Letter, from Antoni van Leeuwenhoek to Henry Oldenburg, dated at Delft Signed autograph Observations of the structure of human blood and the milk of cows, of the structure of human hair and elk's hair, of the fat and flesh of various living creatures, and the anatomy of the louse For summary see 'The correspondence of Henry Oldenburg', ed and trans by A Rupert Hall & Marie Boas Hall, vol 10, p 547 Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, part 1, p 72 English extract printed in 'Philosophical Transactions', vol 9, no 102, p 23 7 April 1674