Wrote Letter L-207 of October 1689 to Christiaan Huygens about sending books to his brother Constantijn Huygens in London

October 1, 1689
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This letter is known only by reference in a letter from Huygens. It was written before 18 October 1689 when Huygens referred to it in a letter to his brother.

In this letter, Leeuwenhoek tells Huygens that he sent four copies of his most recently published book to his brother Constantijn Huygens in London, one for the Queen, one for himself, one for Dr. Stanley, and the last one for the Royal Society.

Letter 2552 of 18 October 1689 from Christiaan Huygens to his brother in London, Oeuvres complètes. Tome IX. Correspondance 1685-1690 (1901)

I have just received a letter from Sr. Leeuwenhoeck, who tells me that he sent you by the ferry boat from Rotterdam to London (Skipper Jeroen Vinck) 4 copies of his last observations, for the Queen, who had some others have to do with what he says, for you, for Dr. Stanley and for the Royal Society which the last said doctor will be able to give to these gentlemen, and he may at the same time complain to them on behalf of the author, which has seven letters that he wrote to them, of which I was the bearer of the last one, never received a response. Which marks that there is nothing going on with you.

This letter is the last of three known letters from Leeuwenhoek to Christiaan Huygens. For an overview of their correspondence, see Letter L-205 of 6 March 1689.

It is not clear which volume Leeuwenhoek sent to London. If he sent a Latin volume, it was Continuatio Epistolarum, Datarum Ad longe Celeberrimam Regiam Societatem Londinensem (Continuation of the letters, given to the most famous Royal Society of London), published in Leiden in 1689 by Cornelis Boutesteyn. However, it contains eight letters (not seven), Letter L-186 [53] of 4 April 1687 through Letter L-194 [60] of 28 November 1687, all written to the Royal Society. Continuatio Epistolarum must have been popular. It was bundled with other volumes and was reprinted four times, in 1696 as an Editio altera, in 1715 as a second edition, and in 1730 as the Editio tertio. None of these letters was published in Philosophical Transactions.

If Leeuwenhoek sent a volume with seven letters, then it would be Tweede Vervolg der Brieven (Second Continuation of the Letters), also published in 1689 but in Delft by Andries Voorstad. It contains Letter L-196 [61] of 25 May 1688 through Letter L-206 [67] of 1 April 1689, all written to the Royal Society. Again, none of these letters was published in Philosophical Transactions.