Wrote Letter L-349 of 1698-12-17 to Harmen van Zoelen to defend himself against Hartsoeker's claim to have been the first to discover sperm

December 17, 1698
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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

Leeuwenhoek wrote this letter to Harmen van Zoelen to defend himself against Nicholas Hartsoeker's claim in Hartsoeker's recently published Proeve der Deurzicht-kunde to be the first person to see sperm. Most of the letter is long quotations from letters he wrote in late 1677 to William Brouncker and early 1678 to Nehemiah Grew as well as a letter Constantijn Huygens wrote to Leeuwenhoek in 1677.

What triggered Leeuwenhoek's reaction was his reading a pre-publication copy of Hartsoeker's Proeve der Deurzicht-kunde, the Dutch translation by A. Block of Hartsoeker's Essai de dioptrique from 1694. Jan Luykens did the cover illustration, which did not appear in the French version. It shows one of Hartsoeker's designs for a long telescope. Earlier Christiaan Huygens had been influenced by Hartsoeker's designs for his own long telescope.