Wrote Letter L-098 of 1680-01-16 to Robert Hooke, enclosing a copy of Letter L-085 of 1679-05-20 to Constantijn Huygens about the number and size of little animals

January 16, 1680
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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 on one quarto page, written and signed by Leeuwenhoek, is preserved at the Royal Society (MS. 1876. L 1. 47.).

Leeuwenhoek wrote this short letter to Hooke, first, to acknowledge receipt of the first number of Philosophical Collections. This first number finally reached Leeuwenhoek in mid-January. Second, he enclosed in this letter an autograph copy of his letter to Constantijn Huygens, dated 20 May 1679. Finally, he gave Hooke a quick follow-up to his observations of little animals in pepper-water and ginger-water.

I saw very distinctly little animals (dierkens) that I judged to be a hundred million times smaller than a grain of sand

Even if his standard of comparison was a very large grain of sand, these animals were so little that they must have been bacteria.

The enclosed copy of the May 20, 1679, letter to Constantijn Huygens with Leeuwenhoek's calculations of the quantities of microbes that he was observing was published after Hooke's death by W. Derham in Philosophical experiments and observations of Dr. Robert Hooke, pp. 55-57.