Period 4 - 1694-1702

This period was the most productive of Leeuwenhoek's career. He was in his sixties and seventies, a widower for the second time in 1694, living with his devoted and unmarried daughter Maria, presumably with more time than ever for his observations.

The situation in London was stable and favorable. In late 1693, Hans Sloane replaced Thomas Gale as one of the two Royal Society secretaries. He also agreed to assume responsibility for the expenses and most losses of Philosophical Transactions. He began publishing Leeuwenhoek after the years when his successor Edmond Halley had not.

Over these eight years, Leeuwenhoek wrote 73 letters with scientific content. The secret to his productiving may have involved getting out of the house and finding a quiet place to work.

The table below summarizes these eight years of Leeuwenhoek's scientific career, from the early May 1694 until late April 1702. Hans Sloane was the editor of Philosophical Transactions.

Letters in Period 4

Of the 99 letters in this period, 18 were addressed to members of the Royal Society, four by name. Hans Sloane and Richard Waller were the Royal Society secretaries and the editors of Philosophical Transactions. John Chamberlayne translated many of Leeuwenhoek's letters.

• 13 to Members of the Royal Society
• 12 to Hans Sloane
• 1 to John Chamberlayne
• 1 to John Somers
• 1 to Richard Waller

The rest were addressed to a variety of people. N.N. indicates "no name", the condition of 8 of these letters, several of whom were addressed to groups or to people who are otherwise unknown. Two of them, however, we do know: Letter 118 of August 5,  1699, was written to the directors of the United East India Company in Delft; the letter of April 8, 1701, was written to the Burgomasters and governors of the City of Delft.

Over four dozen of these letters were written to the following people:

• Antonio Magliabechi - 14, only one with scientific observations
• Frederik Adriaan van Reede - 12
• Anthonie Heinsius - 8
• Pieter Rabus - 7
• Hendrik van Bleyswijk - 5
• Maarten Etienne van Velden - 4, only two with scientific observations
• Harmen van Zoelen - 3
• Nicolaas Witsen - 3
• Johann Wilhelm von Pfalz-Neuburg, Elector Palatine - 2
• Karl von Hessen-Kassel - 2
• Nicolaas Bogaert van Belois - 1
• Jan van Leeuwen - 1

Jan van Leeuwen was one of two men of that name related to Leeuwenhoek's sister Catherine, who had lived in Rotterdam. She married a van Leeuwen and her daughter Rijke married an unrelated man with the same last name.

Once Richard Waller got serious about reviving Philosophical Transactions, he set its goals high -- and he began publishing Leeuwenhoek's letters from Periods 2, 3, and 4 that had been lying around for up to twelve years. In 1693, he published letters from April 5, 1680, and May 13, 1680, in volume 17, number 196. He followed that with five letters over a decade old: one from 1683 and two each from 1685 and 1686, all published in volume 17, 1693. The one from 1683, September 11, had already been translated and published a decade earlier by Robert Plot in volume 14. Waller's was shorter, had the figures reversed, and was missing one. In addition, according to Dobell, the translation is not as accurate.

The next year, for volume 18, Waller continued, publishing two of Leeuwenhoek's letters, from Period 4, July 10, 1686, and March 7, 1692.

Even so, this left a large body of letters from those periods that remained untranslated into English until the late 20th century's Alle de Brieven / Collected Letters.

Note: Birch's History stops at the end of 1687, so we do not have as complete an idea of what happened as we did during earlier years.

On November 30, 1693, Hans Sloane (left), only 33 years old, was appointed Secretary of the Royal Society, joining Waller, who had been there for several years. E.S.J. Brooks wrote in Sir Hans Sloane: The Great Collector and His Circle London, (1954, p. 82):

Sloane was the owner and editor of the journal, responsible for obtaining the items to be published. Profits were rare and losses were made up from his assets.

From then, for the next twenty years (excepting only 1710 when John Harris replaced Waller for one year), these two, Waller and Sloane, were the Royal Society's secretaries and Sloane was Philosophical Transactions' editor. It appeared regularly. For Leeuwenhoek, beginning in 1695, the tide turned.

In Delft

On December 5, 1693, a week after Sloane was appointed Secretary, Leeuwenhoek rented the St. Agnieten Tooren (right) from the City. The results are clear on the table at the top of this page compared to the table at the top of Period 3. In the five years before he rented the tower, Leeuwenhoek averaged three or four letters per year. For the twelve years that the lease ran, he averaged eleven letters per year.

Given the uncertainties in London, Leeuwenhoek continued publishing his letters is Dutch editions and roughly parallel Latin translations. London was far away and his Dutch printer was literally next door on the Hippolytusbuurt, corner of the Nieuwstraat. This involved getting someone to translate the letters into Latin and presumably someone else to copy edit or proofread them because Leeuwenhoek couldn't. In addition, he had to have dozens of plates prepared from the hundreds of red chalk drawings of his observations. They had to be labelled correctly to match the text references. All this while maintaining a full schedule of observations.

By 1697, Leeuwenhoek had caught up with himself. The final letter in the Sesde Vervolg van der Brieven (Sixth Continuation of the Letters) was dated September 27, 1696 (AB 177). According to Leeuwenhoek's numbering system, this was letter 107. He never published the first 27, so these volumes contained all of the 80 letters through the end of September 1696. It took until 1702 for him to write enough letters for the Sevende Vervolg (Seventh Continuation), by far the largest with 39 letters.

Dutch editions

While as a fellow of the Royal Society, Leeuwenhoek never visited London or attended a meeting, in 1700, Hans Sloane visited him. We have no record of that meeting, and we have no reason to think that Sloane spoke Dutch. Perhaps he was accompanied by translator John Chamberlayne, to whom Leeuwenhoek began addressing letters the following year.

Two things happened after Sloane visit that were perhaps caused by it:

• From July 1700 until the end of 1712, almost all the 40 letters that Leeuwenhoek wrote were published by Sloane in volumes 22 through 29.
• Leeuwenhoek stopped self-publishing in Delft. He published a collection of 39 letters, the Sevende Vervolg der Brieven / Seventh Continuation of the Letters, in 1702. After that, he let the English translations in Philosophical Transactions suffice.

Thus, a dozen letters were published in both Philosophical Transactions and the Sevende Vervolg. However, the bulk of the letters written during this period were not published in Dutch/Latin. Some of them remained in the English translation only until the Alles de Briven / Collected Letters project caught up with them at the end of the 20th century and published the full letters in both Dutch and English.