…my work, which I’ve done for a long time, was not pursued in order to gain the praise I now enjoy, but chiefly from a craving after knowledge, which I notice resides in me more than in most other men. And therewithal, whenever I found out anything remarkable, I have thought it my duty to put down my discovery on paper, so that all ingenious people might be informed thereof.

Anton van Leeuwenhoek. Letter of June 12, 1716

Anton van Leeuwenhoek was a one-of-a-kind scientist. He initially traded in Delft, Holland, following the family tradition. He had received no higher education or university degrees and knew no languages ​​other than his native Dutch. This would have been enough to exclude him from the scientific community of his time. However, with skill, diligence, boundless curiosity and an open mind, free from the scientific dogma, he made himself the protagonist of some of the most important discoveries in the history of biology. It was him to discover bacteria, protists, spermatozoa, blood cells, nematodes, rotifers and much more. Two peculiarities that distinguished him were his curiosity to observe everything that could be placed under his lenses and his care in describing what he observed. Because he was not good at drawing, he hired an illustrator to prepare sketches of what he observed, which would accompany his written descriptions. His research, which became widespread and made him very famous at the time, brought a new world of microscopic life to the knowledge of people.

Leeuwenhoek was born in Delft, in the Netherlands on October 24, 1632. In 1648, he was an apprentice in Amsterdam at a textile dealer. There, he worked with magnifying glasses, which were used for quality control of the fabrics, to check the wire density. In 1654, he returned to Delft, where he spent the rest of his life, initially becoming a linen trader. He also worked as a surveyor, wine taster and minor citizen officer. In 1676 he served as trustee of the estate of the bankrupt Jan Vermeer, the famous painter, who was born in the same year as Leeuwenhoek and is believed to be his friend.

In 1668, Leeuwenhoek visited London (the only time in his life), where he probably learnt about Robert Hooke’s Micrographia (1665), the first scientific bestseller that collected drawings of organic samples observed under a microscope, which aroused his interest. Back to Delft, he learned to produce lenses, which he then used to make the first rudimentary microscopes.

One of Robert Hooke’s drawing from Micrographia: the head of a fruitfly with detailed eyes structure

Compound microscopes (i.e. those that use more than one lens) had been invented around 1595, almost forty years before Leeuwenhoek was born. Several of his predecessors and contemporaries, in particular Robert Hooke in England and Jan Swammerdam in Holland, had built compound microscopes that allowed them to make important discoveries. However, due to various technical difficulties in their construction, they could not magnify an object more than 20-30 times. At most Hooke managed to obtain a magnification of 40-50 times. Leeuwenhoek’s ability in lens production, together with his sharp vision and great care in regulating the lighting of the environment, allowed him to build microscopes that magnified the object more than 200 times (possibly up to 270-300), obtaining clearer and brighter images than what his colleagues managed to achieve.

Leeuwenhoek, preferred to use small and spherical lenses to make single lens microscopes. He kept the details of how he made his lenses secret, but today we can be reasonably sure of how he did it. With a flame, he heated the central part of a glass rod until it melted, then he pulled the ends in opposite directions to form a long, thin wire of molten glass. When the thread broke in the middle, he placed one end in the fire, so that a small glass sphere was formed. This sphere was in fact a lens, which then required more work to adjust it. The smaller the sphere, the greater the magnification it could offer.

Leeuwenhoek produced over 500 microscopes, of which only 11 have survived to the present. Compared to modern microscopes, these were extremely simple devices. The lens was mounted on a small hole in the brass plate that made up the body of the instrument. The sample to be analyzed was then placed in front of the lens and its position and focus was adjusted by turning two screws. The whole instrument was only 3-4 inches long and had to be kept close to the eye (as shown in the painting below); it required good lighting and great patience.

Leeuwenhoek with his microscope. Oil painting by Ernest Board. Credit: Wellcome Library, London.

Starting from 1673, Leeuwenhoek wrote informal letters to the newly formed Royal Society of London, describing what he observed with his microscopes: the first letter contained some observations on bee stings. In 1680 he was elected a full member of the Royal Society, thus joining Robert Hooke, Henry Oldenburg, Robert Boyle, Christopher Wren and other luminaries of his day, despite never attending a meeting. In fact, he maintained his collaboration (which continued until his death) exclusively by correspondence. His letters, written in Dutch, were translated into English or Latin and published in the Philosophical Transactions of the Royal Society. Here are some examples of what Leeuwenhoek wrote.

In a letter of September 7, 1674, he reported some observations on lake water, including an excellent description of the Spirogyra green alga: “Passing just lately over this lake, …and examining this water next day, I found floating therein divers earthy particles, and some green streaks, spirally wound serpent-wise, and orderly arranged, after the manner of the copper or tin worms, which distillers use to cool their liquors as they distil over. The whole circumference of each of these streaks was about the thickness of a hair of one’s head… all consisted of very small green globules joined together: and there were very many small green globules as well.”

In 1674, for the first time he also observed some single-cell organisms. Just as happened with Hooke’s Micrographia, many members of the Royal Society refused to believe in the existence of the microscopic creatures described by Leeuwenhoek. It was not until 1677 that their existence was fully accepted, when Robert Hooke returned to his microscopes, which he had given up because of eye strain, and verified Leeuwenhoek’s observations. In 1674 Leeuwenhoek also examined the red blood cells, which had been discovered six years earlier by his Dutch colleague, Jan Swammerdam. With his superior lens, Leeuwenhoek was able to provide a more detailed description of the cells and also to accurately determine their size.

On September 17, 1683, Leeuwenhoek wrote to the Royal Society about his observations on a plaque between his teeth, “a little white matter, which is as thick as if’t were batter”. He repeated these observations on two women (probably his wife and daughter) and on two old men who had never brushed their teeth in their lives. Looking at these samples under a microscope, Leeuwenhoek reported: “I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving. The biggest sort… had a very strong and swift motion, and shot through the water (or spittle) like a pike does through the water. The second sort… oft-times spun round like a top… and these were far more in number.” In the mouth of one of the old men, Leeuwenhoek found “an unbelievably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time. The biggest sort… bent their body into curves in going forwards… Moreover, the other animalcules were in such enormous numbers, that all the water… seemed to be alive.” This was one of the first descriptions we have of bacteria. Actually, the scientist had already observed them in the water 7 years earlier, when he estimated that more than 10,000 would be needed to fill the volume of a small grain of sand. It took a century before anyone else after him was able to view them under a microscope, demonstrating how powerful his tools were.

A letter dated December 25, 1702, instead reports the descriptions of various protists, including the one called Vorticella: “In structure these little animals were fashioned like a bell, and at the round opening they made such a stir, that the particles in the water thereabout were set in motion thereby… And though I must have seen quite 20 of these little animals on their long tails alongside one another very gently moving, with outstretched bodies and straightened-out tails; yet in an instant, as it were, they pulled their bodies and their tails together, and no sooner had they contracted their bodies and tails, than they began to stick their tails out again very leisurely, and stayed thus some time continuing their gentle motion: which sight I found mightily diverting.”

Leeuwenhoek observed under his microscopes tissues of animals and plants, mineral crystals and fossils. He was the first to observe microscopic foraminifera (a type of protozoa), which he described as “little cockles… no bigger than a coarse sand-grain”. He was also the first to observe spermatozoa in animals. He considered the latter one of the most important discoveries of his career, and described the spermatozoa of molluscs, fish, amphibians, birds and mammals, coming to the conclusion that fertilization took place when spermatozoa entered the egg cell. He also discovered microscopic animals, such as roundworms and rotifers. The list of his discoveries could go on and on. In 1698, he showed blood circulation in the capillaries of an eel to Tsar Peter the Great of Russia, who visited his laboratory, and continued to receive visitors (including other monarchs, such as James II of England and Frederick II of Prussia), curious to see the mysterious creatures he described in his letters. He continued his studies until the last days of his life. After his death on August 30, 1723, the pastor of the New Church in Delft wrote to the Royal Society:

“Anton van Leeuwenhoek considered that what is true in natural philosophy can be most fruitfully investigated by the experimental method, supported by the evidence of the senses; for which reason, by diligence and tireless labour he made with his own hand certain most excellent lenses, with the aid of which he discovered many secrets of Nature, now famous throughout the whole philosophical World.”