The Letters
Period 1 - 1673-1677

editor: Henry Oldenburg

According to Alle de Brieven / Collected Letters, van Leeuwenhoek wrote 35 letters during the five years from 1673 through 1677. According to Cole, 31 of those letters were eventually published. Four were lost and are known only through references to them in other letters.

Only 14 of the letters were published during van Leeuwenhoek's lifetime. As you can see on the tables, of the 31 letters:

13 were published in partial English translations in Philosophical Transactions, volumes 8 - 12, at least one each year, each volume.

1 was published by Robert Hooke in Microscopium in 1678 (see table for period 2).

The other 17 were first translated into English and published in the late 19th and early 20th centuries, and are now in Alle de Brieven / Collected Letters, vols 1 and 2.

All but eight of these letters were addressed to Henry Oldenburg. One addressed Robert Boyle about a subject of his specialty. Two, after the death of Oldenburg in September 1677, went to the mathematician Viscount Brouncker, also founding president of the Society. Four went to Constantijn Huygens, and one to his son, Christiaan Huygens.

In Delft

For the first year, the first five letters to Henry Oldenburg, van Leeuwenhoek wrote about his observations of everyday things: molds, bees, lice, trees, air, milk, hair, and nails. He also was using some more powerful lenses to see that blood contained tiny red bodies. He observed bones, brains, spit, and cuticles. On Oldenburg's suggestion, van Leeuwenhoek observed sweat, fat, tears. He discussed his techniques of cutting cross-sections to examine animal tissue and of capillary tubes for examining blood.

But in September of 1674, at the end of a long letter about the structure of the eye and of crystals, he spoke of colorful "animalcules".

And the motion of most of these animalcules in the water was so swift, and so various, upwards, downwards, and round about, that 'twas wonderful to see: and I judge that some of these little creatures were above a thousand times smaller than the smallest ones I have ever yet seen, upon the rind of cheese.

At that point, all the letters containing observations that van Leeuwenhoek had written to Oldenburg had been published in Philosophical Transactions, usually within several months of their having been written.

Only three of next nine letters were published. Two of the unpublished mention the little animals. However, one that was published was about half of a very long letter detailing his observations over many months of "Little Animals by Him Observed in Rain-Well-Sea. and Snow Water; as Also in Water Wherein Pepper Had Lain Infused", according to the title of the letter in the table of contents of volume 12.

These observations caused the members of the Royal Society to go through a formal demonstration replicating and thus validating them. The details are on the "No longer any doubts" page.

What he published himself

Twenty years later, for the Vijfde (Fifth, 1696) and Sevende (Seventh, 1702) volumes of his letters in Dutch, van Leeuwenhoek reached back and published three of his letters from this early period:

to Henry Oldenburg of March 23, 1677, that Oldenburg had excerpted in Philosophical Transactions No. 134 in 1677

to Robert Hooke in October 1677 that Hooke had published in Microscopium in 1678

to Viscount Brouncker in November 1677 about human sperm that had been published in full in Latin translation by newly appointed editor Nehemiah Grew in Philosophical Transactions No. 142 in 1679

Even though he eventually published 165 of his own letters in Dutch and Latin, other than the three listed above, van Leeuwenhoek did not publish any of his letters from these years, 1673-1677. Nor did Samuel Hoole select any for his volumes.

What's odd is van Leeuwenhoek's own numbering system. When he did publish his collected letters, Deel 1, 1685, he began with a letter from April 1679 that he labeled number 28. This letter is Cole's number 28, too, and is number 43 in Alle de Brieven / Collected Letters, volume 3. Van Leeuwenhoek was well aware that at least 27 early letters were not included. Had he not kept copies himself?

What he observed

The titles of the letters from the table of contents of those volumes of Philosophical Transactions show the range of specimens that van Leeuwenhoek stuck in front of his lens, coming from a variety of different plants and animals, some pictured on the right:

air, blood, milk, bones, brain, spitle, cuticula, sweat, fatt, teares, optic nerve, the sap of some plants, sugar, salt, trees, wine, muscle fibres, the cortical and medullar part of the brain, moxa, cotton, and little animals in rain-, well-, sea-, and snow-water, also in water wherein pepper had lain infused

Nehemiah Grew's drawing of Gresham College where the Royal Society met during van Leeuwenhoek's time

Another view would be that the newcomer and outsider from Delft was actively involved with the leading cutting-edge revolutionary scientific movement of the time and in active dialogue with the leading scientists -- Hooke, Grew, Boyle, Willis -- who took him seriously.