Leeuwenhoek's Animalcules

The letter that left Delft on 9 October 1676 ran to 18 pages of Dutch. Antoni van Leeuwenhoek addressed it to Henry Oldenburg in London, and in it he described five kinds of animalcules, tiny living things he had watched in water in which pepper had been left to soak. One of them was possibly a bacterium. He had also examined rain, well, moat, sea and river water and infusions of other spices, and he asked whether such creatures were present in the air.

Leeuwenhoek was a tradesman with no university education who read only Dutch. Born in Delft in 1632, the son of a basket-maker, he trained in a linen-draper's shop and later worked as a draper, surveyor, wine assayer and minor city official. Some time before 1668 he learned to grind lenses. His microscopes had one lens clamped between two small metal plates, with the specimen on a pin moved by two screws, and they magnified more than 200 times. The compound microscopes of the day were of little practical use beyond 20 or 30 times.

His report was hard to credit. Nobody knew such organisms existed, and the American Society for Microbiology notes that his reputation was called into question. Oldenburg translated part of the letter, and the Royal Society printed it in Philosophical Transactions on 25 March 1677. Leeuwenhoek kept writing to the Society, in Dutch, for about fifty years, and his letters were translated into English or Latin.

Later letters went further. In 1683 he described swarms of moving animalcules in the plaque from his own teeth and from the mouths of two old men who had never cleaned theirs, among the earliest recorded observations of living bacteria. He also described blood cells, sperm cells, nematodes and rotifers. The Society elected him a fellow in 1680, though he never attended a meeting, and in 1698 he showed Tsar Peter the Great blood moving through the capillaries of an eel. He died in 1723.