1675: A multitude of unarranged and promiscuous observations

Leeuwenhoek spent the year, at least according to the five letters he wrote, all to Oldenburg, studying a variety of minerals, especially salts, and parts of plants and animals. Most commonly, he soaked things in well water and rain water. Sometimes, he let the things dry before he observed them. Taste became a theme.


Only the letter of August 14 (AB 18) was published in Philosophical Transactions. None of the others were published by Leeuwenhoek in his collections of letters. It wasn't until Veertien tot heden geheel onuitgegeven brieven ... Fourteen Hitherto Totally Unpublished Letters ... in 1930 that the others were first published in the original Dutch and in English translation.


On January 22 (AB 15), he wrote about his improved technique for viewing blood cells and sectioned brain tissue. They seem to have involved what we now call dark-ground microscopy, which he expressed as a metaphor, black silk. Perhaps knowing his persistent observations of blood were a problem for the Royal Society, he wrote:

I can demonstrate to myself the globules in the blood as sharp and clean as one can distinguish with one's eyes, without any help of glasses, sand grains that one might bestrew upon a piece of black taffety silk.


He described salt crystals, vinegar eels, soap, pepper, bile, mustard, optical nerve, fish eggs, and leaf veins, among other things. This letter of February 11 (AB 16) is very long, which he excused in the beginning:

I have drawn the figures made by the salts of herbs as well as I could (but I have not been able to delineate all the particles to perfection) and send you these enclosed, in addition to my trifling speculations; I give the latter unarranged and promiscuously (as put down during my observations) and I hope you will excuse this, bearing in mind etc.

He returned to that excuse at the end:

I hope that you will not be displeased by the trifling observations set forth herein, and that you will not be incommoded by the multitude of my observations.


Thoughout this year, Leeuwenhoek was receiving letters from Oldenburg, now lost. He began this letter of March 26 (AB 17):

You observe that people of great knowledge in Paris and elsewhere1 do not agree on the globules I have discovered in a great many bodies. I do not mind this at all. What I wrote on this subject is perfectly true, and if those learned gentlemen were living in the Netherlands, I should invite them to come and see the globules in most of the parts, as I described.

He described bean and pea meal. He continued his observation of blood serum, discussing the transparency of objects and how sap circulates and muscle has veins.

In closing, he wrote:

You told me (in your letter) that your friends wished the figures of the salt to be more neatly and distinctly drawn, and that possibly my microscope did not show the same more clearly. Sir, be assured that my microscope showed the same as clearly and distinctly as one can imagine to see figures with the naked eye but the fault is mine, since I cannot draw and on the other hand since I have the intention to keep the method I use secret from everybody. Therefore I draw the lines rough and simple only to assist my memory with the sole intention to give the meaning of the figures


The letter of August 14 (AB 18), the only other published in volume 10 of Philosophical Transactions, continued Leeuwenhoek's observation of the texture of blood and the parallel role of sap in plants. He also discussed the structure of sugar and salt and the difference in how they taste.

In the letter's opening (not published in Philosophical Transactions), Leeuwenhoek wrote:

You recommend me to call in the assistance of other persons, capable of judging said things. Allow me to say, Sir, that there are very few people in this town that could assist me, and as to people that come to visit me from other places, well, recently I had one here who rather intended to dress himself in plumage borrowed from me, than to lend me a helping hand.


In the middle of the month, the painter Jan Vermeer died. We don't know much about their relationship while Vermeer lived. But Vermeer's tangled finances and the character of his mother-in-law, Maria Thins, would have great effect on Leeuwenhoek and his scientific work later the following year.

Just before Christmas, on December 20, he wrote a letter (AB 19) to Oldenburg that discussed his theory of globules as well as an aerometer that Leeuwenhoek invented. He noted its similarity to an invention by Boyle in Philosophical Transactions number 115. In the opening paragraph, he noted:

I expect to be contradicted, since the speculations set forth in my letter, will appear strange to some people.

In the next paragraph, he corrects what he called a "serious mistake" from his February letter:

Last summer I carried out many observations on various waters and discovered in most of them a great many small animals that are incredibly minute and different from the animals seen by others in water.

He described live creatures in water as well as his technique for examining nerves and blood cells. He added:

I looked forward to another letter to learn of the opinion of the Gentlemen on my thesis, for I expect to be contradicted since the speculations set forth ... will appear strange to some people. I will be greatly obliged if these objections are communicated to me.

I saw that Mr. Boyle had invented similar glasses and that the said gentleman had thought much farther than I did, ... which I admit is a very important question.

In parentheses, he added

to my regret I do not understand English and in this town there is nobody who is able to translate it into Dutch for me.

Beginnings of science

Leeuwenhoek and his contemporaries did not have the benefit of our hindsight. In their struggles, we see the beginnings of the Enlightenment and the birth of the scientific method. Their hindsight was on the world of authority stretching back to Aristotle and further, to the Old Testament and the beginnings of time. While scholars have found evidence of atheism even before the word was coined in the 1500's, there were no coherent public statements or adherents during Leeuwenhoek's lifetime. The Eastern tradition of Hinduism was godless and not an alternative in Europe. Not everyone was devout, but those who doubted the very existence of the God of Abraham kept it to themselves.

In the world of authority, you learned what was true and had always been true, including the Genesis creation story. You repeated that, giving you authority. That world was full of secrets accessible only by the adept or the elect.

In his first letter, Leeuwenhoek replicated Hooke's observations from Micrographia. Perhaps he did it out of curiousity. Perhaps it was de Graaf who explained the motto of the Royal Society, at least implicitly:

The Royal Society's motto 'Nullius in verba' roughly translates as 'take nobody's word for it'. It is an expression of the determination of Fellows to withstand the domination of authority and to verify all statements by an appeal to facts determined by experiment.

In that context, Leeuwenhoek's replication was publishable. Comparing Hooke's descriptions and Leeuwenhoek's shows that Leeuwenhoek extended Hooke's observations by seeing details that Hooke had either missed or omitted recording. In the world of authority, that might be viewed as heresy. In the new world of observation and experiment, it was viewed as an advancement. By the spring of 1676, Leeuwenhoek was challenging Nehemiah Grew, and the favor was returned. These early letters show that Leeuwenhoek was continually taking suggestions about what to observe and how to observe it. He was open about most, though not all, of his methods and techniques.