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How many letters did Leeuwenhoek write?
Short answer: 285 letters with scientific observations
~ 364 letters sent by Leeuwenhoek
The total number of letters sent by Leeuwenhoek include both those we have in manuscript and/or print as well as those referred to in other letters but now lost. The editors of the nineteen-volume Alle de Brieven / Collected Letters have not completed their task, but it appears as though they will end up with approximately 364 letters numbered 1 through 364. These numbers are the only set to provide a unique number for each letter.
About three-quarters of the total 364 letters in Alle de Brieven / Collected Letters had publishable scientific observations, 285 letters, perhaps a few more depending on how you define "publishable scientific observation".
In a 1937 article in Annals of Science, F. J. Cole expanded on Leeuwenhoek's numbers.
- Cole inferred, no doubt correctly, which were the first 27 letters that Leeuwenhoek did not self-publish but caused him to begin his own numbering with 28.
- Cole retained Leeuwenhoek's numbers 28 to 146 for the letters from April 1679 through April 1702.
- To bridge the gap to Send-Brieven, about four dozen letters, Cole continued this Arabic numbering to Letter  of June 10, 1712, all addressed to member of the Royal Society.
- Then came the 46 letters between November 1712 and November 1717 in Send-Brieven that Leeuwenhoek numbered I through XLVI.
- Finally, Cole continued the Roman numerals from [XLVII] through [LXI] for 15 letters that Leeuwenhoek sent in the last three years of this life to the Royal Society and Philosophical Transactions editor James Jurin.
- Throughout, Cole added 33 copies, cover letters, and notes about mailing addresses, etc., using the number of Leeuwenhoek's previous letter followed by (a) and, several times, (b), (c), and even (d). Most of these were early letters, pre-1680. An example is the letter of April 16, 1674 to Henry Oldenburg above right (click to enlarge). It is number 6 in Alle de Brieven / Collected Letters and number 3a in Cole. This type of letter also accounts for almost all of the 80 letters in Alle de Brieven / Collected Letters that are not in Cole.
Cole's numbers add up to 284 letters: 190 + 46 + 15 + 33. Adding the 80 in Alle de Brieven / Collected Letters brings the total to 364.
The 251 letters with scientific observations in the combined numbering systems of Leeuwenhoek and Cole (190 + 46 + 15). The other 34 letters were written to, among others:
- Constantijn Huygens - 6 letters
- Pieter Rabus - 6, published in De Boekzaal van Europe)
- Anthonie Heinsius - 4, though two were excerpted in Letter 60 of November 28, 1687
- Lambert van Velthuysen - 3
Cole identified 16 of these letters and Alle de Brieven / Collected Letters added the other 18.
From April 1673 until August 1723, Leeuwenhoek sent 223 letters to the Royal Society either in general or to a specific member. The exceptions are letters to people on the continent like Christiaan Huygens or Gottfried Liebniz who were members of the Royal Society but to whom Leeuwenhoek sent letters that he did not expect to be published in Philosophical Transactions. This 223 does include letters to, for example, Francesco Cornaro, the Venetian ambassador to England who was not a fellow or member of the Royal Society. A letter to him was published in Philosophical Transactions in 1705.
~ 200 letters
About two hundred of Leeuwenhoek's original letters, in Dutch, some with specimens, are in the possession of the Royal Society along with 279 of the drawings (and some engravings) that Leeuwenhoek sent to them.
Leeuwenhoek's own numbering system gave special status to 146 letters from 1673 to 1702 and the 46 letters in Send-Brieven from 1712 through 1717. He self-published 165 of them (192 minus the first 27). The other letters with scientific observations were all written in Periods 1, 5, and 7 when he was not self-publishing.
About 85% of the letters Leeuwenhoek sent to the Royal Society contained scientific observations. Most of them had opening and closing sections with personal or contextual matters that were omitted in the Philosophical Transactions excerpts of 114 of them in 117 articles. Since Leeuwenhoek published only about a fifth of them himself, the manuscripts and the transcriptions in Alle de Brieven / Collected Letters provide our only access to this information.
Leeuwenhoek published 165 letters himself in pamphlets and books in the original Dutch and in Latin translation, with irregular paginations and 865 figures in 110 of them. He started with Letter 28 of April 25, 1679 and continued to Letter 146 of April 20, 1702 to Karl von Hessen-Kassel. This was also the final letter in Sevende Vervolg der Brieven. Skipping four dozen letters, he began numbering again with Send-Brieven, Letter I of November 8, 1712 through Letter XLVI of November 20, 1717, for a total of 46 letters.
Of the 285 letters with publishable scientific observations, 163 letters, a little over half, had a total of 1,227 figures
- Originals at Royal Society: 279 drawings in 43 of the first 294 of the 364 letters
- Leeuwenhoek's self-published letters: 869 figures in 110 of 165 self-published letters in Dutch and Latin
- Philosophical Transactions: 604 figures in 86 of 117 articles
- French, German, other Latin: 136 figures in 22 of 83 letters
- other Dutch and English: 22 figures in 5 of 24 letters
- English (Hoole): 378 figures in 57 of 79 letters with 635 figures; the 57 have only 545 figures because Hoole omitted 90 of them.
The editors in London published excerpts from 114 of Leeuwenhoek's letters in Philosophical Transactions in 117 articles. Several times, they split a long letter into two articles. Several other times, they combined parts of two letters into one article. Though the second part may have had a note that it came from a different letter, the two parts were treated as one letter by the editors. Of these articles, 86 had a total of 604 figures.
In the short-lived Philosophical Collections, Robert Hooke published five other letters that some writers consider part of Philosophical Transactions. He also published two other letters in Microscopium.
It is common -- and erroneous knowledge -- that "Leeuwenhoek wrote hundres of letters and they were published in Philosophical Transactions" or words to that effect. In fact, he sent 189 letters containing scientific observations to the Royal Society, 41 of which he published himself. He published 124 letters with scientific observations that he did not send to the Royal Society.
Of the 165 letters that Leeuwenhoek published himself, 108 had 869 figures out of the 1,127 figures in all the letters. Another letter, sent to Constantijn Huygens in 1679, had 3 figures, making 109 letters with 872 figures.
For unknown reasons, Leeuwenhoek used Roman numerals I through XLVI for the letters in Send-Brieven.
Only 41 letters appeared in both Philosophical Transactions and Leeuwenhoek's self-published volumes, almost all of them in Periods 2 and 4. For the most part, when Leeuwenhoek was being published regularly in Philosophical Transactions, he did not publish his own work. Of these 41 letters, seven of them were published in Philosophical Transactions after Leeuwenhoek published them himself in Dutch and Latin.
Leeuwenhoek (and Cole) numbered all but 34 of the 285 letters he wrote that had scientific observations and explained his thinking.
Why does Leeuwenhoek's numbering begin with 28? What happened to the first 27? I find Dobell's argument persuasive (1932, pp. 356-359), that Leeuwenhoek did not begin to keep fair copies of what he sent until he realized they were worth keeping, about the time his work was validated by the Royal Society and he was elected a member.