Letter, from Antoni van Leeuwenhoek to the Royal Society, dated at Delft Signed autograph Experiments with bezoar stone (concentration found in the intestines of certain ruminant animals); experiments with monkey stone (a special type of bezoar stone); investigation of gout tubercles; experiments with red coral and white coral used in medicine Original figures no longer exist; engraved figures for publication are attached Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 7, p 241 6 July 1688