Letter, from Antoni van Leeuwenhoek to Christopher Wren, dated at Delft Signed autograph Examining the ovaries of lambs; observations of the flea: the muscle tissue, testicles, tracheids, sting; observations of the muscle tissue of the louse, gnat, fly; the effects of sal volatile oleosum; drinking tea and mild beer for curing a fever and thinning the blood; observing the structure of erythrocytes; theory on the formation of blood Original figures no longer exist Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 4, p 3 English extract printed in 'Philosophical Transactions', vol 13, no 145, p 74-81 22 January 1683