Translation of a letter, from Antoni van Leeuwenhoek to Robert Hooke, dated at Delft Observing the lees in wine; experiments with fermenting wine; comparing the structure of yeast cells with erythrocytes (red blood cells); examining yeast in syrups from an apothecary's shop; observing particles in rain-water; an analysis of chyle from a cow, fat globules in milk, composition of urine; examining particles in air; discusses the function of the heart and the circulation of blood in the body; observations of the tracheae of a fly, a flea, and a cockroach; observations of the copulation of cockchafers and of dragonflies, the spermatozoids of the grass-hopper, the gnat, the flea, and of the fly; examination of mites; calculating the number of micro-organisms in a grain of sand Translation of EL/L1/62 1680