Translation of a letter, from Antoni van Leeuwenhoek to Henry Oldenburg Theory for the existence of different skin colours; observations of the reproduction of eels and their blood; observations of the reproduction of lice; compares the larvae of fleas with silk-worms; discusses Mr Swammerdam's views on fleas; demonstrating a grain of millet as a measure of capacity and states that he sends 8 testimonials about this (these have ref code EL/L1/29 and are found after this letter); numbers of organisms living in a drop of water Translation of EL/L1/29 1677