Key to the Letters pages

Each of Leeuwenhoek's 350+ letters has its own page on Lens on Leeuwenhoek. The pages have a standard format that reflects some arbitrary choices explained below.


Leeuwenhoek did not give his letters a title in the sense of a unique short identifying phrase. They are best distinguished and referred to by the date that Leeuwenhoek put on them and the person or group he addressed them to.


The sidebars give access to the figures that illustrated that letter. Each has a thumbnail, a linked label, and a caption: what Leeuwenhoek called it, not what a scientist would call it today.

Each thumbnail is linked to a larger pop-up window that can be moved and re-sized to make it easier to compare the figures.

The label beneath the thumbnail -- example: "Letter 30 Figure 1" -- is linked to a page with more information about the figure.

What did Leeuwenhoek call it?

The Dutch in the caption under the link comes from Alle de Brieven / Collected Letters, thus from the manuscript. The English is a translation of that Dutch, not necessarily a transcription from Alle de Brieven / Collected Letters. When the letters were printed in Dutch, the spelling, capitalization, and grammar were edited, apparently not by Leeuwenhoek, or he would have written it that way in the first place.

Using the language from the manuscript tells us more about Leeuwenhoek, so the English translations try to be more literal and less interpretive.

Standard reference information

Cole's number

Leeuwenhoek gave unique numbers to 165 of his letters. In a 1937 article in Annals of Science, F. J. Cole expanded Leeuwenhoek's numbering system to include letters, usually without scientific observations, that Leeuwenhoek did not self-publish. Cole added a lower case a, b, c, or d to Leeuwenhoek's preceding number.

Cole also filled in the ten years between 1702 and 1702, that is, between Sevende Vervolg and Send-Brieven. In that time, Lens on Leeuwenhoek's Period 5, Leeuwenhoek wrote over fifty letters but neither numbered nor published them. Sevende Vervolg ended with Letter 146 of April 20, 1702, and Send-Brieven picked up with Letter I of November 8, 1712. Between them, Cole continued with Letter [147] of April 28, 1702, through Letter [190] of June 10, 1712.

Cole's additions have either a letter after Leeuwenhoek's number or they are in square brackets.

AB/CL number, volume

Alle de Brieven / Collected Letters began numbering afresh and included several dozen more letters than Cole did.

This is the only set with a unique number for each letter. Many references in the literature about Leeuwenhoek use these numbers.

While Alle de Brieven / Collected Letters uses Leeuwenhoek's numbers, where applicable, it makes no acknowledgement of Cole's numbers.

Publication history

For edited periodical publications such as Philosophical Transactions, the left column of the table has the year it began, not necessarily the year(s) it published Leeuwenhoek. For Leeuwenhoek's self-publications, the table has the year of first editions only. For later printings and editions, see Related events under Learn more.

Leeuwenhoek's summary

Except for the first six letters that he published, every one of the 165 letters that Leeuwenhoek numbered and published was preceded by a summary. For those six early letters, the summary was not published until the "Register der Saaken in the voorgaende Brieven vervat" (Index of Topics contained in the foregoing Letters) at the beginning of Vervolg der Brieven in 1687.

The summaries first appeared in the Dutch editions and were translated for the Latin editions. Whether the summaries were added by Leeuwenhoek, the bookseller, or the typesetter, we do not know.

These summaries were not part of the letter manuscripts, so they were not included in Alle de Brieven / Collected Letters. Thus, they have never been translated into English until now.

The Dutch on the left has preserved the original spelling and capitalization. The English on the right does not try to reproduce that.

Text of the letter

The Koninklijke Bibliotheek (Royal Library) in Den Haag has digitized the first 15 volumes of Alle de Brieven / Collected Letters, up to the letter of July 12, 1707, their No. 269. The link goes directly to that letter at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren (Digital Library for Dutch Letters).

Volume 16, which was published in 2014, volume 17, in press in mid-2017, and the forthcoming volumes 18 and 19 will not be available for digitizing until four years after print publication. Depending on budgets, they may be available at the DBNL several years after that.

What follows on each letter page is a discussion of passages from the letter. They emphasize information about Leeuwenhoek's life and career, with less emphasis on his science. The quotations come from the Alle de Brieven / Collected Letters translation of the manuscript, as edited to avoid words that were not used in English in Leeuwenhoek's time. Looking back, of course, we have a more accurate understanding of what Leeuwenhoek was describing. Lens on Leeuwenhoek, however, is a work of history, not modern scientific interpretation.

Note: Not all of these topics were addressed in every letter.

The manuscript

This section of each letter's page also notes the status of the manuscript. Most of those that have survived are in the possession of the Royal Society. Many of those were not in Leeuwenhoek's hand. They were written by a copyist and signed by Leeuwenhoek, who may have kept the original for his own records and for typesetting in Leiden and Delft.

Reception in London

For the letters through the end of 1687, Birch's History of the Royal Society volumes III and IV note when the letters were received and read at a weekly meeting in London. This section of each letter's page gives those entries as direct quotations, along with whatever members' comments followed.

Specimens and methods

Leeuwenhoek often recounted where he obtained his specimens and how he prepared them for observation. Doing so, he often revealed details about his life and his lifestyle.

Response to other researchers

One of the characteristics that leads people to call Leeuwenhoek a dilletante is how much of his research agenda was a response to other researchers. Another way to look at that is to abandon the lone genius theory of scientific advancement and recognize that in this area, as in so many others, Leeuwenhoek was developing modern science as we know it. Science can be seen as an intensely social enterprise by a community of scholars whose research is indeed driven by a response to other researchers.

The public and their opinions

Especially early in his career, Leeuwenhoek's letters frequently portray his relationship with the people of Delft and Holland as an encounter between his new-found knowledge and their received wisdom that opposed contrary evidence, sometimes to the point of what Leeuwenhoek saw as frustratingly absurd denial. In these passages, we can see the beginnings of the never-ending friction between data-based factual reality and ideas that are convenient, appealing and deeply assumed by the public, who would feel it a form of betrayal to abandon them.


Two of Leeuwenhoek's ongoing questions about what he saw through his lens: size and quantity. The first was a very small number and the second a very large one, both of them unbelievable by those who held to the received wisdom of the ancients. Why would God make anything that small? As to the quantity, Leeuwenhoek frequently noted how unbelievable and inconceivable the numbers were.

At the time, arithmetic was necessary for commerce, so the basic skills of addition, subtratction, multiplication, and division were common enough. However, anything more complicated required special training. In 1669, Leeuwenhoek became certified as a surveyor after the learned some of that higher mathematics. That enabled him to become one of Delft's wine gaugers, using his skills to compute the liquid content of barrels, the sizes of which were not standardized.

It was thus easy for him to include some of his calculations, usually multiplication, in the text and margins of his manuscripts.

Visitors to his house

In the letters, Leeuwenhoek occasionally noted that a visitor had come to his house. For the four decades before his death, Leeuwenhoek was the most internationally famous person living in Delft. It is not surprising that he had many visitors, only some of whom he noted in the letters. Added to the list of printed works by other researchers, the list of visitors shows that Leeuwenhoek was deeply embedded in his community and his time.


This section answers the questions:

  • Where are the original drawings? Most are lost.
  • Who drew them? In what medium?
  • Referring to the sidebars, how many figures accompanied the letter? What specimens did they illustrate? In which printed edition did they appear? Since Leeuwenhoek used the same engraved plates for all of his Dutch and Latin editions, the sidebars show the set for which the available scans are clearest.
  • What passages in the letter refer to the making of these figures? As above, these English quotations are based on the Alle de Brieven / Collected Letters translation, as edited to preserve the historical context.
  • What other publications had versions of these figures, with special attention to Philosophical Transactions?


This section discusses issues of translation and terminology.

Learn more

This section has links within Lens on Leeuwenhoek relevant to this letter.

Other letters that are mentioned in the sections above.

Pages such as overviews of the period in which the letter was written.

People mentioned in the sections above, especially the person or group to whom Leeuwenhoek addressed the letter.

Events in Leeuwenhoek's life related to this letter. They include the publication of later editions and printings in which the figures appeared.


Each letter is tagged with one or more terms to make it easier to find letters and figures on related topics. This broad grouping uses the terms that Leeuwenhoek himself used.


  • Circulation
  • Digestion
  • Fermentation
  • Growth
  • Movement
  • Reproduction
  • Respiration
  • Sight
  • Taste

Tools and Techniques


  • Number
  • Size

A search will produce more targeted results.