursuits.

His deep curiosity led him to examine without hesitations or embarrassments. Once he was sick and had diarrhea. Yes, he studied that too. And, of course, he found “little animals” there, as well. He never seemed to have guessed, though, that the little worm-like creatures he saw might have made him sick. He was the first to discover micro-organisms inside the human body.

His own body provided source material for other discoveries. He examined his skin, his hair, his mouth and his teeth. One day he noticed “…a little white matter, which is as thick as if ‘twere batter”15Dobell, p 239 in his mouth. Ever curious he scraped some off and mixed it with his saliva. Voilà! More of his “little animals”. He then went to other people, asking for samples. He got a sample from one man who said that he had never washed his mouth in all his life. (Can you imagine Mr. Leeuwenhoek trolling the streets asking, “Excuse me, but may I scrape your gums, sir?”) He noticed more “little animals” in those who had not washed their mouths than those who had. He guessed that these creatures caused people’s stinking breath, but acknowledged that even clean mouths held “animalcules” by the millions.

Leeuwenhoek was buried in the Old Church of Delft on Sunday, August 31, 1723

During his lifetime he studied sixty-seven kinds of insects, recording the different stages of their growth. There was no end to different types of insects he found in his yard and house. When he could not find lice, he bought them, remarking that, “I had plenty of them brought to me for my money.”16Hoole, p. 57 He was so taken with the lowly house fly that he wrote, “…there is greater cause for admiration…in… so small…an animal, than in…a horse or an ox.”17Ibid., p. 67 He would go to great lengths not only to get different kinds of specimens to study, but also to nourish and keep them alive. During the cold winters he carried around a box of worms in his pocket to keep them warm. He enlisted his wife to keep a box of silkworm eggs under her clothes. Another time he persuaded her to do the same with a box of mites. (“He who finds a wife finds what is good….”!) Once, did he go too far. He wanted to learn how many lice would reproduce in a certain period of time. So, he put two lice on a black sock and wore it for a few days. He made a count of the eggs then put the same sock on. After a few more days he counted how many lice had hatched. He put the sock on again for a few more days. After ten days he had had enough. Disgusted, he threw the sock into the street and took to scratching his leg to kill any remaining lice.

The publication of his numerous discoveries prompted many to come calling at his humble home in Delft, including some of the most notable and powerful: Peter the Great (he gave him one of his microscopes); Queen Catherine of England, wife of Charles II; King James II of England; Czar Peter of Russia; King Frederick I of Prussia; the Elector of Saxony, Augustus II; and, Queen Mary of England. The numerous visits wearied him, especially as he grew old. Yet, he continued making his microscopes, looking and observing and notating until the very end of his life. Indeed, a few hours before he died, in 1723 at 90 years of age, he asked a friend to translate two more letters.

An engraving of Leeuwenhoek’s tomb by J.C. Phillips (1740) after a drawing by the designer, T. Jelgersma.

In his will he ordered that twenty-six microscopes be sent to the Royal Society. He had rarely given any of his microscopes away. For whatever reason—whether pride, insecurity or stubbornness—he never shared the secrets of his microscopes, or more specifically, how he made his lenses. Thus, he unwittingly contributed to a decline in the study of microbiology. No one else had the quality of lens that he had made. Without this essential tool science lost interest in the “little animals”. Amazingly, it was not until the 1840s, nearly 150 years later, that compound microscopes were finally made that rivaled Leeuwenhoek’s.

Nevertheless, the impact of Antoni van Leeuwenhoek’s work was immense. As one biographer marveled: “How Leeuwenhoek was able to discover, with the limited means at his disposal…must remain for ever a marvel. On reading these passages [his letters] the modern protozoologist, knowing the patience and perseverance needed to make such observations—even with adequate instruments, and with the accumulated information of the next 200 years to help him—can only regard this extraordinary old man, as he regarded his ‘little animacules,’ with dumfounded admiration.”18Dobell, p. 239 For Leeuwenhoek, his little animals were more than just curiosities, hobbies or scientific discoveries. Rather, they were part of “the unconceivable Providence, perfection, and order, bestowed by the Lord Creator of the Universe….”19Dobell, p. 267


1 Clifford Dobell, Antony van Leeuwenhoek and His ‘Little Animals’ (New York: Russell & Russell, Inc., 1958) facing p. 113
2 Dobell, p. 172
3 Dobell, p. 41
4 Dobell, p. 41, 42
5 A. Schierbeek, Editor, Measuring the Invisible World (London and New York: Abelard-Schuman, 1959), p. 195
6 Ibid., p. 66
7 Ibid., p. 171
8 Dobell, p. 37
9 Ibid., p.74
10 Ibid., p. 64
11 Samuel Hoole, trans., The Select Works of Anthony van Leeuwenhoek, Vol. 1 (first printed 1798; reprint New York: Arno Press, 1977), p. v.
12 Schierbeek, p. 196
13 Ibid., p. 183
14 Ibid., p 20
15 Dobell, p 239
16 Hoole, p. 57
17 Ibid., p. 67
18 Dobell, p. 239
19 Dobell, p. 267