Wrote Letter L-134 of 1683-09-16 to Anthonie Heinsius about whether Heinsius would like a copy of his speculations on the living organisms in mouths and on the structure of the skin
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 on two quarto pages, written and signed by Leeuwenhoek, is preserved at the Rijksarchief (National Archive) in Den Haag (Inv. No. 13). The letter was not published during his lifetime. It was not accompanied by any drawings.
The first part of this letter reported on his most recent correspondence with the Royal Society.
The second part mentioned two French visitors to his house. (See below under Visitors.)
The third part reported on his recent observations that would be in upcoming letters to the Royal Society about little animals in the human mouth and about human skin, which Leeuwenhoek thought had no pores.