Wrote Letter L-015 of 1674-09-07 to Henry Oldenburg about the anatomy of the eye and optic nerve of a cow, minerals eg. salt, clay, English and Flemish earth; first mention of protozoa in stagnant water

September 7, 1674
Standard reference information
Leeuwenhoek's number: 
Collected Letters number: 
Collected Letters volume: 

Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

The original manuscript on six folio pages, written and signed by Leeuwenhoek, is preserved at the Royal Society (MS. 1835. Early Letters L1.7).

An excerpt was published in Philosophical Transactions, vol. 9, no. 108, dated 23 November 1674. See Publication history below.

Leeuwenhoek wrote this letter to Henry Oldenburg about the eye, optic nerve, and crystals; little animals in water from Berkelse Meer.

It is an example of the kind of letter that gives him a reputation for disorganization. He began with the eye of a cow, which was hard to mount behind his lens. He found that if he let it dry, until it:

in hardness almost resembles a Nutmeg preserved, I have with a razor cut asunder, and observing it in parcels ....

In other words, the difficulty of mounting and lighting his specimens led him to be the first microscopist to use sectioning. After relating a visit from Jan Swammerdam, he noted how he "experienced a very painful treatment" from some people in Delft who pointed out a mistake he made. He discussed how his friend Dr. Cornelis 's-Gravesande explained the difference between tendons and nerves. Then he moved on to the optic nerve, encouraged by 's-Gravesande to explore other nerves. He wrote, "To pass on to other objects" and described burning salt. That reminded him of his trip to England six years earlier when he "tried to discover the component parts of chalk" from the famous white cliffs. He also studied the yellowish Earth there that the Dutch potters used. That led to the clay around Delft, called Black Earth by the potters.

Finally, at the end of this long letter, Leeuwenhoek changed subjects abruptly. He tacked on a paragraph that about a casual observation that would change his life.

About two leagues from this Town there lyes an Inland-Sea, called Berkelse-Lake. ... and observing the water ... I took up some of it in a Glass-vessel which having viewed the next day, I found moving in it several Earthy particles, and some green streaks, spirally ranged, ... among all of which there crawled abundance of little animals some of which were roundish; those that were somewhat bigger than others were of an Oval figure: On these latter I saw two legs near the head and two little fins on the other end of their body.

Little animals? What's the difference between a "particle" and an "animal"? They both moved. But the particles were moved and the animals moved themselves.

The motion of most of them in the water was so swift, and so various, upwards, downwards, and round about, that I confess I could not but wonder at it. I judge, that some of these little creatures were above a thousand times smaller than the smallest ones, which I have hitherto seen.

From the description of shapes and colors and heads and fins, Dobell concluded that Leeuwenhoek saw rotifers, ciliates, and a common species of Euglena. More recently, Wim van Egmond has developed that description.

At the time, Leeuwenhoek said nothing else about their size and number, perhaps recognizing how unbelievable others might find his claim and the "very painful treatment" that he might again suffer.

These letters do not show a secretive amateur. They show a forthright but insecure researcher who was astonished by what he saw and was eager to help others see, too.