1674: Perhaps will to many seem incredible

In 1674, Leeuwenhoek wrote eleven letters, three to Constantijn Huygens, the rest to Henry Oldenburg.


In the spring of 1674, Leeuwenhoek was ready with new observations. In April's letters to Constantijn Huygens and Henry Oldenburg, two each, he first turned his lens on the human body. The first letter (AB 4) was saved by Huygens:

When I was at your house some time ago, I told you ... that I had observed blood from my hand, which I found to consist of red globules, also floating about in a wheylike fluid. ... I have since examined my blood once more, this time with greater accuracy. I now find that the fluid in which the globules float are more like a crystalline than like a wheyish fluid.

This letter has several other more accurate descriptions to clarify claims he had made during his visit to the leading intellectual in the Republic. Leeuwenhoek was not afraid to correct himself. Two days later, he wrote (AB 5) to Oldenburg:

I have observ'd, taking some Blood out of my own hand, that it consists of small round globuls driven thorough a Crystalline humidity or water: Yet, whether all Blood be such, I doubt. And exhibiting my Blood to my self in very small parcels, the globuls yielded very little colour.

Both Marcello Malpighi and Jan Swammerdam saw these structures before Leeuwenhoek, but Leeuwenhoek was the first to recognize what they are: red blood cells.

What he observed

Leeuwenhoek looked more closely at things he could already see, as did the other microscopists of the time. He began one of his enduring themes, how fluids moved in plants and animals, especially blood. He 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, Leeuwenhoek observed sweat, fat, and tears. He discussed his techniques of cutting cross-sections to examine animal tissue and of capillary tubes for examining blood. All that, in only three letters, from April 1673 to April 1674.

Birch's History as well as the letters themselves record that many of these specimens were suggested either directly by members of the Royal Society or indirectly as responses to observations published by others in Philosophical Transactions or elsewhere. Oldenburg encouraged this sort of interaction as part of the ethos he was helping to develop, what we now call science but he called natural philosophy. He was also taking advantage of the revolutionary technology of the time -- the printing press -- as well as reliable postal services to distribute this new knowledge.

June - July

In a letter to Oldenburg dated June 1 (AB 8), Leeuwenhoek began to deal with an ongoing problem: what he was observing had not only never been suspected by anyone. It was also, on its face, incredible or unbelievable (ongelooffelijck). The first part of the problem was size.

The red Globules of the Blood I reckon to be 2500 times smaller than a grain of sand; which perhaps will to many seem incredible.

Leeuwenhoek showed how he calculated the size and did so as part of a discussion of his techniques. Oldenburg published this letter, but he must have begun to hesitate.

The following letter of July 6 (AB 9) continued these observations of particles in various fluids: sweat, fat, and tears. Leeuwenhoek enclosed one of his thin tubes, similar to one he had taken to Constantijn Huygens. However, neither Huygens nor his son Christiaan were able to see the red blood cells with their microscopes. Perhaps they corresponded with Oldenburg about it.

Up to that point, Oldenburg had published all the letters containing Leeuwenhoek's observations in Philosophical Transactions, usually within several months of their having been written. After that flush of quick success, only three of next dozen letters were published.


Two important events in Leeuwenhoek's life happened in the summer of 1674. Jan Swammerdam visited him from Amsterdam. Swammerdam, though five years younger, had a medical degree from Leiden and had published three books in Latin about insects. The two microscopists had their first Philosophical Transactions publications in the same volume, Swammerdam's beginning two pages after Leeuwenhoek's:

  • A Specimen of Some Observations Made by a Microscope, Contrived by M. Leewenhoeck in Holland
  • Extracts of Two Letters of Dr. Swammerdam, Concerning Some Animals, That Having Lungs are Yet Found to be without the Arterious Vein; together with Some Other Curious Particulars

Both of them knew the Huygens, father and son, but it was not until the following summer that these two leading Dutch microscopists met. They had a contentious relationship that would last until Swammerdam's death in early 1680.

The second event was more momentous: the discovery of microbes.

At the beginning of the following month, Leeuwenhoek wrote to Oldenburg (my translation):

Ontrent twee uren gaens van dese Stadt, leijt een binnelantse meer, de Berckelse meer genaemt ... nu laest inde voornoemde meer varende ... Around two hours distant from this City lies an inland lake named Berckel lake. ... sailing recently over this lake ...

This lake, or rather two small lakes joined by a river, was drained in the 1700's for farmland. Oldenburg translated "twee uren" as two leagues instead of two hours. The lake was about four miles from Delft, and Leeuwenhoek may have been there with Jacob Spoors on a surveying assignment.


At the end of a long letter (AB 11) dated September 7 about the structure of the eye and of crystals, Leeuwenhoek spoke of colorful "diertgens", little animals that he had found in the lake at Berkel. (Oldenburg translated "meer" as sea instead of lake. More ominously for the public's knowledge of Leeuwenhoek today, he translated "diertgens" as animalcula, italicizing it as Latin. Two years later, he translated in animalcules, as an English word.)

Leeuwenhoek referred to them as "seer veel kleijne diertgens", literally "extremely many small little animals", doubling for emphasis both the quantitiy and size. He continued:

And the motion of most of these little animals 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.

This is the famous Berkelse Meer letter that marks the first time anyone had seen, let alone imagined, a world of living creatures a thousand times smaller than the smallest the eye could see, so small and so numerous that they would defy belief.

Of Leeuwenhoek's next dozen letters of observations, only three were published in Philosophical Transactions. He did not publish them in his Dutch/Latin collected works, either. They remained unpublished until 1930 in Veertien tot heden geheel onuitgegeven brieven ... Fourteen Hitherto Totally Unpublished Letters ... and then in 1939 in volume 1 of Alle de Brieven / Collected Letters.Philosophical Transactions

The unpublished letters increasingly revealed the little world that Leeuwenhoek was the first to see. The three published letters

  • December 4, 1674 AB 13
  • August 14, 1675 AB 18
  • April 21, 1676 AB 22

dealt with subjects that were understandable in the everyday world: the optic nerve, plant sap, trees, sugar, salt. During that time, Oldenburg published letters by other researchers with implausible claims about freaks of nature that violated what we now know as laws of physics and biology. To his mind, were Leeuwenhoek's unpublished descriptions even further beyond the threshold of plausibility?


The letter that Leeuwenhoek wrote on October 19 (AB 12) was the first that did not get published. It describes worm eggs, the structure of metals and bladders, and Leeuwenhoek's ideas about taste.

We don't have any direct evidence about the Royal Society's editorial process that would explain why some of Leeuwenhoek's letters went unpublished during Oldenburg's tenure as editor. We know that Oldenburg did not publish every letter he received from his many correspondents around the world. We also know that few correspondents had as many articles published in their whole careers as Leeuwenhoek had during his first few years alone.

It is difficult to understand what was happening at the Royal Society during these years. The letters to Leeuwenhoek are lost. Birch's History volume III, filled with the Society's experiments and discussions, mentions Leeuwenhoek only twice in these years, on pages 88 and 143. The first acknowledges the receipt of Leeuwenhoek's very first letter in 1673. The second notes that at the meeting on November 12, 1674:

Mr. OLDENBURG was directed to produce Mr. LEEWENHOECK'S observations concerning air, blood, etc.

What is true?

In the two letters of late 1674 to Oldenburg, Leeuwenhoek reacted to criticisms with some sensivity. In the Berkel Lake letter of September (AB 11), he wrote:

Meanwhile excuse my boldness in sending you these observations and please to remember that in my inconsiderate youth I did not feel inclined or eager to apply myself to studies.

The letter of December 4 (AB 13) about the optic nerve and his technique of observing cerebral structures was published the following year in volume 12 of Philosophical Transactions. That article omitted the last part of the letter, which included:

I shall be obliged to you for sending me the objections raised to my observations, that I may forward my further observations to your address. But please to remember who I am and to take my opinions for what they are worth. I have always intended to stand by my speculations and my considerations till I should be better instructed or more experienced and then to abandon my previous opinions and to accept my latest views and put them down in writing.

While Leeuwenhoek took these remarks personally, he was also exhibiting the nature of all scientific knowledge, subject to better instruction and more experience. In the world of authority and received wisdom where these proto-scientists worked, they were still distinguishing "absolute truth" from "provisional truth".