Daily Archives: 9 October 2014

October 9, 1676

Anton Van Leeuwenhoek

In 1673 Regnier de Graaf, a brilliant young physician in Delft, Holland, wrote a letter of introduction about Anton Van Leeuwenhoek to Henry Oldenburg, Secretary of the Royal Society in London. De Graaf said that Leeuwenhoek had devised microscopes that were far superior to any then known.  Accompanying De Graaf’s letter was the first letter to the Royal Society written by Leeuwenhoek, which dealt with observations on the structure of mold, as well as the structure of the bee and the louse.  Leeuwenhoek’s letter was published in Philosophical Transactions of the Society, and Oldenburg wrote to the author requesting further communications. Over the next fifty years, Leeuwenhoek wrote more than three hundred letters to the Royal Society.

On today’s date, Anton Van Leeuwenhoek wrote his most famous letter to the Royal Society, communicating the results of a series of experiments on water infused with pepper. Leeuwenhoek began by examining some snow-water that he had kept sealed for three years. He noted no creatures. He then added some peppercorns to the solution in what might have been an attempt to discover “the cause of the hotness or power whereby pepper affects the tongue.” Three weeks later, on 24 April 1676, Leeuwenhoek discovered the sudden appearance of a tremendous number of “very little animals.” Judging by his calculations of their number and size, historians have surmised that Leeuwenhoek had become the first person to see bacteria. Leeuwenhoek wrote:

The 31th of May, I perceived in the same water more of those Animals, as also some that were somewhat bigger. And I imagine, that [ten hundred thousand] of these little Creatures do not equal an ordinary grain of Sand in bigness: And comparing them with a Cheese-mite (which may be seen to move with the naked eye) I make the proportion of one of these small Water-creatures to a Cheese-mite, to be like that of a Bee to a Horse: For, the circumference of one of these little Animals in water, is not so big as the thickness of a hair in a Cheese-mite.

Previously, the existence of single-celled organisms was entirely unknown. Thus, even with his established reputation with the Royal Society as a reliable observer, his observations of microscopic life were initially met with skepticism. Eventually, in the face of Van Leeuwenhoek’s insistence, the Royal Society arranged to send an English vicar as well as a team of respected jurists and doctors to Delft to determine whether it was in fact Van Leeuwenhoek’s ability to observe and reason clearly, or perhaps the Royal Society’s theories of life itself that might require reform.

Finally, in 1680, Van Leeuwenhoek’s observations were fully vindicated by the Society. Although he neither lectured nor wrote formal scientific papers, he was recognized as an original scientist and was admitted as a Fellow to the Royal Society. Given contemporary medical theories, it did not occur to Leeuwenhoek that what he saw with his microscope was in any way connected to disease, but his observations laid a foundation on which further investigations were born.


  • Letter to H. Oldenburg, 9 Oct 1676. In The Collected Letters of Antoni van Leeuwenhoek (1957), Vol. 2, 75.