Using his handcrafted microscopes, he was the first to see and describe single celled organisms, which he originally referred to as "animalcules", and which we now refer to as microorganisms. He was also the first to record microscopic observations of muscle fibers, bacteria, spermatozoa and blood flow in small blood vessels. Van Leeuwenhoek did not write books, but sent letters to the Royal Society in London. The letters were published in the Royal Society's journal "Philosophical Transactions of the Royal Society".