Letter, from Antoni van Leeuwenhoek to Henry Oldenburg, dated at Delft Copy by Leeuwenhoek Observations of human and horse sweat, structure of human and animal hair, the motion of blood in narrow glass-pipes; concerning the structure of fat in cows, sheep and fish Figures of glass-pipe in the margin For summary see 'The correspondence of Henry Oldenburg', ed and trans by A Rupert Hall & Marie Boas Hall, vol 11, p 38 Printed in 'Philosophical Transactions', vol 9, no 106, p 128 Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, part 1, p 116 6 July 1674