1673: Exceedingly curious and industrious

In 1673, Leeuwenhoek wrote three letters, all to Henry Oldenburg of the Royal Society in London. One letter is lost.



"About two years ago I observed the sting of a bee," he wrote in his first letter, dated April 28, 1673 (AB 1). Thus, by early 1671, Leeuwenhoek had learned how to grind lenses and work metal as well as almost anyone. Two years later, Reinier de Graaf had mentored Leeuwenhoek to write down his observations and send them to Henry Oldenburg (below left). Oldenburg published Philosophical Transactions, the only periodical other than the French Journal des Scavans that would consider such a letter. This first letter  replicated and expanded on several of Robert Hooke's observations in Micrographia: mould, the stinger of a bee, and an eye of a bee.


Within a month after Leeuwenhoek sent it, Oldenburg had translated the letter and read it for discussion at a meeting of the Royal Society.

On May 7, 1673, the Royal Society met in London. Birch's History notes:

Mr. OLDENBURG produced a book of Dr. de GRAAF dedicated to the Society, ... together with a letter to Mr. OLDENBURG, dated at Delft in Holland, 28th April, 1673, communicating some microscopical observations of Mons. LEEWENHOECK.

Oldenburg published this letter on May 19 in Philosophical Transactions number 94. He added a sentence anticipating more observations and apparently sent a letter to Leeuwenhoek in the same vein.


Leeuwenhoek was ready with his second letter (AB 2). He had drawings to accompany his first letter, at Oldenburg's request. He had new observations and the description of an experiment. Right away, he set his style of working multiple threads into one letter. Subheadings would have helped. The second letter covered these topics:

  • Plant - deal (pine), oak, beech, sugar cane; ash root; mold; tylose
  • Animal - bee, louse, gnat, wasp sting; louse eye, mouth, stomach, gut; louse, valves
  • Processes - fluid movement, fire; digestion, drawing blood, force of piston
  • Techniques - specific gravity of deal and oak
  • Other - composition of air; globules; personal details about education and language

Note Leeuwenhoek's reference to "my recently invented microscope". The Dutch is mijn nieuw gevonden microscopix. The Dutch gevonden is a form of the verb vinden, to find. In Sewel's 1735 dictionary, uitvinden means to find out, to discover. In the other direction, invent is uytvinden, verzinnen, bedenken, and verdichten in Dutch. In spite of Leeuwenhoek's place in the popular imagination as "inventor" of the microscope, he was not and he did not claim it here. He was writing to the Royal Society, after all, of whom Robert Hooke was an active member. Having repeated and extended Hooke's observations from 1665 himself, Leeuwenhoek would not have claimed any primacy.

Constantijn Huygens, who lived not far from Delft, visited Leeuwenhoek and read the letter. A week before Leeuwenhoek sent it, Huygens sent his own letter to Robert Hooke that acted as a cover letter and recommendation similar to de Graaf's letter in April.

Two days after the date of Leeuwenhoek's second letter, Reinier de Graaf died on August 17, at age 32. Two months later Leeuwenhoek celebrated his 41st birthday, If only his close family and their spouses and children had come to the party, three dozen people would have been there. None of them had any idea that their kinsman, a guy who used to have a shop and was now working for the city government, was about to become the most famous person in Delft.

What he accomplished

Encouraged by the response to his first letter, Leeuwenhoek felt the need to explain himself. In the long excerpt below, he sounds insecure, doubting, tentative, apologetic. That first letter was so risky that he blames it on de Graaf. He wants to make sure that they understand that he is a tradesman without the kind of education they have.

Remember that he is addressing famous scholars in a country that his own country was at war with. Not only that, de Ruyter's Dutch fleet was soundly defeating the English and French navies combined. A week later, the English would suffer their decisive defeat just off the Dutch coast.

From Leeuwenhoek's point of view, he was trying out something that was taking a lot of his time, that had no prospects of remuneration, and that he could explain to only a few of his fellow Delftenaars, one of whom died two days later. His mentor Constantijn Huygens had the week before written to the Royal Society to prepare the way for Leeuwenhoek's letter:

I could not forbear by this occasion to give you this character of the man, that he is a person unlearned both in sciences and languages, but of his own nature exceedingly curious and industrious, ... so diligent a searcher as this man is, though allways modestly submitting his experiences and conceits about them to the censure and correction of the learned.

Leeuwenhoek seemed willing to express that modesty. He did not want to appear impertinent. He wanted to temper expectations. Without preamble, the second letter opens (text from Alle de Brieven / Collected Letters vol. 1, p. 43, my emphasis):

I have several times been pressed by various gentlemen to put on paper what I have seen through my recently invented microscope. I have constantly declined to do so, first because I have no style or pen to express my thoughts properly, secondly because I have not been brought up in languages or arts, but in trade, and thirdly because I do not feel inclined to stand blame or refutation from others.

Pressed by Dr. Reg. de Graaf I have thought better of my intention and given him a memorial of what I observed concerning mould, the sting and some articulations of the bee, and also the sting of the louse, which memorial he (Mr. de Graaf) has forwarded to you and informed me of your reply. I see from this that my observations were not unwelcome to the Royal Society, and that these Gentlemen were anxious to see the figures of the sting and articulations of the bee which there I state to have seen.

As I am not a draughtsman myself, I have had them drawn for me, but the proportions have not been observed as accurately as I could have wished. Also each figure has been seen and drawn through a particular magnifying glass; I am sending you these enclosed.

I beg you and the Gentlemen under whose eyes this happens to come, to bear in mind that my observations and opinions are only the result of my own impulse and curiosity and that there are in this town no amateurs who, like me, dabble in this art. Take my simple pen, my boldness and my opinions for what they are; they follow without any particular order.

Considering what happened over the following half of a century, we might think of this letters as the second of the hundreds of installments of his observations "... without any particular order". The man needed a blog!

Of course, the insecurity did not last, even though Leeuwenhoek never lost his formal deference to his distinguished correspondents, a flourish common at the time. Before long, Leeuwenhoek was the most famous person in Delft, receiving vists from royalty. His style didn't hinder him from becoming Philosophical Transactions's most published author, a distinction he still holds. Later letters still covered multiple topics, but usually had a focus or common thread.

To the modern writer with the most negative assessment of Leeuwenhoek -- Baas Becking in his Immortal Dilettant article of 1924 -- it seemed that Leeuwenhoek was but:

a pair of eyes, a pair of hands, directed by other minds. For when his own mind tried to direct, he could produce nothing but chaos.

The more positive view: two of the few world-class scientists in the Dutch Republic, de Graaf and Huygens, had an eye for new talent. They were encouraging Leeuwenhoek right from the beginning. Leeuwenhoek must have returned the visit to Hofwijk, Huygens' house in Voorburg, because the following spring he wrote, "When I was at your house some time ago."

The newcomer and outsider from Delft was actively involved with the leading cutting-edge revolutionary scientific movement of the time. He was in active dialogue with leading scientists -- Oldenburg, Hooke, de Graaf, Huygens -- who took him seriously.