This page presents assorted comments on just some of the microscopic objects that van Leeuwenhoek was the first ever to observe. Many are his own words in letters he wrote to the Royal Society, interspersed with the comments of others, especially of Clifford Dobell (1922). Given the vagueness of many of Antonj's descriptions, the specificities of some of Dobell's identifications do often seem to be stretches, even though Dobell was an experienced and noted protozoologist and microscopist. Nonetheless, it is clear that van Leeuwenhoek saw all of these microorganisms or their relatives.

It should be noted that van Leeuwenhoek expressed the relative "littleness" and "bigness" of microscopic objects in terms of their estimated volumes. He treated them as spheres or cylinders and then applied basic geometry. To measure small objects, he estimated how many of them lined up in a row would appear to equal the diameter of a larger object that he had already determined. (His comparisons often started with the sizes of "large grains of sand" and "millet seeds", so in truth we can only estimate the accuracy of his findings.) From this he calculated how many would fit in the larger object's volume.

About two hours distant from this Town there lies an inland lake, called the Berkelse Mere, whose bottom in many places is very marshy, or boggy. Its water is in winter very clear, but at the beginning or in the middle of summer it becomes whitish, and there are then little green clouds floating through it; which, according to the saying of the country folk dwelling thereabout, is caused by the dew, which happens to fall at that time, and which they call honey-dew. This water is abounding in fish, which is very good and savory. Passing just lately over this lake, at a time when the wind blew pretty hard, and seeing the water as above described, I took up a little of it in a glass phial; and examining this water next day, I found floating therein divers earthy particles, and some green streaks, spirally wound serpent-wise, and orderly arranged, after the manner of the copper or tin worms, which distillers use to cool their liquors as they distil over. The whole circumference of each of these streaks was about the thickness of a hair of one's head (Spirogyra; size suggests not relatives Arthrospira or Spirulina - Dobell 1922.). Other particles had but the beginning of the foresaid streak; but all consisted of very small green globules joined together: and there were very many small green globules as well. - vL 1674

It may be remarked here that Leeuwenhoek himself had a very good idea of the actual diameter of a red corpuscle, though he could not express it exactly in terms of any micrometric unit: for he notes in one place that he had satisfied himself that 100 diameters of a red corpuscle amounted to something less than that of a coarse grain of sand (which he had just assessed at the equivalent of 1/30 inch). Consequently, he imagined the diameter of a corpuscle to be "rather less" than 1/3000 of an inch (or about 8.5 micrometers - ed.) - an astonishingly good estimate. - Dobell 1922

I affirmed that the Animalcula fastened to the small Roots of the Green stuff found on the top of the Water in our Ditches and little Canals, which we call Duckweeds, had two small Wheels issuing out of their Bodies. - vL 1702

Among these there were, besides, very many little animalcules, whereof some were roundish, while others, a bit bigger, consisted of an oval. On these last I saw two little legs near the head, and two little fins at the hindmost end of the body. Others were somewhat longer than an oval, and these were very slow a-moving, and few in number. - vL 1702

These animalcules had divers colors, some being whitish and transparent; others with green and very glittering little scales; others again were green in the middle, and before and behind white. (Dobell thought that this probably describes Euglena viridis, as the peculiar arrangement of the photosynthetic chloroplasts in this species gives the flagellate the appearance he described under a low magnification. If correct, this is the first mention of Euglena, whose discovery is usually attributed to Harris, 1696 - ed.).  Others yet were ashen grey. And the motion of most of these animalcules in the water was so swift, and so various, upwards, downwards, and round about, that it was 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, in wheaten flour, mould, and the like. - vL 1674

About six years ago, being in England, out of curiosity, and seeing the great chalk cliffs and chalky lands at Gravesend and Rochester, it oft-times set me a‑thinking; and at the same time I also tried to penetrate the parts of the chalk. At last I observed that chalk consists of very small transparent particles ("klootgens") and these transparent particles lying one upon another, is, methinks now, the reason why chalk is white. - vL 1674

The fourth sort of animalcules, which I also saw a-moving, were so small, that for my part I can't assign any figure to them. These little animals were more than a thousand times less than the eye of a full-grown louse (for I judge the diameter of the louse's eye to be more than ten times as long as that of the said creature), and they surpassed in quickness the animalcules already spoken of. I have divers times seen them standing still, as it were, in one spot, and twirling themselves round with a swiftness such as you see in a whip-top a-spinning before your eye; and then again they had a circular motion, the circumference whereof was no bigger than that of a small sand-grain; and anon they would go straight ahead, or their course would be crooked. - vL 1674

van Leeuwenhoek discovered human "blood corpuscles" in 1674. In 1698 he demonstrated circulation in the capillaries of an eel to Tsar Peter the Great of Russia. - Dobell 1922

I saw the very small animalcules, as yesterday; and besides these, a little animal that had nearly the figure of a mussel-shell, lying with its hollow side downward (probably Stylonychia - Dobell 1922). It was of a length anigh that of a louse's eye. In the morning, I found the said very small animalcules in greater number, and I saw also one bigger animalcule, like that just spoken of. - vL 1675

The Vinegar-Eel (the nematode Anguillula aceti, which van Leeuwenhoek described in 1676 and repeatedly used as a standard of size - ed.) is assigned various sizes in the text-books. I have cultivated and studied this worm at various times, and find that ordinary large individuals (females) may measure anything from about 1.2 mm. to 1.7 mm. in length. "A full-grown eel such as we see in vinegar" is approximately 1.5 mm. long, and this agrees quite well with all Leeuwenhoek's references. - Dobell 1922

These creatures were provided with exceeding short thin legs in front of the head (although I can make out no head, I call this the head for the reason that it always went in front during motion). This supposed head looked as if it was cut off aslant, in such fashion as if a line were drawn athwart through two parallel lines, so as to make two angles, the one of 110 degrees, the other of 70 degrees. Close against the hinder end of the body lay a bright pellet, and behind this I judged the hindmost part of all was slightly cleft. These animalcules are very odd in their motions, oft-times tumbling all around sideways; and when I let the water run off them, they turned themselves as round as a top, and at the beginning of this motion changed their body into an oval, and then, when the round motion ceased, back again into their former length. - vL 1676  

And lastly, I also saw some 3 (though very few) which were a good 20 times bigger than the biggest sort spoken of above. These were long, and bent crooked, the upper part of the body round, but flat beneath, looking much after the fashion of an 1/8 part of the peel of a large citron. Their motion was all a-wallowing, on their back as well as on their belly. I could discern no little feet or legs on them; and after this time, moreover, I never saw them any more. - vL 1676

Besides these, many little worms, or little eels, which were even smaller than the very tiny eels spoken of before. A little animal that was 3 or 4 times as long as broad. - vL 1676

I observed some very little animalcules, a bit longer and bigger than the small animalcules seen by me in the pepper-water. Nevertheless, these had a different form and motion; for while the animalcules in the pepper-water went forward all winding-wise, these animalcules all advanced in jumps, hopping like a magpie: yet were they very few in number. - vL 1676

The 23rd of May, I discovered, besides the foresaid animalcules, living creatures that were perfectly oval, like plovers' eggs. I fancied that the head was placed at the pointed end, which at times was stuck out a bit more. Their body within was furnished with some 10, 12, or 14 globules, which lay separated from one another. When I put these animalcules on a dry place, they then changed their body into a perfect round, and thereupon oft-times burst asunder; and the globules, together with some watery humor, flowed out on all sides, without my being able to discern any other remains. - vL 1676

I also gently pressed the dirt out of two new-killed pigeons, that were about a month old; and in the first I couldn't find any living animalcules at all. But in the dirt from the second pigeon (which was much clearer than the first's) I saw many animalcules; so that I judged there were quite 100 of them in a bit as big as a sand-grain. These moved among one another very prettily, and were all of one and the same bigness, having the figure of an egg, and being in my judgment as big as a sixth part of one of our blood-globules. - vL 1676

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. vL 1677

I now saw very plainly that these were little eels, or worms, lying all huddled up together and wriggling just as if you saw, with the naked eye, a whole tubful of very little eels and water, with the eels squirming among one another; and the whole water seemed to be alive with these multifarious animalcules. This was for me, among all the marvels that I have discovered in nature, the most marvelous of all; and I must say, for my part, that no more pleasant sight has every yet come before my eyes that these many thousand of living creatures seen all alive in a little drop of water, moving among one another, each several creature having its own proper motion. - vL 1678

Some accounts credit van Leeuwenhoek as the first person to discover the Amoeba in pond water in 1674. Many histories give the honor to August Johann Rösel von Rosenhof, giving the year of discovery as 1757.

The blood aforesaid was very watery, and not pure blood by any means: for as soon as I separated a good bit of the frog's skin from the flesh, some watery juice began to run out of the skin, as well as the flesh; and still more when I opened the belly. And as I had well-nigh bashed in the frog's head, trying to make it keep quiet, I thought some watery juice might well have been squashed out of its mouth or stomach too, and from this juice the animalcules might have come; for when I afterwards took blood out of the frog's veins clean, I could discover no animalcules therein; neither when I viewed the watery matter which came from between the skin and the flesh, nor yet in that from the hollow of the belly. In the foresaid watery matter, I noticed some irregular particles, most of which looked to me round, and were about as big as the globules of our blood. In some of these I could make out that they were composed of 6 lesser globules; and there were besides particles that seemed only about 1/6 of the bigness of the others. And when I examined the said watery blood of divers frogs, that had run out into the dish (for I took a clean dish for each several frog), a very few animalcules were to be seen in it once more. - vL 1683

I took a little white matter, which is as thick as if it were batter. I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving. The biggest sort had a very strong and swift motion, and shot through the water (or spittle) like a pike does through the water. The second sort oft-times spun round like a top and these were far more in number. In the mouth of one of the old men, an unbelievably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time. The biggest sort bent their body into curves in going forwards. Moreover, the other animalcules were in such enormous numbers, that all the water seemed to be alive. - vL 1683

Discovered banding in 1682. Of major importance were his description of the optic lens in many species of animals. Leeuwenhoek studied the structure of the optic lens, and he demonstrated the striation in skeletal musculature. In 1719 he introduced histological staining, using saffron for investigating muscle fibers. - Dobell 1922

I had got the foresaid water taken out of the ditches and runnels on the 30th of August: and on coming home, while I was busy looking at the multifarious very little animalcules a-swimming in this water, I saw floating in it, and seeming to move of themselves, a great many green round particles, of the bigness of sand grains. When I brought these little bodies before the microscope, I saw that they were not simply round, but that their outermost membrane was everywhere beset with many little projecting particles, which seemed to me to be triangular, with the end tapering to a point : and it looked to me as if, in the whole circumference of that little ball, eighty such particles were set, all orderly arranged and at equal distances from one another; so that upon so small a body there did stand a full two thousand of the said projecting particles. - vL 1700

Further, I discovered a little animal whose body was at times long, at times drawn up short, and to the middle of whose body (where I imagined the undermost part of its belly was) a still lesser animalcule of the same make seemed to be fixed fast by its hinder end. Such a little animal, because of its wonderful structure and manner of propagation, I have had drawn, and at least twice as big as it looks to the naked eye when you see it in the water and attached to the root of a bit of duckweed. (Fig. 4: ABCDEFG shows this creature, whereof A is the hind end that it hangs on by, while at CDE are shown its eight horns (though others a bit smaller had six horns), as it looked when it had straightened itself out, for otherwise it can scarce reach to a quarter of this length; and its horns seemed to my eye to be made in so marvelous a manner, that the draughtsman's art isn't competent to portray them. - vL 1702

When these animalcules bestirred themselves, they sometimes stuck out two little horns, which were continually moved, after the fashion of a horse's ears. The part between these little horns was flat, their body else being roundish, save only that it ran somewhat to a point at the hind end; at which pointed end it had a tail, near four times as long as the whole body, and looking as thick, when viewed through my microscope, as a spider's web does to the naked eye. - vL 1673

In structure these little animals were fashioned like a bell, and at the round opening they made such a stir, that the particles in the water thereabout were set in motion thereby. And though I must have seen quite 20 of these little animals on their long tails alongside one another very gently moving, with outstretched bodies and straightened-out tails; yet in an instant, as it were, they pulled their bodies and their tails together, and no sooner had they contracted their bodies and tails, than they began to stick their tails out again very leisurely, and stayed thus some time continuing their gentle motion: which sight I found mightily diverting. - vL 1702

van Leeuwenhoek discovered freshwater diatoms in 1702. Tabellaria (stacked rectangular boxes in the lower right corner of the photomicrograph) was his first. Viewing the minute (usually less than 1.0 µm diameter) pores in their frustules (siliceous shells) has long been a favorite test of the resolution of microscopes. Although he could clearly describe the overall appearance of diatoms, he did not mention those pores.

Mr. Leeuwenhoek showed us further the eye of a fly, which appeared very remarkable under the microscopium, and had the appearance of veritable hexagona lying alongside one another; which Mr. Leeuwenhoek considers actually are eyes, and consequently makes flies into something better than so many Arguses; for 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. (Antonj often had occasion to state that this sort of unthinking rejection of his ideas by "experts" did not much bother him. -ed.) Further, he showed us the wing of a fly, which also appeared very wonderful. The sting of a fly appeared also very singular. - von Uffenbach 1710

Viviparous parthenogenesis ("live virgin birth") alternates with sexual reproduction in some aphid species. van Leeuwenhoek discovered this first known instance of the phenomenon and carefully followed its development.

Yeast cells metabolize polysaccharides to form carbon dioxide gas and ethyl alcohol. For centuries, various strains have been selected and propagated by humans to produce different ratios of those two main products, thereby adapting them for use in leavening bread and brewing beer. Many of the earliest discoveries in enzymology (ενζυμον, enzymon = "in leaven", i.e., "in yeast") were made by Louis Pasteur and Eduard Buchner while studying the fermentation of sugars to make beer. Under moderate magnification, yeast cells appear as featureless egg-shaped particles. They reproduce by budding, the daughter buds often remaining loosely attached and strung together in pairs or chains.

Gout is a debilitating disease of humans and Dalmatian dogs characterized by the formation of masses in the joints called tophi. van Leeuwenhoek conjectured correctly that it is the needle-like "microcrystals" he found in tophi that are largely responsible for the pain of gout. Over a century would pass before these were determined to be composed of sodium urate deposited as the result of either faulty excretion or abnormal production of uric acid, or both.  

Greatly extending the discoveries of Robert Hooke, van Leeuwenhoek described the vessels in plants. He made both longitudinal and transverse sections, noting numerous fine details. One of his most famous illustrations is of a transverse cut of a one-year-old ash tree.