1677: To confirm his observations


It was one thing for Leeuwenhoek to count how many and calculate how small. It was another thing for those without the lenses and the expertise, the members of the Royal Society, to believe him.

As Birch recounts in his History of the Royal Society, the members, in fact, did not believe Leeuwenhoek. The process by which they came to accept the truth of his claims was an early test of the power of the developing discipline that we call science and they called philosophy, hence Philosophical Transactions.


At the regular meeting on February 1:

There was read part of a very long letter of Mr. LEEWENHOECK to Mr. OLDENBURG, dated at Delft, 9th October, 1676, which had not been produced before, because it could not be sooner translated into English out of the Low Dutch language, in which it was written.

The contents thereof were a great number of observations made by Mr. LEEWENHOECK with his microscope, concerning certain little animals found by him in vast quantities in common water, snow-water, well-water, and such water. ...

It was ordered, that the sequel of these observations should be read at the next meeting; and that the author be desired to communicate his method of observing.

"Little animals" in "vast quantities".

Two weeks later, at the weekly meeting on February 14:

Mr. OLDENBURG produced the sequel of Mr. LEEWENHOECK'S letter concerning the great plenty of very little animals observed in rain, well, sea, and snowwater, as also in water, in which pepper had lain infused.

The remainder of this paper was referred to another meeting; and the secretary was again desired to procure from Mr. LEEWENHOECK his method of observing, that by making use of the same, the Society might be enabled to confirm his observations.

The ability to "confirm his observations," to replicate results, is what we now call reliability of the evidence.

Birch noted that the final part of Leeuwenhoek's long October 1676 letter was read at the meeting on February 22. Even extracted for publication finally the following year at about half its length, this long letter ran to eleven pages in Philosophical Transactions. In all of volume 12, where most articles ran between two and five pages, only one other exceeded the length of this one by Leeuwenhoek.


The next letter that Leeuwenhoek wrote to Oldenburg is dated March 23 (AB 31), and it was read at the meeting of March 29, 1677. It was also the first that Leeuwenhoek included in his self-published collected letters. Birch noted:

A letter of Mr. LEEWENHOECK to Mr. OLDENBURG, dated at Delft, 23d March 1677 was read, giving some account of his observing live animals in water.

In that letter, van Leeuwenhoek refers to two letters to him, now lost, of February 12 and 22, asking for information about his "method of observing". He proceeds to give it to them.

I was not a little pleased that my Observations about Water had not displeased your leaned Philosophers. Nor do I wonder, they could not well apprehend, how I had been able to observe so vast a number of living Creatures in one drop of water, that being very hard to conceive without an ocular inspection.

This letter of March 23, 1677, was published in volume 12 in the number following, and thus a month or so before the long letter from October 1676. At the end of the March 23 letter, van Leeuwenhoek added a postscript of complaint that was not published:

Having again and again observed that you cannot find time to translate my letters in English.

After his early successes, he seemed to think that the problem was one of translation only. It seems it was also a question of validity and reliability. After the members heard Leeuwenhoek's letter on March 29, Birch noted:

It was ordered, that Dr. GREW should be desired to try what he could observe in the like waters; and that for this purpose an extract should be given him by Mr. OLDENBURG of Mr. LEEWENHOECK'S observations formerly read to the Society.


On May 14, 1677, Leeuwenhoek wrote another letter to Oldenburg,

Yours of the 22th of February mentions, that some of your Friends did wish, I would with all possible exactness observe the Carneous Fibres of a muscle, and also the Cortical and Medullar part of the Brain.

Trying to be agreeable, Leeuwenhoek does as requested, describing muscles and brains, moxa and cotton and his dissecting technique:

I took the flesh of a cow; this I cut asunder with a sharp knife, and using a microscope I severed before my eyes the membrane from it.

This letter has no mention of swarming little animals, but Leeuwenhoek does insist on another very large number.

These globuls, of which I saw that the Carneous filaments do consist of, are so small, that, if I may judge by my sight, I must needs say, that ten hundred thousand of them would not make one grain of gravel-Sand.

This letter ends with an unpublished request to relieve what must have been his consternation over the reception he was receiving at the Royal Society:

I beg you to let me know in what respect my observations agree with or differ from those of others.

This letter, published in Philosophical Transactions volume 12 later that year, would turn out to be the final of van Leeuwenhoek's letters published by Oldenburg, who died on September 5, 1677.

Nehemiah Grew, himself an accomplished user of the microscope, was not able to replicate Leeuwenhoek's results. After Oldenburg's death, at the next annual election of Society officers, he and Robert Hooke were elected co-secretaries, replacing Oldenburg and Thomas Henshaw.

"several testimonials
from persons of good credit" in Delft
May 18 Benedict Hahn and Henry Cordes, Lutheran pastors in Delft, wrote in Latin to the Royal Society attesting to having seen microbes in Leeuwenhoek's infusions (translations from the Collected Letters, vol. 2, pp 257-271). "We saw at least 200 living creatures in this 50th part of water; little animals which moved and swam in the water."
June 2 Robert Gordon, medical student in Delft, wrote to the Royal Society, also in Latin. "I saw as many as 20,000 of those little animals in a quantity of pepper-water not exceeding the size of a grain of millet."
August 13 Aldert Hodenpijl wrote in. "In that small Quantity of water, I saw above thirty thousand Living Creatures."
August 21 Johannes Boogert, notary, Robert Poitevin, doctor, and W. van der Burch, lawyer, wrote to the Royal Society in Latin. "We have observed with our own eyes that each of the 90 parts contained more than 500 little animals."
August 30 Alex Petrie, pastor of the English Congregation in Delft, wrote in English to Royal Society. Leeuwenhoek sent a Dutch translation, too. "... that in so small a quantity of water I should see such  vast number of those little animals."

Hooke would continue in his unofficial position as Curator of Experiments. But who would replace Oldenburg, who had published Philosophical Transactions himself -- its founding editor and the person who paid for the printing out of his own pocket? (Philosophical Transactions did not become an official Royal Society publication until the 1753.)


The unflappable Leeuwenhoek sent another letter dated October 5 (AB 33) to the Society along with a cover letter to its president, Viscount Brouncker. Birch noted that at the next meeting, the first without Oldenburg, on October 15, 1677:

Mr. HOOKE read a letter, which had been delivered him by Mr. HENSHAW, to whom it was sent by the president, in which Mr. LEEWENHOECK understanding, that Mr. OLDENBURG was dead, desired to know, to whom he might address his letters for the future; including several testimonials of his former experiments, and an account both in Dutch and Latin of some new observations.

The consideration of this was adjourned to the next meeting. And in the mean time Mr. HOOKE was desired to make a microscope after a way, which he proposed as very likely to do as much, if not in the same manner as that of Mr. LEEWENHOECK.

This is the first mention of any Latin translations provided by Leeuwenhoek. Philosophical Transactions going into a hiatus after Oldenburg's death, Leeuwenhoek's "new observations" were published the following year by Robert Hooke in Microscopium along with a long response. (See Period 2.)

The "several testimonials" from ministers, a public notary, and "other persons of good credit" were no doubt appreciated, but they were irrelevant, perhaps redundant is a better word because of the success Hooke was about to have.

While we have no evidence of any reaction by van Leeuwenhoek, a quick summary reveals the time delay that may well have been a source of frustration.

A Year's Wait

By October 1674, van Leeuwenhoek had developed sufficiently strong and difficult-to-use lens to begin to see the world of microorganisms. Meeting initial skepticism, he independently performed an extended series of experiments and controlled observation, he made careful records, and he sent them in October 1676, with illustrations, to the Royal Society. It was a another whole year before the Society got around to "considering" his claims, and even then, they wanted to be able to see for themselves.

Could they have asked Leeuwenhoek to join them? Could they have sent a delegation to Delft? Yes, but that's not how science was learning to work. The members of the Royal Society had to have someone else independently duplicate Leeuwenhoek's results from scratch.

Fortunately, they had at their disposal probably the only person on earth capable of this feat of replication and verification: Robert Hooke. Even so, it took Hooke three tries before he succeeded.