‘Your most humble servant’: the letters of Antony van Leeuwenhoek

Abstract

Antony van Leeuwenhoek's entire output is contained in the hundreds of letters that he wrote from 1673 to 1723. This article discusses the content, features, and circumstances of the letters and their contemporary publishing history, especially in the Royal Society's Philosophical Transactions, as well as a brief history of the project begun in 1932 to publish a complete edition of Leeuwenhoek's letters in Dutch and English translation with linguistic, scientific, and historical annotations.

Leeuwenhoek, letters, Royal Society, Philosophical Transactions, Alle de Brieven, Collected Letters

Introduction

Antony van Leeuwenhoek (1632–1723), the Dutch pioneer microscopist, has long frustrated those who work with knowledge found in treatises, single-volume distillations of years of thought, organized within a logical framework and full of self-contained arguments. By contrast, Leeuwenhoek's only surviving writings are the almost four hundred letters that he wrote regularly over 50 years, from 1673 to 1723. Most of the letters discuss multiple, often disparate topics. They consist largely of descriptions of what he had recently observed. As his career advanced, Leeuwenhoek would return to a specimen without reference to earlier notes on purpose, so as not to prejudice his eyes. He made no attempt to publish any logical framework of ideas or to give the letters any level of organization higher than simple chronology. While this situation is convenient for understanding Leeuwenhoek's development and growth, it is inconvenient for almost every other purpose.

In the early 20th century, parts of Leeuwenhoek's work were accessible in three languages: the 165 letters that Leeuwenhoek published himself in Dutch, the same letters that he published in Latin translations, and the excerpts from about 120 letters in English translations published by the Royal Society of London. There was also a two-volume English translation of parts of 80 of the 182 letters written between January 1680 and April 1702 organized by topic, not chronology, that Samuel Hoole published in 1800 (Hoole 1800). All of Hoole's letters were among those published by Leeuwenhoek himself, but only a quarter also appeared in Philosophical Transactions.

Around the 200th anniversary of Leeuwenhoek's birth in 1932, a group of Dutch scientists formed a committee to publish a complete edition of all of Leeuwenhoek's letters in Dutch and English translation with copious illustrations and detailed linguistic, scientific, and historical annotations. That edition is nearing completion, a propitious opportunity to look at Leeuwenhoek's corpus as a whole and to clarify Leeuwenhoek's relationship with the Royal Society.

Alle de Brieven/Collected Letters

The idea of producing a modern edition of all of the letters written by Leeuwenhoek arose in 1923 during the commemoration of the 200th anniversary of his death (Palm 2005). In 1927, Gerard van Rijnberk (1875–1953), professor of physiology and the longest serving editor-in-chief in the history of the Nederlands Tijdschrift voor Geneeskunde (Dutch Journal of Medicine), initiated the project to publish all of Leeuwenhoek's letters in Dutch and in English translation. In 1931, the Nederlandse Akademie van Wetenschappen (Royal Dutch Academy of Arts and Sciences) set up a committee of scientists, not historians, led by Van Rijnberk, to prepare the letters for publication, and work began the following year. In 1939, the first volume of Alle de Brieven van Antoni van Leeuwenhoek/The Collected Letters of Antoni van Leeuwenhoek (hereafter, Collected Letters) was published. In its introduction, Van Rijnberk (1939) explained that Leeuwenhoek's language, devoid of modern scientific terms, was difficult for modern readers to understand.

In order to elucidate Leeuwenhoeck’s writings and line of thought, a great number of notes and modern illustrations have been added to the text. The figures are partly photographs of modern microscopic preparations, partly, in cases where we do not possess an original drawing sent by Leeuwenhoeck, of photographs of preparations and objects representing what Leeuwenhoeck must have seen. By using as much as possible his own methods and by eventually producing the same artefacts, we have tried to obtain preparations materializing what Leeuwenhoeck saw and described. Reproductions of these preparations enable the reader to form a clearer opinion of Leeuwenhoeck’s observations. He can also compare with them the objects which he studied as seen by the modern microscopist. Moreover, modern terminology has been supplied throughout. Leeuwenhoeck’s opinions, which sometimes appear to us childish, sometimes incomprehensible, also require to be recast in modern language. By explaining them in connexion with the philosophic systems and physical doctrines universally accepted in his day, they become intelligible. …

The whole edition will be concluded by a volume dedicated exclusively to Leeuwenhoeck and his scientific work. It is to contain a complete life of Leeuwenhoeck, based on the most recent data and biographical discoveries. Moreover, a number of Dutch scientists will sketch the merit of Leeuwenhoeck’s investigations in as many objective summaries.

Half a dozen scientists, and now historians have edited the volumes, resulting in myriad inconsistencies in formatting.

• Volumes 1–2: Gerard Carel Heringa.

• Volumes. 3–5: Abraham Schierbeek.

• Volumes 6–8: J. J. Swart.

• Volume 9: Johannes Heniger.

• Volumes 10–16: Lodewijk C. Palm.

• Volume 17: Huib J. Zuidervaart, Douglas Anderson, and Lizzy Entjes.

• Volumes 18–19: Douglas Anderson and Huib J. Zuidervaart

Too easily forgotten is the contribution of those who transcribed Leeuwenhoek's handwriting and those who translated his 17th and 18th century Dutch into English. These transcriptions were begun by Judith Mendels (1906–1995) and the bulk of them were done by B.C. (Kees) Damsteegt (1915–2003). They did not work from the original manuscripts. Instead, the Royal Society, which has graciously met every request of the editors over the decades, sent the letters in the format shown in Fig. 1. As they did their work, Mendels and Damsteegt also wrote the linguistic footnotes, which led to a series of helpful articles about Leeuwenhoek's language, two of them in English (Mendels 1952; Damsteegt 1982).

Figure 1.

Reproduction of the first part of Leeuwenhoek's letter of 7 July 1722 to James Jurin, secretary of the Royal Society. (Photograph by the author.).

Adriaan Swaen and Arie Querido were the translators for the first four volumes, and Emile van Loo for volumes 5–8. Carry Dikshoorn translated the most letters, about 150 letters in volumes 9–15. Elze Kegel-Brinkgreve translated the letters written after July 1707, volumes 16–19, and I modernized her rough translations of the letters in the final two volumes.

Van Rijnberk's original vision turned out to be too ambitious. The latter volumes have been edited by historians, who have scaled back. Gerrit Lindeboom (1976), an internist-radiologist, at the time chairman of the Leeuwenhoek Committee, announced this new policy in the preface of volume 10.

The number of scientific explanatory notes gradually declined and still declines further. The more so since 3 years ago, the decision has been made to drop this type of notes completely. The rest, of course, did not change.

Volumes 18 and 19, now expected to be published in 2023, have only the illustrations from Philosophical Transactions, blended into the text of both the Dutch and the English translations. Van Rijnberk's concluding volume of biography and scientific summaries has been abandoned.

The earlier translators struggled with vocabulary (Palm 1989b; Henderson 2012), especially when Leeuwenhoek described things that no one had ever seen before. For example, Henry Oldenburg first translated Leeuwenhoek's diertgens (little animals) as ‘animalcula’ in 1674, in one of Leeuwenhoek's early letters (Leeuwenhoek 1939). Oldenburg put it in italics, as he would a word in a foreign language, giving Leeuwenhoek's plain Dutch word a fancy Latin or Latinized translation.

The translators of the Collected Letters also struggled with how to best represent in English Leeuwenhoek's often confusing syntax and ambiguous punctuation. Their solution was to preserve as much of Leeuwenhoek's syntax and punctuation as possible, which made the English translations difficult for the modern reader who expects a period at the end of every sentence. Leeuwenhoek tended to put a period only at the end of a paragraph, often a very long paragraph. The early translators also preserved Leeuwenhoek's capitalization of nouns within sentences.

For the final volumes, editors Huib Zuidervaart and I have modernized the English punctuation and capitalization to make the text more easily readable. We have retained the linguistic footnotes, the historical footnotes, the identification of people mentioned, and the cross-referencing to help researchers find the other letters in which a certain topic, specimen, or phenomenon is also discussed. We have avoided the previous practice of using modern English scientific terms anachronistically as translations of Leeuwenhoek's 18th century Dutch.

Volume 19 has 23 letters from 1720 to 1723, to the Royal Society and to and from the Philosophical Transactions editor James Jurin. It also has 13 letters written in the year after Leeuwenhoek's death in August 1723, most of them involving a lacquered case with 26 microscopes, specimens still attached, that Leeuwenhoek bequeathed to the Royal Society. The case and microscopes have since gone missing (Ford 1983; Robertson 2015; Zuidervaart and Anderson 2016). Volume 19 also has about 40 letters missed in the previous volumes, including five from Leeuwenhoek to various people, and almost 2 dozen from Leeuwenhoek known only by reference in other letters. Finally, volume 19 lists almost a hundred and fifty letters to Leeuwenhoek between 1674 and 1717 that are also known only by reference in other letters.

A Leeuwenhoek letter

Most of the hand-written manuscripts that Leeuwenhoek sent to the Royal Society are still in its library and several dozen others are in libraries in the Netherlands, Germany, Italy, and England. Before 1679, Leeuwenhoek wrote letters on folio pages, but after he was elected to membership in the Royal Society in 1680, he wrote on quarto-sized pages, roughly 24 by 30 cm. Many letters exceeded 20 quarto pages, not counting the figures. He wrote on both sides of the paper in a black ink that has not faded. Most of the Dutch manuscripts are in his hand, as shown in Fig. 2, though some are fair copies made by others. The handful of letters in Latin and Italian were written by others. Half a dozen times in the early 1690s, Leeuwenhoek sent to the Royal Society the printed letter in Dutch from one of his own publications. The Royal Society published none of them in the Philosophical Transactions.

Figure 2.

Leeuwenhoek's handwritten Letter 264 of 20 April 1706 to the members of the Royal Society. (Photograph by the author.)

Each of the 165 letters that Leeuwenhoek published himself in Dutch and Latin is preceded by a summary of its contents. These summaries are not in the manuscripts, so they are not included in the Collected Letters. Leeuwenhoek numbered 119 letters himself, beginning with letter number 28 of 25 April 1679 through letter number 146 of 20 April 1702. (For the fate of the first 27 letters, see Dobell (1930).) Leeuwenhoek switched to the Roman numerals I–XLVI for the 46 letters in the Send-Brieven between 1712 and 1717. By convention, Leeuwenhoek's numbers are in square brackets following the serial number in Collected Letters.

Leeuwenhoek often began a letter with the honorifics common at the time, e.g. ‘highly learned sir’ (Hoog Geleerde Heer) and ‘very noble gentlemen’ (Hoog Edele Heeren). He addressed letters to members of the Royal Society, usually collectively but also to individual secretaries, presidents, and editors: Henry Oldenburg until his death in 1677, then Robert Hooke, Thomas Gale, Christopher Wren, Francis Aston, Richard Waller, Hans Sloane, James Jurin, and his translator after 1700, John Chamberlayne. He addressed letters to foreigners, usually after they visited him, such as Gottfried Wilhelm Leibniz, Francesco Corner, and Johann Wilhelm von Pfalz-Neuburg. Especially later in this career, he addressed letters to well-known Dutch people like Antonie Heinsius, Peter Rabus, and Herman Boerhaave.

The first paragraph, especially of the letters to the Royal Society, usually dealt with business: thanks for recent letters received, notes of recent letters not replied to, copies of Philosophical Transactions received or not, and grateful recognition of the Royal Society's acknowledgment of his work. This opening paragraph was never included if the article was printed in Philosophical Transactions. The bulk of every letter was the description of Leeuwenhoek's specimens and methods and an often daily, even hourly account of his observations. When he departed from what he literally saw, he usually marked his inferences and speculations with ‘I thought’ or ‘I imagined’ (beeld ik mijn selven in). Typically, each letter was an account of Leeuwenhoek's often disparate recent activity. An example is Letter 17 [11] of 26 March 1675 to Henry Oldenburg about marrow fats, peas, and runners; the structure of venous blood from a pregnant woman; an explanation of the transparency of objects; an account of the circulation in oak leaves and a comparison to veins in humans; the circulation of sap in an oak leaf; an examination of the veins in the connective tissue between the muscles and how these muscles are nourished by the blood; and explanations of an enclosed ink impression of oak leaves and drawings of the vein of an oak leaf and of a louse's leg.

More than half of the letters, 163 of them, were illustrated by 1227 figures, although several of them are ambiguously labeled. They are often arranged on fold-out plates, as in Figs 4 and 5 below. Leeuwenhoek drew some of the simple 2D figures early on, but he employed draughtsmen for almost all of them. Dutch historian Sietske Fransen (2019) recently examined the surviving original red-chalk drawings at the Royal Society (most are lost) and found that Leeuwenhoek used four different draughtsmen. Only the final one is known by name: Willem van der Wilt (1691–1727). Boitet (1729) wrote (my translation) that Van der Wilt drew ‘most all of the plates in the celebrated work of Mr. Leeuwenhoek, through magnifying glasses wonderfully drawn from life.’ He noted Van der Wilt's early talent that he was so ‘advanced that few were his match’.

Several other features are common in the letters. Frequently, Leeuwenhoek's observations involved a response to other researchers, either citing or implicitly referring to their publications, so much so that Laurens Baas-Becking (1924) famously observed that Leeuwenhoek was but ‘a pair of eyes, a pair of hands, directed by other minds. For when his own mind tried to direct, he could produce nothing but chaos.’ Listing 33 books mentioned in Leeuwenhoek's letters, Lodewijk Palm (1989a) disagreed, seeing Leeuwenhoek ‘as a loner, a student of natural history working in his well-chosen isolation, which was a prerequisite for the independent valuation of the wonders of nature he saw through his small lenses.’ A more charitable view is that Leeuwenhoek was a curious researcher who was actively involved in the learned discourse of his day. The letters from and to Leeuwenhoek give evidence that at the Royal Society, Henry Oldenburg, Robert Hooke (Leeuwenhoek's most enthusiastic supporter), Nehemiah Grew, Robert Boyle and later Richard Wallter, Hans Sloane, and James Jurin influenced Leeuwenhoek's research agenda. He had an extended exchange of letters with Gottfried Wilhelm Leibniz, Antonio Magliabechi, and Antonie Heinsius. In Delft, he worked with physicians such as Regnier de Graaf, Cornelis's Gravesande, Theodore Craanen, and Cornelis Bontekoe, and with scholars such as Constantijn Huygens and his son Christiaan earlier in his career and with Frederik Adriaan van Reede van Renswoude, Peter Rabus, Abraham van Bleyswijk, and Herman Boerhaave later in his career.

The public and their opinions and misconceptions were of so much concern to Leeuwenhoek that he wrote about his frustrations in dozens of letters, such as these two examples from Letters 152 and 157 [95] in 1695 about people who thought that life could be generated spontaneously (Leeuwenhoek 1983):

I shall indeed feel greatly satisfied if I should happily have achieved by my labors that some people, thus better informed … give up these old wives’ tales of foolish things.

We must be sorry that today there are still found among us so many people—some of whom are considered scholars and ought to oppose the old errors and to embrace the truth—who are so stupid.

In most of the letters, Leeuwenhoek quantified the objects and structures that he observed by comparing their dimensions to common objects such as a grain of sand or a hair from his beard. Each of the volumes of Collected Letters has a page titled ‘Weights and measures used by Leeuwenhoek’ among the back matter. For more on Leeuwenhoek's micrometry, see Davis (2020) and Dobell (1932). Leeuwenhoek was especially concerned with communicating the size and number of the little animals that he saw in water from various sources. In 3 dozen letters, Leeuwenhoek performed calculations for which he wrote out the arithmetic in the margin of the page. For example, in a 1679 letter to Nehemiah Grew, Leeuwenhoek calculated the number of little animals in the milt of a cod as 150 000 000 000 and compared it to his calculation of the number of people on Earth, 13 385 106 000 (Leeuwenhoek 1948).

A final common feature of Leeuwenhoek's letters were his references to visitors to his house. Almost all of them were unnamed (‘a certain learned gentleman’ or ‘two very famous professors’), but dozens of them can be identified. In 1711, the 78-year-old Leeuwenhoek (Leeuwenhoek 2014) wrote to James Petiver of the Royal Society, ‘in a period of 8–10 days before your visit in 4 days’ time 26 people, all of whom had a letter of recommendation, except a duke and an earl with their tutor, came to visit me, which tired me so much that I was in a sweat.’

Most commonly, Leeuwenhoek closed his letters with ‘Your humble servant’ (uw onderdanige dienaar) or an even more flowery variant such as ‘Your most obligated and humble servant’ (uw seer verpligt en alderonderdanigste dienaar). Through the letter of 14 October 1683 to Anthonie Heinsius, Leeuwenhoek signed his name Antonj Leeuwenhoeck, with an occasional A. Leeuwenhoeck. He signed Letter 40 to Francis Aston on 28 December 1683 Antoni Leeuwenhoek, the first signature without the c, which he never used again. At the time, i and j were the same letter; thus, Antoni and Antonj were the same spelling. On 5 January 1685, he signed Letter 43 to the members of the Royal Society A. van Leeuwenhoek, the first use of ‘van’. For the following five letters, he returned to A. Leeuwenhoek and Antoni Leeuwenhoek. Finally, with Letter 50 of 14 May 1686 to the members of the Royal Society, he used Antoni van Leeuwenhoek and either that or A. van Leeuwenhoek for the rest of his career.

The letters in the Collected Letters, accounted for in Table 1, are not the only letters Leeuwenhoek wrote, of course. His correspondence with his Leiden printer and bookseller Cornelis Boutesteyn alone must have been extensive. He would have corresponded with visitors before and after their visits. He would have corresponded with his sister Catharina in Rotterdam and friends like Maria Duyst, who lived in The Hague and Utrecht. None of this correspondence has been found and has probably not survived. Of the several dozen letters from Leeuwenhoek known only by reference in other letters, it is not always clear which contained publishable content.

Table 1.

Analysis of Leeuwenhoek's letters in Collected Letters in seven periods.

Period	11673–1679	11679–1686	11687–1694	11694–1702	11702–1712	11712–1719	11720–1723	11673–1723 Totals
Collected Letters volume	1, 2	3–6	6–10	10–14	14–16	17, 18	19
Letters with publishable observations	32	37	33	72	49	47	16	286
… with figures	16	24	28	35	29	22	9	163
Number of figures	65	219	315	194	244	144	46	1227
Letters to Royal Society	39	46	37	27	50	17	19	230
… to RS with observations	28	36	32	23	47	14	16	191
… in Philosophical Transactions	15	22	1	21	44	2	15	120
Letters in Dutch and Latin editions	0	25 in Part 1	31 in Part 2	63 in Part 3	0	46 in Part 4	0	165

When and where did Leeuwenhoek write?

Table 1 shows that Leeuwenhoek stayed active throughout his 50-year career. In his most active period, he wrote 2 dozen letters between April and December 1695, once every 10 days, on average. On the other hand, he wrote no letters between April 1689 and September 1691, over 2 years and 5 months. Between April 1689 and January 1692, he addressed only one letter to the Royal Society. From November 1714 until March 1717, he addressed no letters to the Society.

He dated two-thirds of his letters on either a Tuesday or a Friday. On Wednesdays and Saturdays, his job as an official (camerbewaarder) for the city's magistrates (schepenen) clearly took up most of his day. He did not write letters on Sundays until the mid-1690s, after his second wife died.

Leeuwenhoek lived in the center of Delft, on the section of the Nieuwe Delft gracht called the Hippolytusbuurt. His house, called the Gouden Hoofd (golden head), was the second house from the corner of the Nieuwstraat. In the early 1800s, a century after Leeuwenhoek's death, his house and the house on the corner were combined into one and given the name and address of the house on the corner, the Suikerhuis (sugar house).

As shown in Fig. 3, the Gouden Hoofd had a ground floor, an upper floor, and an attic. During one of the 19th century renovations, another floor was added, further concealing the original Gouden Hoofd within the Suikerhuis. For many years, one of Leeuwenhoek's publishers, Henrik Cronevelt, occupied the Suikerhuis. On the upper floor of the Gouden Hoofd, Leeuwenhoek partitioned off about a third of his bedroom to make a narrow room with one window. It was approximately 140 cm wide, about the width of a medium-sized mattress. In dozens of letters, Leeuwenhoek called it his comptoir, his word for the tiny room where he worked, where he ground lenses, constructed magnifying glasses, prepared specimens, made observations, and wrote letters. See Bolt et al. (2018) and Cocquyt et al. (2021) for the composition and construction methods of the microscopes, and Robertson (2017, 2019) for replications of some of Leeuwenhoek's observational methods.

Figure 3.

3D model of the Gouden Hoofd made by Tijm Landow of the University of Amsterdam's 4D Research Lab. Leeuwenhoek's comptoire is on the upper floor behind the right-hand window. (Reproduced with permission.)

Next to the Gouden Hoofd, as noted in Delft tax and notary records, the third house from the corner was called the Rode Zee (Red Sea). The fourth house was called the Nieuwe Vischmart (new fish market). Both of those buildings are, in 2021 a large bakery. On the outer wall of the fourth house, close to the fifth house, is a plaque that reads, ‘Here stood the house ‘Het Gouden Hoofd’ where 91-year-old Antoni van Leeuwenhoek, the discoverer of microbes, died on 26 August 1723.’ Leeuwenhoek would not, in fact, have turned 91 until 2 months after his death. And, of course, the plaque is on the wrong house.

Overview of the letters

Leeuwenhoek himself is of some help in grouping his letters. While the 165 letters that he published between 1685 and 1719 come in many bundles, both Dutch and Latin, the most common is a four-volume set, which he called Opera Omnia in Latin.

•  Part 1: 25 letters written from 1679 to 1686.

•  Part 2: 31 letters written from 1687 to 1694.

•  Part 3: 63 letters written from 1694 to 1702.

•  Part 4: 46 letters written from 1712 to 1717.

Grouping the letters written before Part 1, the letters in the 10-year gap between Parts 3 and 4, and the letters after Part 4 produces the seven periods shown on Table 1. Of the total, at least 286 of them had microscopic and other observations and descriptions and were meant to be published. The other letters were cover letters, copies of letters, query letters, thank-you letters, and dedications.

The letters were not always published in Philosophical Transactions in the same period in which they were written. For example, Letter 57 [30] of 5 April 1680 and Letter 60 [31] of 13 May 1680 were published as one article in 1693. A 1673 letter was published as two articles. The table counts every letter in that period that was at some point abstracted or excerpted and printed. The numbers for 1679–1686 include the five letters that Robert Hooke excerpted in Philosophical Collections, the short-lived successor to Philosophical Transactions after its owner and editor Henry Oldenburg died.

What did Leeuwenhoek write about?

Leeuwenhoek observed specimens of hundreds of species of animals and plants. He never organized the observations, but as Van Berkel (1982) has argued, Leeuwenhoek's work was much less chaotic than it appears at first sight. A total of four themes account for almost all of the letters. Since modern interest in Leeuwenhoek began in 1932, dozens of researchers have painstakingly extracted and compiled everything Leeuwenhoek wrote about a certain topic, a process that can be exhausting, to say the least. For a comprehensive bibliography of this secondary research, see the website Lens on Leeuwenhoek 2021. Terms like microbiology or protistology would have made no sense to Leeuwenhoek. In his terms, he was interested in:

• Generation and growth of plants and animals.

• What he called diertgens (literally, ‘little animals’), now called microbes, especially their size and number, although he called sperm diertgens, too.

• Circulation of blood and other fluids in plants and animals.

• Comparative anatomy—looking for analogous structures among species of plants and animals, especially how muscles work.

Figure 4 illustrated Letter 110 [65] of 7 September 1688, which Leeuwenhoek published (1688) separately as a 30-page booklet called ‘On the true circulation of the blood, also that the arteries and veins are continuous blood vessels, set ready for the eyes.’ The booklet was meant to be given to people to whom he demonstrated red blood cells making the turn in the fish's tail. See within Fig. 4, Leeuwenhoek's ‘Fig: 6A’ in the middle left. He let the visitors peer through what he called his aalkijker (literally: ‘eel viewer’) as a showcase, ‘set ready for the eyes’ and gave them this pamphlet as a souvenir of their visit.

Figure 4.

Illustration from Letter 110 [65] of 7 September 1688, engraving made from the red-chalk originals. (Leeuwenhoek 1688.)

In his following letter to the Royal Society in January 1689, Leeuwenhoek described and provided detailed drawings of his eel viewer (Fig. 5), even showing a half-dead eel in the tube in ‘Fig: 13’. The eel needed to be alive for blood to keep flowing, but not so alive that the tail moved out of focus of the lens that the visitors were looking through. Neither of these letters was published in Philosophical Transactions.

Figure 5.

The design and use of Leeuwenhoek's eel-viewer, his showcase for visitors, to demonstrate the circulation of blood, illustrating Letter 113 [66] of 1 January 1689. (Leeuwenhoek 1967.)

The major headings in the table of contents of Abraham Schierbeek's two-volume biography (1950) indicate the breadth of Leeuwenhoek's research, even including what is now called chemistry: Leeuwenhoek investigated the healing properties of substances, he smelted metals, and he used heat, solvents, and distillates as they were used by the (al)chemists of his day. Schierbeek's other headings are mathematics, physics, geology, microbiology, lower animals, entomology, vertebrates, spermatozoa, histology and physiology, medicine, botany, and pharmaceutical plants.

Leeuwenhoek's letters at the Royal Society

In all seven periods, Leeuwenhoek wrote letters to specific officers of the Royal Society, but beginning in 1684, he most often addressed the letters to the ‘members of the Royal Society’ in general. In London, the letters were often read aloud at a weekly meeting of the Society. The readings were noted in the Society's journal books, but it is possible that some readings were not noted. Beginning with the letter of 9 October 1676 on Leeuwenhoek's discovery of little animals in infusions of spices, the Society read most of the letters aloud until August 1687. The readings were sporadic until 1697, when they began reading every letter again until 1715. However, they resumed when James Jurin became secretary and beginning in 1721, the final dozen letters were read soon after they were received.

As shown on Table 1, of all the letters that Leeuwenhoek wrote with publishable observations, he sent about two-thirds to the Royal Society. However, the Royal Society did not respond in kind. Only 120 of the 189 letters with publishable observations were excerpted in Philosophical Transactions, including the five in Philosophical Collections. The titles usually began ‘An Abstract of a Letter from …’, ‘The extract of a Letter from …’, ‘Part of a Letter of …’, or something similar. Those letters were accompanied by 604 figures in 86 Philosophical Transactions articles. Most of the original red-chalk drawings that Leeuwenhoek sent with his letters are lost.

The table shows that between 1687 and 1694, Leeuwenhoek sent 32 letters to the Royal Society with observations and only one was published; many were not even translated and almost all of the drawings are lost. Between 1712 and 1719, he sent 14 letters with observations, only four were translated and read, two of them were published, and all of the original drawings are lost.

The explanation for this publishing drought lies with Edmond Halley, the mathematician and astronomer. During his two stints as editor of Philosophical Transactions, Halley's stated policy was to publish articles about what he called ‘Physical, Mathematical, and Mechanical Theories or Observations’ (Halley 1686). He considered what we now call biology and botany as less precise, based mostly on observation rather than controlled, repeatable experiment. In 1887, the Royal Society solved that conflict by splitting Philosophical Transactions into two parts, one for mathematics, the physical sciences, and engineering and the other for biology. However, Halley was the editor for volume 16, issued between 1686 and 1692, in which he published no letters by Leeuwenhoek. He resumed the editorship for volumes 29 and 30, issued between 1714 and 1719, in which he published only one letter.

Leeuwenhoek's response was decisive. During the period when the Royal Society published only one letter, he published 31 himself. When it published only two letters, he published 46 himself. He began in 1684 with booklets containing one, two, or three letters. By the end of the 1680s, he was publishing volumes containing dozens of letters each. He ended up with a total of 25 titles, 18 in Dutch and seven in Latin, with a summary preceding each of the 165 letters in both languages, a separately published subject index for the pre-1695 volumes (Anon 1695), and an index at the end of each of the following volumes, both Dutch and Latin. Subsequent editions doubled the number, including the fourth edition of Continuatio Epistolarum (Continuation of the Letters) published posthumously. For a comprehensive bibliography of these publications, see the website Lens on Leeuwenhoek 2021.

Were all of Leeuwenhoek's letters published?

In a 1715 letter to Gottfried Leibniz, Leeuwenhoek (2018) wrote:

I intend to have nothing printed in our language, but after my death more than a hundred letters will be made generally known in print and to that end I have already ordered eleven copper plates to be cut.

When Leeuwenhoek died in 1723, his daughter Maria (1654–1745), who lived her whole life in the Gouden Hoofd, inherited everything (Zuidervaart and Anderson 2016). When she died 20 years later, her father's microscopes were sold at auction. The last page of the auction catalogue (Rees 1747) added a note (my translation):

In the estate of the late Miss Maria van Leeuwenhoek are found some bequeathed manuscripts of letters of her father, Mr. Anthony van Leeuwenhoek, which were written by his honor during his life and set in a neat and good order, in order to be printed as a continuation of his preceding published letters. All the plates belonging to them are present and already engraved in copper, as is the Latin translation of these letters. Anyone being inclined to buy this to have it printed as a continuation of his already published series of letters can address themselves to the executors of this estate.

Apparently, no one accepted that offer and the letters, more than a hundred of them, are lost. Which letters could they be? The letter to Leibniz announcing the hundred unpublished letters was written in 1715, so the four dozen letters from 1702 to 1712 are good candidates, but that is not ‘more than a hundred’. All of the pre-1715 letters with observations that he had not already published add up to around a hundred. That is a possibility, but we do not know for sure.

Conclusion

Now that all of the available letters written by Leeuwenhoek are accessible in both Dutch and English, we can begin to work with the corpus as a whole. Computerized textual analysis of the corpus has the potential to help us discover patterns that on our own, we can dimly see at best. What questions do we not know to even ask? This level of analysis will let us confirm, or amend, what we know about Leeuwenhoek, e.g. his relationship with the Royal Society.

It is too simple to say that Leeuwenhoek sent letters to the Royal Society, which published them in Philosophical Transactions. That is true only to a limited extent. To put it into context, fewer than half of all of Leeuwenhoek's letters with publishable observations were excerpted in Philosophical Transactions accompanied by fewer than half of all of the figures. Of the 165 letters that Leeuwenhoek published in Dutch and Latin, only 41 of them were also published in Philosophical Transactions. When confronted with the 18 folio pages of what turned out to be Leeuwenhoek's most famous letter, containing his descriptions of little animals in infusoria, Oldenburg roughly translated and published less than half of it (Leeuwenhoek 1677). It was not until 1937 that the whole letter was published, in a facsimile edition with an English translation (Cohen 1937). On the other hand, when the Royal Society was given a copy of Leeuwenhoek's 1697 Continuatio Arcanorum Naturae Detectorum (Continuation of nature's mysteries disclosed), which had 15 letters, only one of them to the Society, Robert Hooke (1697) prepared extensive summaries of all of the letters that he then read aloud at several weekly meetings during the summer of 1697. The Royal Society, and especially Hooke wanted to stay current on Leeuwenhoek's research, even if it did not involve them. In the end, no author in the history of Philosophical Transactions has had more articles published in that journal than Leeuwenhoek has, by far.

Work on the Collected Letters began on the 200th anniversary of Leeuwenhoek's birth, in 1932. It seems as though it will finally end on the 300th anniversary of his death, in 2023, 91 years later, which, as it happens, was how long Leeuwenhoek lived.

Conflicts of interest

None declared.

Author notes

This article is an expanded version of a presentation made to the World Microbe Forum, June 2021.