Wrote Letter L-085 of 1679-05-20 to Constantijn Huygens about the number and size of the little animals he was observing

May 20, 1679
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Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren. The original manuscript is lost, but there are two signed autographic copies. One is in London, Royal Society, MS. 1869. Ll. 41. The other is in Leiden University's library, dated May 21. It has three quarto pages with one drawing in the text. In the Leiden copy, the one drawing is not hatched.

Leeuwenhoek wrote this letter to Constantijn Huygens about his speculations on microscopic and submicroscopic dimensions. Starting from the size and the organization of the little animals that he observed, Leeuwenhoek concludes that the component parts (molecules) of water are unimaginably small.