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Send-Brieven / Epistles
Cabinet of Wonders
This "cabinet of wonders" has all of the 140 figures, originally copperplate etchings, that accompanied half of the 46 letters of the Send-Brieven / Epistles.
This page has Letters XII, XV, XVI, XIX.
Clicking on the underlined Roman numerals on the table below will take you to that letter's summary and figures.
| page |
letters |
| Period 6 |
I |
| Using the Microscopes |
II, III, V, XI |
| Counting the Animalcules |
XII, XV, XVI, XIX |
| No Longer Any Doubt |
XXIV, XXV, XXVI, XXVIII, XXX |
| As Science Began |
XXXII, XXXIII, XXXIV, XXXVI, XXXVII |
| Theater of Nature |
XLI, XLIII, XLIV |
 Click thumbs to enlarge. Click- drag to move. Open several.
Letter XII
to: the Royal Society in London
October 26, 1714
Dutch title: Drie soorten van Muggen.
English title: Three types of gnats.
Letter XII
of October 26, 1714
to the Royal Society in London
Van Leeuwenhoek's summary of the contents:
Three types of gnats. Gnats without hooks. Gnats with hooks, and their form. Larger type of gnats, that also have hooks. The flesh from the legs of the last type examined. Every flesh fiber also provided with circular ridges.
Remarks made on the rose beetle. The fleshfibers of the rose beetle do not appear to be wrapped with little membranes; but each flesh fiber appears to be covered with a special little skirt. Whether the fibers of such a leg are attached from within to the hard horny part of the leg. Thorn-like pointy parts stand on the legs of a rose beetle. The membranes examined, arranged from within against the horny part of the leg: how thickly these membranes lie on one? The membranes described.
A fly examined: the horny part of the leg, and the little membrane that covers the horny part, appear to be composed of round globules. Muscles discovered in the legs of the fly.
Strange experiences concerning a flying creature, named the Spek-eeter [lit., pork-eater]. Continual relaxing and stretching in the flesh fibers of the mentioned Spek-eeter. The same experiences observed in various so-called Spek-eeters. The relaxing and stretching were not observed in the flesh fibers of flies.
New remarks concerning the wild honey bee. The wild honey bee does not let its hooks stick in the body, as does the tame bee. The previously mentioned movement, of stretching and relaxing, also examined in the flesh fibers of the wild honey bee: but in vain. Many little strands of flesh lie obliquely in the legs of flying creatures. In the flesh parts of a gnat leg no movement discovered.
Observations concerning the flesh of a very fat cow. Why the mentioned movement of flesh fibers is seen more in small creatures, like flies, than in large creatures; namely because in the small bees each flesh fiber is wrapped with a membrane, and thus can provide for the muscle, which is not like that in four-footed animals.
Further remarks on the flesch fibers of the mentioned cow. The circular ridges of the flesh fibers are not composed of cicles. How the same are shaped.
It will be found that the ridges, that the pullers [tendons; Dutch "Trekkers"] of the whale have, also occur spirally or screw-like. The same also found in the puller, that was taken from the back leg of a horned beast. The puller comes to the same thing as the puller of a whale. The same found in the pullers of the hen and the mouse.
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Letter XII
of October 26, 1714
to the Royal Society in London
Van Leeuwenhoek's summary of the contents:
Drie soorten van Muggen. Muggen zonder angels. Muggen met angels, en hun maakzel. Grooter soort van Muggen, die ook angels hebben. Het vleesch uyt de Pooten van de leste soort onderzocht. Yder vleeschfibertje ook met ringswyze inkrimpingen voorzien.
Opmerkingen op een goude Torre gedaan. De vleeschfibertjens van een goude Torre schynen niet omwonden te zyn met membraantjes; maar yder vleeschfibertje schynt met een byzonder rokje bekleedt te zyn. Of de fibertjens van zoo een Poot van binnen aan het harde hoornachtige deel van de Poot gehecht zyn. Doorensgewys puntige deelen, op de Pooten van een goude Torre staande. De membraanen, van binnen tegen het hoornachtige deel van de Poot aanleggende, onderzocht: hoe dik die membraanen op een leggen? Die membraantjens beschreven.
Een Vlieg onderzocht: het hoornachtige deel van de Poot, en het membraantje dat het hoornachtige deel bekleede, scheenen te besteen uyt rondachtige bolletjes. Musculs in de Pooten van een Vlieg ontdekt.
Zeldzeeme ondervindingen omtrent een vliegend Schepzel, Spek-eeter genoemd. Gedurrige inkrimpingen en uytrekkingen in de vleeschfibertjes van de gemelde Spek-eeter. Dezelve ondervinding in verscheyde zoo genaamde Spek-eeters waargenomen. De inkrimpingen en uytrekkingen wierden niet waargenomen in de vleesfibertjes van Vliegen.
Nieuwe opmerkingen omtrent de wilde Honigby. De wilde Honigby laat haare angels in de lichaamen niet steeken, gelyk de tamme Byen. De voorgemelde beweging, of uytrekking en inkrimping, ook in de vleeschfibertjes van de wilde Honigby gezocht: maar schier te vergeefs. Veele vleschstriemtjes leggen schuyns in de Pooten der vliegende Schepzelen. In de vleeschdeelen van een Mugge-poot geene beweging ontdekt.
Waarneemingen omtrent het vleesch van een zeer vette Koe. Waarom de gemelde bewegingen der vleeschfibertjes meer gezien worden in kleyne Schepzelen, gelyk Vliegen, dan in groote Schepzelen; namelyk om dat in de kleyne beesjes yder vleeschfibertje met een membraantje is omvangen, en dus voor een muscul kan verstrekken; het welke zoo niet gelegen is in de viervoetige Dieren.
Vordere opmerkingen op de vleeschfibertjes van de gemelde Koe. De ringsqyze inkrimpingen van de vleeschfibertjes bestaan niet uyt eene circulaire rondte. Hoe dezelve van maakzel zyn.
Men zal bevinden dat de inkrimpingen, die de Trekkers van een Walvis hebben, ook omwentelenderwyze of schroefgewys geschieden. Het zelve ook bevonden in den Trekker, die uyt het achterbeen van een Hoornbeest was genomen. Die Trekker koomt over een met den Trekker van een Walvis. Het zelve bevonden in de Trekkers van een Hoen en van een Muys.
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Figure 1: Two pullers [tendons; Dutch "Trekkers"] from the leg of the larger type of gnat showing pieces of flesh
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
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Figure 2: Scraping from the horny part of the leg of a flying creature
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
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Figure 3: Bit of flesh from the leg of a fly
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
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Figure 4: Flying creature called a Spek-eeter [lit: ham eater]
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
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Figure 5: Flesh fiber from the leg of a wild honey bee
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
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Figure 6: Flesh fiber from a fat horned beast
Letter XII
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
October 26, 1714
Dutch title: Drie soorten van muggen
English title: Three types of gnats
note: Figure 7 of a single flesh fiber is missing.
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Letter XV
to: the Royal Society in London
November 20, 1714
Dutch title: Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker.
English title: It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller [tendon].
Letter XV
of November 20, 1714
to the Royal Society in London
Van Leeuwenhoek's summary of the contents:
It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller [tendon; Dutch "Trekker"]. On the contrary most flesh fibers, as thick as they are, are joined with pullers, sheaths, and blood vessels.
The back legs of a mouse examined. Twelve pullers discovered in some parts of the leg, where one can not recognize fleshy parts with the naked eye. The pullers have flesh fibers on the back side, that also lay in an orderly arrangement. Description of the orderly arrangement of the flesh fibers. The force, which, for example, was applied to the topmost part of the mentioned pullers; was also applied to the six on the undermost side: and how?
The pullers from the back foot of a mouse are thinner than a hair from our head. They have nevertheless their spiral ridges. The flesh fibers, fixed to the pullers, are on the other side also attached to other pullers. Thoughts of the author about the firm attachment of the topmost pullers to other smaller pullers. The muscle of a mouse cut through the middle, in order to examine the ends of the largest pullers. On that occasion a large membrane cut through, from which many little membranes originate. The pullers and the flesh fibers similarly formed.
The flesh fibers are in certain senses also pullers. Where the pullers have their end. The large flesh muscles compared to the trunk of a tree, that itself spreads out into various branches: but there the branches end, the flesh fibers having on the contrary no end. How a puller can be separated from another without splitting.
Flesh from the large muscle of the back leg of a mouse brought in front of the magnifying glass. Two pullers discovered there, that were not thicker than a hair from our head. In such a puller various spiral ridges traced: the same pullers provided with obliquely descending flesh fibers. To such a puller is no force done, without another bearing that force also; similarly, all the pullers and flesh fibers that come out of a larger puller. All the flesh fibers and pullers of a muscle are moved similarly.
Amazing perfection of the structure of muscles, pullers, flesh fibers, etc. The flesh fibers appear joined to no other pullers, as to the pullers on the opposite side. How difficult it is to show the mentioned structure of the pullers and flesh fibers. Two, even three flesh fibers, sometimes join into another flesh fiber. All of the force, that is done in the pullers, is also done in the flesh fibers. How a puller nevertheless bears even greater force than a flesh fiber. Demonstration of the structure of pullers and flesh fibers.
Further indication how through the movement of a puller other pullers also are moved with the flesh fibers. Why in the pictured puller no spiraling ridges were traced. One can better follow the ends of the pullers in small flesh muscle. The pullers have flesh fibers on all sides.
Shorter muscles in a mouse investigated. Flesh fibers originate from other flesh fibers. Indication how a shoot comes from a puller, and from the shoot little flesh fibers, and from these again a shoot. The flesh of a Bonsem investigated.
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Letter XV
of November 20, 1714
to the Royal Society in London
Van Leeuwenhoek's summary of the contents:
Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker. In tegendeel zyn de meeste vleschfibertjes, zoo dik als ze zyn, vereenigt met de Trekkers, Vliezen, en Bloet-vaten.
De achterste Pooten van een Muys onderzocht. In eenige deelen van de Poot, waar aan men met het bloote oog geene vleeschdeelen konde bekennen, twaalf Trekkers ontdekt. Die Trekkers aan weerszyde bezet met vleeschfibertjens, die ook zeer ordentlyk geschikt lagen. Beschryving van die ordentlyke schikkinge der vleeschfibertjes. Het geweld, 't welke by voorb. aan de bovenste der gemelde Trekkers word aangedaan; dat word ook aangedaan aan de ses onderste: en hoedanig?
De Trekkers uyt de achtervoet van een Muys zyn dunder dan een hair van ons hooft. Zy hadden evenwel hunne omwentelende inkrimpingen. De vleeschfibertjes, aan die Trekkers vast, zyn aan de andere kant ook gehecht aan ander Trekkers. Gedagten van den Auteur over het vasthechten van de bovenste Trekkers aan andere kleyndere Trekkers. De muscul van een Muuys in 't midden doorsneeden, om de eynden van den grooten Trekkers te ontdekken. By die gelegenheit een groote Membraan doorsneden, uyt dewelke veele membraantjes voortquamen. De Trekkers en de vleeschfibertjes gelyk geformeert.
De vleeschfibertjes zyn in een zekeren zin ook Trekkers. Waar dat de Trekkers hun eynde hebben. De groote vleesch-muscul vergeleken by de stam van eenen Boom, die zich in verscheyde takken uytspreydt: maar daar de takken hun eynde hebben, hebben de vleeschfibertjes in tegendeel geen eynde. Hoe dat de Trekker van een kan scheyden zonder scheuritage.
Vleesch uyt de groote muscul van 't achterbeen van een Muys voor het Vergroot-glas gebragt. Daar in twee Trekkers ontdekt, die niet dikker waren als een hair van ons hoofd. In zoo eenen Trekker verscheyde omwentelende inkrimpingen bespeurt: dezelve Trekkers voorzien met schuyns nederdaalende vleeschfibertjes. Aan zoo eenen Trekker word geen geweld aangedaan, of de andere lydt dat geweld ook; gelyk ook alle de Trekkers en Vleeschfibertjes, die uyt een grooter Trekker voortkoomen. Alle de Vleeschfibertjes en Trekkers van een muscul worden gelyk bewogen.
Verwonderens-waardige volmaaktheit van 't zamenstel der Musculen, Trekkers, Vleeschfibertjes, enz. De Vleeschfibertjes schynen aan geen endere Trekkers vereenigt, als aan de tegen overstaande Trekkers. Hoe moeyelyk het is het gemelde 't zamenstel der Trekkers en Vleeschfibertjes te vertoonen. Twee, ja drie Vleeschfibertjes, vereenigen zich somwylen in een ander Vleeschfibertje. Al het geweld, dat aan de Trekkers word aangedaan, word ook aangedaan aan de Vleescvhfibertjes. Hoe een Trekker evenwel veel grooter geweld lydt als een Vleeschfibertje. Aanwyzing van 't zamenstel der Trekkers en Vleeschfibertjes.
Voerdere aanwyzing hoedanig dat door het beweegen van den eenen Trekker ook de andere Trekkers, met de Vleeschfiertjes, bewogen worden. Waarom in den afgebeelden Trekker geene omwendelende inkrimpingen bespeurt worden. Men kan de eynden van de Trekkers beter vervolgen in kleyne vleesch-musculen. De Trekkers zyn rontom met vleeschfibertjes bezet.
Korter musculs in een Muys nagespeurt. Vleeschfibertjes uyt andere vleeschfibertjes voortkoomende. Aangewezen hoe uyt een Trekker een Sprank, en uyt die Sprank vleeschfibertjes, en uyt deze wederom Spranken voortkoomen. Het vlees van een Bonsem onderzocht.
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Figure 1: flesh fiber from the lowest shank of a mouse near the foot showing three pullers [tendons; Dutch "Trekkers"] with spiraling ridges and one puller (right) without them.
Letter XV
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
November 20, 1714
Dutch title: Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker.
English title: It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller.
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Figure 2: flesh fiber from next to the spine of a mouse showing connection to spine (lower) and muscle spreading (right) by attachment to another muscle
Letter XV
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
November 20, 1714
Dutch title: Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker.
English title: It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller.
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Figure 3: a puller [tendon; Dutch "Trekker"] and its flesh fibers showing where they join
Letter XV
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
November 20, 1714
Dutch title: Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker.
English title: It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller.
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Figure 4: puller [tendon; Dutch "Trekker"] from the breast of a wild honey bee showing puller through the middle, flesh fibers lower left, and two shoots, middle right and top, also with flesh fibers.
Letter XV
to: Myn heeren die van de Koninglyke Societeit te Londen - Royal Society
November 20, 1714
Dutch title: Het is altyd niet waar dat de vleesdeelen in een kleyn Diertje in dunte toeneemen en eyndigen aan den Trekker.
English title: It is not always true that the fleshy parts in a small animal increase in thickness and end in a puller.
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Counting the Animalcules
How small? How many?
How van Leeuwenhoek calibrated and counted
the multitude of tiny things he discovered.
It was an extraordinary claim, and no one believed it. Yet the man making the claim, Antony van Leeuwenhoek, trusted his eyes, amplified by his lenses. These lenses were so powerful that they let him break past the limits of the macroscopic world, the world of everyday things that we can see and touch. The first human to cross this threshold, he entered the microscopic world, a world that no one in his time, the 17th century, even suspected.
The more van Leeuwenhoek looked, the more he was surprised and delighted. He says that his earliest sighting of thousands of animalcules in a drop of water:
... was for me, among all the marvels that I have discovered in nature, the most marvellous of all; and I must say, for my part, that no more pleasant sight has ever yet come before my eyes than this of so many thousands of living creatures in one small drop of water, all huddling and moving, but each creature having its own motion.
The claims
The claims by Galileo -- that it was the earth moving, not the sun -- violated the most obvious common sense, that the earth is standing still. Van Leeuwenhoek's claims also violated the most obvious common sense. He claimed that:
 the world we could see was composed of tiny living things and indeed whole societies of intricately formed animals
the different kinds of animals were very, very tiny
there were millions, no billions, even trillions of them in a glassful of water or the guts of an animal, all animals, including humans
"I do but tell fairy-tales"
On November 12, 1680, as a newly elected Fellow of the Royal Society, van Leeuwenhoek wrote to Robert Hooke, by then its co-secretary and curator of experiments (Dobell's translation),
Whereas I suffer many contradictions, and oft-times hear it said that I do but tell fairy-tales about the little animals, and that there are people in France who do not scruple to say that those are not living creatures which I exhibit, ... I make bold to say, that people who say such things have not yet advanced so far as to be able to carry out good observations.
So after he figured out how to make the lenses, then how to make the microscopes, and finally how to patiently see what he called "animacules" and other tiny structures, van Leeuwenhoek had two other questions to answer.
How little were these animacules? They had no names, though we now call them bacteria, protozoa, and cells.
How many of them were there?
Having trained himself to be a surveyor, van Leeuwenhoek knew enough arithmetic to be able to answer both questions.
The numbers did not have to be "inconceivable" for others to doubt. In 1710, he was kind enough to spend some time with Zacharias von Uffenbach, a German jurist, who wrote (Dobell translation):
Mr. Leeuwenhoek showed us further the eye of a fly, which appeared very remarkable under the microscopium. ...
He is of opinion that a fly, according to his view, has more than a hundred, nay, more than a thousand, eyes; which is only one of this good man's extraordinary notions, which seem wont to have more of ingenuity than foundation.
You can see why van Leeuwenhoek would be less than patient with visitors. He would show them something that they could see with their own eyes, and then they would call what they saw an "opinion", an "extraordinary notion".
How he measured and counted
Van Leeuwenhoek lived before micrometers, before standards of measurement for small things. Such standards like the inches and miles had long been used by surveyors for large things. Birch's History recounts the November 8, 1677, meeting of the Royal Society at which, for the second week in a row, Hooke was unable to replicate van Leeuwenhoek's results. Birch continues that Hooke:
then shewed a way of measuring the bigness of any object seen through
the microscope, which was by opening the other eye, and seeing some other object with the left eye, whilst the right eye sees the object through the microscope:
and it was evident, that a pipe not bigger than a pig's bristle appeared a
cylinder of about three inches diameter.
Given these constraints and crude solutions, van Leeuwenhoek compared his tiny animalcules to common but still visible things: grains of sand, millet seeds, and hair. The claims from October 1676 that Hooke was trying to replicate had been followed by a letter earlier in 1677 titled: "Wherein Some Account is Given of the Manner of His Observing So Great a Number of Little Animals in Divers Sorts of Water".
How great a number? In a drop of water, van Leeuwenhoek computed 2 ,730,000 (see image), and underestimating on purpose, claimed only "1000000 living Creatures in one drop of water". The details are discussed on the "No Longer Any Doubt" page.
A few years later, he was still patiently explaining his methods. His letter of November 12, 1680, continues:
As they'll say 'tis not credible that so great a many of these little animalcules can be comprehended in the compass of a sand-grain, ... I have figured out their proportions thus, in order to exhibit them yet more clearly to the eye:
He enclosed this drawing with his letter and then explained it.
Let me suppose, for example, that I see a sand-grain but as big as the spherical body ABGC and that I see, besides, a little animal as big as D, swimming, or running on the sand-grain; and measuring it by my eye, I judge the axis of the little animal D to be the twelfth part of the axis of the supposed sand-grain AG; consequently, according to the ordinary rules, the volume of the sphere ABGC is 1728 times greater than the volume of D.
He then compared objects of progressively smaller size.
Now suppose I see, among the rest, a second sort of little animals, which I likewise measure by my eye ...; and I judge its axis to be the fifth part, though I shall here allow it to be but the fourth part (as Fig E), of the axis of the first animacule D; and so, consequently, the volume of Fig D is 64 times greater than the volume of Fig E.
This last number, multiplied by the first number 1728 comes then to 110,592, the number of the little animals like Fig E, which are as big (supposing their bodies to be round) as the sphere ABGC.
His comparison progressed one more level.
But now I perceived a third sort of little animalcule, like the point F, whereof I judge the axis to be only a tenth part of that of the supposed animalcule E; wherefore 1000 animalcules such as F are as big as one animalcule like E.
This number, multiplied by the one foregoing, then makes more than 110 million little animals [like F] as big as a sand-grain.
Using simple yet inexorable arithmetic, van Leeuwenhoek calculated that a drop of water could easily contain millions of moving, eating, reproducing animals, far more tiny animalcules than there were people in the Dutch Republic. A glass of water had more animalcules than there were people on Earth.
On September 17, 1683, he wrote:
I have had several gentlewomen in my house, who were keen on seeing the little eels in vinegar: but some of 'em were so disgusted at the spectacle, that they vowed they'd ne'er use vinegar again.
But what if one should tell such people in future that there are more animals living in the scum on the teeth in a man's mouth, than there are men in a whole kingdom? ... All the people living in our United Netherlands are not as many as the living animals that I carry in my own mouth this very day.
Scientist or conjurer?
This claim defied common sense as much as Galileo's claims that the earth moved and, relative to it, the sun stayed still.
Unfortunately, Galileo's claims challenged Church doctrine as well as a literal reading of the Bible. Galileo's confrontation with authority got him ex-communicated. The equally revolutionary claims of van Leeuwenhoek challenged only academic doctrine. His claims brought him honors and still made him, like Galileo, one of the most famous scientists of his time.
But was he a scientist? Or a conjurer?
In 1677, having just begun his observations, van Leeuwenhoek wrote:
I'm well aware that these my writings will not be accepted by some, as they judge it to be impossible to make such discoveries: but I do not bother about such contradictions. Among the ignorant, they're still saying about me that I'm a conjuror, and that I show people what does not exist: but they're to be forgiven, they know no better. I well know there are whole Universities that won't believe there are living creatures in the male seed: but such things don't worry me, I know I'm in the right.
Early in his career, van Leeuwenhoek had to calculate these specific numbers to satisfy other scientists. Later, having established himself and become trusted, he used words like menigte, multitude, as an indeterminate number.
But as late as July 1684, in a letter addressed to Francis Aston, secretary of the Royal Society, he was still working through his calculations so that others could understand that his huge numbers were more than his imagination. In this case, he was examining the brains of a sheep and the tiny vessels that it contained.
While I viewed the incomprehensible number, of these small Vessells; It was very delightfull to me, to contemplate how every one of them, spread it self into severall branches. ...
For the better representing the inconceivable thinness of the blood Vessells, I made the following Calculations. viz. 100 red Globules lyeing side by side, do not equall the Axe of a [grain of] Sand: let then a Million of them be equall to its solid content.
There are blood Vessells in the brain, which I judge 1/64 part of a blood Globule would be too big to pass thro'; so that the Diameter of the Vessell, is to that of the Globule, as 1 to 4, and if a coarse [grain of] Sand be divided into 64 Millions of parts, 1 of the parts (if it be stiff and unplyable,) will not pass thro' one of the smallest Vessells of the brain.
While a grain of sand would not satisfy today's scientists as a standard unit of measurement, it was the best van Leeuwenhoek had. It was enough to make his point about the relative size of the creatures he observed.
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Letter XVI
to: A. Cink, etc., etc., residing in Louvain
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
Letter XVI
of March 26, 1715
to A. Cink, etc., etc., residerende te Loven
Van Leeuwenhoek's summary of the contents:
Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers. The flesh fibers from the back leg of ayoung mouse examined to their end. In a well formed muscle no pullers are subject to any force but that the flesh fibers must also bear that force. Calculating how much the flesh fibers have born, of a certain degree of force.
Two pullers that climb up from the bottom of the muscle; and two that descend into the muscle from above: not being joined with each other as by flesh fibers. Of the spiral ridges and stretchings, which are found in the pullers and flesh fibers. Why the same are not seen generally. The flesh fibers must all bear the force that is done to the pullers. And the pullers, that come from below, bear no force that the highest pullers don't also bear.
The sheaths, or membranes, running through the muscles, and themselves spreading branch-like through the flesh fibers, make it so that the same flesh fibers do not touch each other; and thus over time are protected from injury or pushing from the flesh fibers. How the stretching and relaxing of the pullers and flesh fibers happen. Representation of the stretching and relaxing through a copper wire, that was wound around a writing pen.
The fish fibers of a cod also wrapped with sheaths. The puller close to the large fin, near the cod's head, examined: and found that the flesh fibers there are supplied with pullers. The same happens generally with all the flesh fibers, that at both its ends is attached to the sheaths of the pullers.
Each flesh fiber also covered with a sheath. Various thin fibers, which a flesh fiber is composed of. Examinations regarding the skin of a cod.
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Letter XVI
of March 26, 1715
to A. Cink, etc., etc., residerende te Loven
Van Leeuwenhoek's summary of the contents:
Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden. Het Vleeschfiertjes uyt het achterbeen van een jonge Muys ten dien eynde onderzocht. In een welgestelde muscul word geenen Trekker eenig geweld aangedaan, of de Vleeschfiertjes moeten dat geweld ook lyden. Uytreekening hoe veel de Vleeschfiertjes, in een zekere groote van geweld, te lyden hebben.
Twee Trekkers die van beneden in den muscul opklimmen; en twee die van boven in den muscul daalen: zynde met malkander niet vereenigt als door Vleeschfiertjes. Van de omwentelende inkrimpingen en uytrekkingen, dewelke in de Trekkers en Vleeschfiertjes gevonden worden. Waarom dezelve doorgaans niet gezien worden. De Vleeschfiertjes moeten al het geweld ook lyden dat aan de Trekkers word aangedaan. En aan de Trekkers, die van beneden komen, word geen geweld gedaan, or de bovenste Trekkers lyden het ook.
De vliezen, of membraanen, door de musculen loopende, en zich taksgewys door de vleeschfiberen verspreydende, maaken dat dezelve Vleeschfiberen malkenderen niet aanraaken; en aldus beschermt zyn in tyde van quetzinge of stootinge van de vleeschfiberen. Hoe de uytrekkingen en inkrimpingen van de Trekkers en vleeschfiberen geschieden. Verbeelding van die uytrekkingen en inkrimpingen door een koperdraadje, dat om eene schryfpenne was gewonden.
De Visfibertjes van een Kabeljauw ook met vliesjes omwonden. De Trekker omtrent de groote vinne, dicht by een Kabeljauws-hoofd, nagezocht: en bevonden dat de Vleeschfiertjes aldaar voor Trekkers verstrekken. Het zelve doen doorgaans alle de Vleeschfiertjes, die met beyde haare eynden in de vliezen van Trekkers zyn gehecht.
Yder Vleeschfiertje ook omkleedt met een vliesje. Menigvuldige dunne fibertjes, waar uyt een Vleeschfiertjes bestaat. Onderzoekingen omtrent de huyt van een Kabeljauw.
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Figure 1: back leg and foot of a young adult mouse
Letter XVI
to: Den Heere A. Cink, etc., etc., residerende te Loven
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Figure 2: flesh from the leg of a mouse showing flesh fibers and pullers [tendons, Dutch "Trekkers]
Letter XVI
to: Den Heere A. Cink, etc., etc., residerende te Loven
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Figure 3: copper thread wound around a writing pen to show the stretching and relaxing of the pullers
Letter XVI
to: Den Heere A. Cink, etc., etc., residerende te Loven
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Letter XIX
to: the Royal Society in London
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
Letter XIX
of November 18, 1715
to Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
Van Leeuwenhoek's summary of the contents:
Vessels discovered in a sugar pear sliced through. Thickness of the vessels. From the vessels come elongated parts: from those parts again come other parts: through which the pear gets its size. Observations concerning the stem of the sugar pear. One must establish that from every part of such a pear a large vessel comes forth, that one may call an artery. No circulation in the fruits. The noted vessels provided to make the seed casing in the pears. Countless branches from the vessels spread out to all the edges of the pear. Wonderful perfection of such a small seed.
A small section of a seed of a sugar pear cut lengthwise, and brought before the magnifying glass. The vessels, and the valve sheaths of the vessels, seen in that section of a seed. The valve sheaths lie very close to each other in the little vessels. What are the valve sheaths for? Secondary vessels and valve sheaths in the seed of a pear; the first carries the sap upwards; the other brings the same back down. Amazing form of such a seed. What great wisdom is is required to make such a tree.
Observations concerning the Poire de seigneur, or the gold pear. The plant seen in the seed, etc. The seeds become large through strings, that are attached firmly, just like the animals in the uterus. The string does not go inwards into the little seed.
The outer rind of the seed investigated. It is composed of small vessels. The little sheaths, that lie inward, are likewise composed of very small vessels. The little plant, that lies in such a seed, cut through and depicted. A multitude of vessels in such a little plant.
Leaf-like parts of the little plant. What the vessels in apples and pears are like discovered; the knowledge of the bark of the coconut described. The so-called threads in the coconut are nothing other than vessels. From these vessels are made bladder-like parts, etc. The vessels once again discovered in a peer. The vessels covered with sheaths. Description of the sheaths. The whole pear is composed mostly of vessels and sheaths. Where do the vessels of the pear, and the thread-like vessels of the coconut, come from?
The vessels of the peer shown. The vessels lie in great multitude in the topmost skin of the pear. Geometry calculates how many vessels lie enclosed in a pear. What use is the topmost skin of the pear for? that sheath is composed of little beds. Description of the beds. The parts, that the upper surface of the pear are made from, comes to many hundreds of millions.
About the spiraling ridges on relaxing and contracting of the pullers and flesh fibers. Many spiraling parts in wood, leaves of trees, tea leaves, etc. The spiraling parts are very easy to recognize. What use are the vessels for example in tea leaves? The spiraling shape is the most perfect for stretching and relaxing.
The membranes have no hollows; as few as the pullers and fibers. How are they nourished? There is no blood pouring through the ends of the arteries in the flesh fibers.
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Letter XIX
of November 18, 1715
to Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
Van Leeuwenhoek's summary of the contents:
Aderen in eene doorgesneede Zuyker-peer ontdekt. Dickte van die aderen. Uyt die Aderen koomen voort langwerpige deelen: uyt die deelen koomen wederom andere deelen voort: waar door de peer haare groote krygt. Waarnemingen omtrent de steel van de Suyker-peer. Men moet vast stellen dat uyt yder deel van zoo eene peer een groote ader voortspruit, die men vel eene Arterie mag noemen. Geene circulatie in de vruchten. De gezeide vaten verstrekken om het zaadhuysje in de peeren te maaken. Ontelbaare takken van die vaten verspreiden zich aan alle kanten in de Peer. Wonderlyke volmaakheit van zoo een kleyn zaadje.
Een kleyn deeltje van een zaadje van een Zuyker-peer in de lengte doorsneden, en voor het vergrootglas gebragt. De vaten, en de klapvliezen der vaten, in dat deeltje van een zaad gezien. Die klapvliezen leggen in de vaatjes zeer dicht by malkander. Waar toe die klapvliesjes dienen? Tweederley vaten en klapvliezen in het zaad van een Peer; de eerste die de sappen opwaarts voeren; de andere die dezelve naar beneden toe brengen. Verwonderens-waardig maakzel van zoo een zaadje. Wat eene groote wysheit vereyscht is geweest om zoo eenen Boom te maaken.
Waarnemingen ontrent de Poire de zeigneur, of de Goudpeer. De plant in het zaad gezien, enz. De zaaden worden groot door strengen, daar zy vast aan leggen, even gelyk de dieren in de Baarmoeder. Die streng gaat niet binnewaarts in het zaadje.
De buytenschors van het zaad doorzocht. Dezelve bestaat maar uyt kleyne vaatjens. De vliesjes, die binnewaarts leggen, bestaan insgelyks uyt zeer kleyne vaatjes. Het plantje, in zoo een zaadje leggende, doorgesneden en afgetekent. Menigte van vaten in zoo een plantje.
Bladgewyze deelen van het plantje. De aderen in Appelen en Peeren hoedanig ontdekt; de weten met de bast van de Cocosnoot te beschouwen. De zoogenoemde draaden in de Cocos-noot zyn niet anders als vaten. Uyt die vaten worden gemaakt blaasachtige deelen enz. De aderen wederom in eene Peer ontdekt. Die aderen met vliezen bekleedt. Beschryving van die vliezen. De gansche Peer bestaat meerendels uyt Aderen en vliezen. Waar de vaten van de Peer, en de draatachtige vaten van de Cocos-noot, uyt voort koomen?
De Aderen van een Peer voor 't gezicht vertoont. Die vaten leggen in groote menigte in het opperste huytje van de Peer. Meetkundige uytrekening hoe veele vaten dat in een Peer besloten leggen. Waar toe de opperste huyt van de Peer dienstig is? dat vliesje bestaat uyt parkjes. Beschryving van die parkjes. De deeltjes, die de oppervlakte van de Peer uytmaaken, beloopen op veele honderden van millioenen.
Van de omwentelende inkrimpinge, en uytrekkinge van de Trekkers en Vleesfibertjes. Menige omwentelende deelen in 't hout, bladeren van Boomen, Three bladen, enz. Die omwentelende deelen zyn zeer maakt te bekennen. Waar toe die vaten by voorbeeld in de Thee-blasden dienen? De omwentelende gestalte is de volmaakste tot de uytrekkingen en inkrimpingen.
De membranen hebben geene holligheden; zoo weynig als de Trekkers en fibertjes. Hoe zy dan gevoed worden? Daar word geen bloed door de eynden der Arterien in de Vleesfibertjes gestort.
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Figures 1 and 2: seed from golden pear with bare outer rind and string for nourishment; seed from golden pear with string broken off and loose wrapper
Letter XIX
to: Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Figure 3: seed laid open showing showing little vessels
Letter XIX
to: Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Figure 4: cross-section of a vessel from a pear
Letter XIX
to: Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Figure 5: spiraling vessels in wood and leaves
Letter XIX
to: Den Heer G. G. Leibniz, lid van de Koninklyke Societeit te Londen, Professor in Hanover, en Bibliotekaris te Bronswyk - member of the Royal Society in London, professor in Hanover, and librarian of Brunswick
March 26, 1715
Dutch title: Vordere bewyzen dat de Vleeschfiertjes zoo wel als de Trekkers ook Trekkers mogen genoemt worden.
English title: Further demonstration that the flesh fibers as well as the pullers [tendons; Dutch "Trekkers"] may also be called pullers.
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Units of measurement
grains of sand
The excerpts in the middle column come from letters that van Leeuwenhoek wrote to the Royal Society, some not published there.
Dobell's biography has a long illustrated excerpt from a letter van Leeuwenhoek wrote to Christiaan Huygens on April 27, 1679. Using a grain of sand as a unit of measurement, this letter details van Leeuwenhoek's method of purposefully under-estimating the vast numbers of little creatures he saw through his lenses.
A transcript of the original Dutch letter is available at Ad Davidse's Christiaan Huygens in het Nederlands. It also uses arithmetic to supports van Leeuwenhoek's claims about the unbelievable numbers of little animals he was seeing with his lenses.
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