Constantijn Huygens wrote Letter L-004 to Leeuwenhoek, enclosing an extract of a letter from Oldenburg that asked Huygens to encourage L.’s nature study

February 1, 1674

This letter is known only by reference in Letter L-006 of 7 April 1674 to Henry Oldenburg.

About two months ago Mr. Constantijn Huygens van Suijlichem sent me an extract from your letter to this gentleman, from which I saw that my observations were welcome to the Royal Society, that you ask for further correspondence concerning my observations in nature-study, and that you have printed part of my observations in the Philosophical Transactions of the Royal Society.

This letter is the first known letter from Huygens to L., who replied with Letter 4 L-005 of 5 April 1674, Collected Letters, vol. 1. The next letter from Huygens is Letter L-007 of 11 April 1675. In total, their known correspondence over the eleven years up to December 1685 includes seven letters from Huygens to L. and seven letters from L. to Huygens. However, they lived just a few miles apart and visited each other often. The known letters are not all of the letters between them, noting L.’s words in Letter L-349 of 17 December 1698 to Harmen van Zoelen: “I used to send the minutes of my dispatched discoveries to Mr. Constantijn Huygens of Zuylighem, who took great pleasure in my discoveries.”

The letter from Oldenburg to Huygens mentioned by Leeuwenhoek in Letter L-006 of 7 April 1674 is noted in Huygens’s reply of 12 February 1674 to Oldenburg, in which Huygens remarks that his own portable microscope is superior to the microscopes used by Johannes Swammerdam and Arent Seyn, both of whom visited Leeuwenhoek around this time. The letter was written in French; the English translation below comes from Hall and Hall, The Correspondence of Henry Oldenburg, vol. X, June 1673–April 1674, pp. 458–9.

I shall have an extract of your letter sent to Mr. Leeuwenhoek, who will be very glad of the warm welcome you give his observations and will thence find himself stimulated to push his industry further. Mr. Swammerdam and the botany professor Van Seyn ... are very much struggling with the microscope and ferreting out the intimate nature of plants, animals, etc. ...

These gentlemen use a very laborious microscopical device, which turns in every dimension on balls properly worked in copper. Nothing more finished can be seen, but I make light of them, opening their eyes with a very simple little device which I always carry in my pocket which gives me all that one could wish for by way of movements, with a needle to view objects in the air, or a plate to lay them flat, and nothing is easier.

After an initial period of skepticism in the 1670s, Constantijn and his sons Christiaan and Constantijn jr. were soon vouching for L. in London and Paris and became his most important Dutch supporters.