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Telescope
From dioptra to theodolite
Until the 16th century, land surveyors commonly used a dioptra, usually on a tripod, that pointed at a distant object and then measured the horizontal and vertical angles. A series of these measurements from different locations created topographic precision that helped build the Roman aqueducts across valleys and through mountains.
The pointing was done by the unaided human eye until the English surveyor and author Leonard Digges (1520 - 1559) mounted a spyglass on one of these dioptra and made what he called a theodolite. While one lens had been used since the 1300's to correct human vision, it was not until the 1500's that people like Digges realized the power of two lenses working together. In the surveying textbook Pantometria (1571, published posthumously), Digges referred to his "topographical instrument" as "perspective glasses," probably a reflecting telescope.
The spyglass greatly extended the range and accuracy of finger pointing, and many early surveyors added a compass, too.
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Dutch spyglass developers
Geodetic measurements
Cornelis de Judaeis; Levinus Hulsius:
De quadrante geometrico libellus.
Nuremberg: Christopher Lochner 1594.
Hans Lippershey (1570-1619) (inset left) was a German who lived his adult life in Middelburg, in the Netherlands. He developed and in 1608 made public designs for a "perspective glass", "an instrument for seeing at a distance". At either end of a paper tube, it had a weak convex objective lens and a strong concave eyepiece lens that magnified three to four times. Lippershey was denied a patent because another Dutchman, Jacob Metius, filed a competing claim.
These glasses were easy to copy, and became popular throughout Europe within months for their military and commercial application of identifying ships from afar. Later, one of Lippershey's close neighbors in Middelburg, Zacharias Jansen (inset right), made a credible claim to have invented the telescope before Lippershey.
But these men were lens grinders, not scientists. By the following summer, Galileo had heard about Lippershey's design, made a similar spyglass of his own, and turned it to the stars. It was another several decades before the first microscopes used this double lens system to look closely at little things. From the point of view of the lens, however, there isn't any difference between very small and very far away.
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Galileo's and Kepler's designs
When he learned about the Dutch perspective glasses Galileo Galilei (1564-1642) en | nl  spent a year from the summer of 1609 improving the magnification from 3 to 8 to 20 and even 30 times, right-side up. In the fall of 1609, he used these spyglasses to examine the planets and stars, and in spring 1610, just 18 months after Lippershey applied for his patent, Galileo published his observations. He used the Dutch refracting design: a plano-convex objective lens at the end of a paper tube and a smaller plano-concave eyepiece lens in a separate tube. The narrower tube with the eyepiece lens, which fit snugly inside the wider tube, slid in and out for focusing.
Because of the crude lens grinding techniques of the time, sufficient for spectacles, these lenses did not have good resolution. To compensate, the aperture had to be restricted, which made the field of view frustratingly narrow.
A year after Galileo's first publication of his celestial discoveries, in 1611, German mathematician Johannes Kepler
(1571-1630) en | nl  improved Galileo's design. In his books Astronomiae Pars Optica and Dioptrice, he described the advantages and disadvantages of replacing Galileo's spherical concave eyepiece lens with a hyperboloidal convex eyepiece lens. Compared to Galileo's design (left), the converging light from Kepler's convex eyepiece (right) allows for a much wider field of view, higher magnification, and less eyestrain. As a trade-off, long focal lengths minimize spherical and chromatic aberrations, but the 150 foot (45 meter) telescope built by Johannes Hevelius showed the impracticality of building long telescopes. Also note that while the Kepler image is inverted where the Galileo image is right-side up, after we depart the human scale for the astronomical and microscopical, "up" and "down" don't matter very much.
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Huygens' design
Using the Keplerian design and understanding that the most important property of a telescope is its aperture (the larger the objective lens, the more light and the better the resolution), Dutch scientist Christiaan Huygens (1629-1695) constructed increasingly long telescopes. Between 1655 and 1659, he built a 12-foot telescope with 2-inch aperture, then one with a 23-foot focus and even one with a 123-foot focus.
The 23 footer had an objective aperture of several inches and magnified about 100 times. Note the cloth cord connecting the objective lens and the eyepiece lens and the portability of the eyepiece lens, which made this telescope easy to manipulate. With it, Huygens announced in Systema Saturnium (1659), he observed Saturn's rings.
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Hooke's microscope
Galileo's microscope
This microscope, of the design of Galileo, was probably made by Giuseppe Campani.
It did not take Galileo long to realize that getting a longer focal length with his telescopes was a big problem. However, using lenses with a shorter focal length, he could, in effect, turn the telescope around and magnify little things. His first microscopes, in 1609, were basically little telescopes with the same two lenses: a bi-convex objective and a bi-concave eyepiece.
By 1624, Galileo had developed an occhiolino (the word microscope was not coined by Giovanni Faber until the following year) that had three bi-convex lenses. It did not magnify much more than his telescopes, about 30 times, but Galileo was far more interested in the multitude of stars he could see through his telescope than in the insects he examined with his microscope.
The Istituto e Museo di Storia della Scienza, the Institute and Museum of the History of Science, in Florence, has two terrific online exhibits, one on Galileo's telescopes and the other on his microscopes.
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Cock microscope
These three-lens microscopes were designed and used by Robert Hooke and made by Christopher Cock, London, around 1665. The main tube of the microscope on the right is 7 inches long and 4 inches in diameter, made of leather-covered cardboard. The brass rod it slid up and down on is 15 inches high. Starting with Kepler's design of two bi-convex lenses, these microscopes added a field lens mounted close to the eyepiece lenses to widen the field of view. It was focused by moving the tube, not the specimen. This was basically the same design that Galileo had presented to Prince Federico Cesi in 1624.
In Micrographia, Hooke discussed these lenses (emphasis added): The Microscope, which for the most part I made use of, ... was contriv'd with three Glasses; a small Object Glass, a thinner Eye Glass, and a very deep one:
This I made use of only when I had occasion to see much of an Object at once; the middle Glass conveying a very great company of radiating Pencils, which would go another way, and throwing them upon the deep Eye Glass. But when ever I had occasion to examine the small parts of a Body more accurately, I took out the middle Glass, and only made use of one Eye Glass with the Object Glass. Three lenses just compounded the aberrations, so two were better, trading "accuracy", what we probably call resolution, for size.
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Micrographia:
or, Some physiological descriptions of minute bodies made by magnifying
glasses title page (left)
illustrations showing Hooke's experimental method accompanying Observ. IX. Of the Colours observable in Muscovy Glass, and other thin Bodies (center) and Observ. X. Of Metalline, and other real Colours (right)
This is Robert Hooke's most influential (and beautiful) book, published when he was 30. The cover not only mentions "the Royal Society" twice, it also has the Society's coat of arms and motto: nullius in verba, on the word of no one. The popularity of the book helped further the Society's image as the most scientifically progressive organization in the world.
For Micrographia, Hooke devised the experiments, often ingeniously given the low-tech means at hand. He developed his own technique for slicing thin sections of a specimen so that he could examine it with his crude illumination system. The book is organized as a series of observations, which is the same Dutch words van Leeuwenhoek used, waaarneming and observatie, eight years later when he began sending letters to the Royal Society.
In addition, Hooke drew all the images himself: a bee's stinger, a razorblade, snow crystals, wood, cork and insects. The originals are copperplate engravings, some of them fold-outs of an already large (folio) volume. These drawings gave the book much of its power.
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Hooke's
microscope
The first illustration, drawn by Robert Hooke, in his Micrographia (1665). He described it, "which for the most part I made use of" (emphasis added): The Tube being for the most part not above six or seven inches long, though, by reason it had four Drawers, it could very much be lengthened, as occasion required; ...
My way for fixing both the Glass and Object to the Pedestal most conveniently was thus:
Upon one side of a round Pedestal AB, ..., was fixt a small Pillar CC,
on this was fitted a small Iron Arm D, which could be mov'd up and down, and fixt in any part of the Pillar, by means of a small Screw E;
on the end of this Arm was a small Ball fitted into a kind of socket F, made in the side of the Brass Ring G, through which the small end of the Tube was screw'd; by means of which contrivance I could place and fix the Tube in what posture I desir'd (which for many Observations was exceeding necessary) and adjusten it most exactly to any Object.
For placing the Object, I made this contrivance; upon the end of a small brass Link or Staple HH, I so fastned a round Plate II, that it might be turn'd round upon its Center K, and going pretty stiff, would stand fixt in any posture it was set;
on the side of this was fixt a small Pillar P, about three quarters of an inch high, and through the top of this was thrust a small Iron pin M, whose top just stood over the Center of the Plate;
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What Hooke saw
Observ. LIV. Of a Louse
from Robert Hooke's Micrographia (emphasis added)
This is a Creature so officious, that 'twill be known to every one at one time or other, so busie, and so impudent, that it will be intruding it self in every ones company, and so proud and aspiring withall, that it fears not to trample on the best, and affects nothing so much as a Crown; feeds and lives very high, and that makes it so saucy, as to pull any one by the ears that comes in its way, and will never be quiet till it has drawn blood:
It is troubled at nothing so much as at a man that scratches his head, as knowing that man is plotting and contriving some mischief against it, and that makes it oftentime sculk into some meaner and lower place, and run behind a mans back, though it go very much against the hair; which ill conditions of it having made it better known then trusted, would exempt me from making any further description of it, did not my faithful Mercury, my Microscope, bring me other information of it. ...
It does not seem to have any eye-lids, and therefore perhaps its eyes were so placed, that it might the better cleanse them with its fore-legs; and perhaps this may be the reason, why they so much avoid and run from the light behind them, for being made to live in the shady and dark recesses of the hair., and thence probably their eye having a great aperture, the open and clear light, especially that of the Sun, must needs very much offend them; to secure these eyes from receiving any injury from the hairs through which it passes, it has two horns that grow before it, in the place where one would have thought the eyes should be; each of these CC hath four joynts, which are fringed, as 'twere, with small brisles, from which to the tip of its snout D, the head seems very round and tapering, ending in a very sharp nose D, which seems to have a small hole, and to be the passage through which he sucks the blood.
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Observ. XLVII. Of the Shepherd Spider, or long legg'd Spider
from Robert Hooke's Micrographia (emphasis added)
The Carter, Shepherd Spider, or long-legg'd Spider, has, for two particularities, very few similar creatures that I have met with, the first, which is discoverable onely by the Microscope.
Plainly describ'd, is the curious contrivance of his eyes, of which (differing from most other Spiders) he has onely two, and those plac'd upon the top of a small pillar or hillock, rising out of the middle of the top of its back, or rather the crown of its head, for they were fix'd on the very top of this pillar. ... The two eyes, BB, were placed back to back, with the transparent parts, or the pupils, looking towards either side, but somewhat more forward then backwards. ...
The second Peculiarity which is obvious to the eye, is also very remarkable, and that is the prodigious length of its leggs, in proportion to its small round body, each legg of this I drew, being above sixteen times the length of its whole body, ...; the eight leggs are each of them jointed, just like those of a Crab, but every of the parts are spun out prodigiously longer in proportion; each of these leggs are terminated in a small case or shell, shap'd almost like that of a Musle-shell ....
To supply therefore each of these leggs with its proper strength, Nature has allow'd to each a large Chest or Cell, in which is included a very large and strong Muscle, and thereby this little Animal is not onely able to suspend its body upon less then these eight, but to move it very swiftly over the tops of grass and leaves.
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Observ. XXXIX. Of the Eyes and Head of a Grey drone-Fly, and of several other creatures
from Robert Hooke's Micrographia (emphasis added)
I took a large grey Drone-Fly, that had a large head, but a small and slender body in proportion to it, and cutting off its head, I fix'd it with the forepart or face upwards upon my Object Plate (this I made choice of rather then the head of a great blue Fly, because my enquiry being now about the eyes, I found this Fly to have, first the biggest clusters of eyes in proportion to his head, of any small kind of Fly that I have yet seen, it being somewhat inclining towards the make of the large Dragon-Flies. Next, because there is a greater variety in the knobs or balls of each cluster, then is of any small Fly.) ...
First, that the greatest part of the face, nay, of the head, was nothing else but two large and protuberant bunches, or prominent parts, ABCDEA, the surface of each of which was all cover'd over, or shap'd into a multitude of small Hemispheres, plac'd in a triagonal order, that being the closest and most compacted, and in that order, rang'd over the whole surface of the eye in very lovely rows, between each of which, as is necessary, were left long and regular trenches, the bottoms of every of which, were perfectly intire and not at all perforated or drill'd through.
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Hooke's cork cell
Hooke was the first person to see, and name, the cell, what we now know as the fundamental unit of an organism. (emphasis added)
Observ. XVIII. Of the Schematisme or Texture of Cork, and of the Cells and Pores of some other such frothy Bodies.
"Judging from the lightness and yielding quality of the Cork, that certainly the texture could not be so curious, but that possibly, if I could use some further diligence, I might find it to be discernable with a Microscope, I with the same sharp Penknife, cut off from the former smooth surface an exceeding thin piece of it, and placing it on a black object Plate, because it was it self a white body, and casting the light on it with a deep plano-convex Glass, I could exceeding plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular; yet it was not unlike a Honey-comb in these particulars.
"First, in that it had a very little solid substance, in comparison of the empty cavity that was contain'd between, as does more manifestly Schem. 11.
"Fig. 1. appear by the Figure A and B, for the Interstitia, or walls (as I may so call them) or partitions of those pores were neer as thin in proportion to their pores, as those thin films of Wax in a Honey-comb (which enclose and constitute the sexangular celts) are to theirs.
"Next, in that these pores, or cells, were not very deep, but consisted of a great many little Boxes, separated out of one continued long pore, by certain Diaphragms, as is visible by the Figure B, which represents a sight of those pores split the long-ways.
"I no sooner discern'd these (which were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this) but me thought I had with the discovery of them, presently hinted to me the true and intelligible reason of all the Phænomena of Cork."
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Legacy of Lenses
Van Leeuwenhoek bequeathed his microscopes and lenses to his daughter Maria, who died twenty-two years later, in 1745. In 1747, her estate auctioned her father's remaining microscopes.
While a copy of the sale catalog has not survived, Harting had one in front of him in 1850 when he gave this accounting in his Het Mikroskoop:
how many?
in total, 419 lenses
 247 completely finished microscopes, each with a lens, and usually with a specimen
172 lenses mounted between little plates
lenses made from what?
all from glass except:
three from quartz
one ground from a sand-grain
how were they mounted?
all single lens except:
two have two lenses
one has three lenses
plates made from what?
approximately 160 were silver
approximately 85 were brass
three were gold
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Tiny Microscopes
Van Leeuwenhoek made palm-sized microscopes
from brass and silver to hold his tiny lenses
and position his specimens.
In the long tradition of pioneers and innovators, Antony van Leeuwenhoek's discoveries were only as good as his tool. His tool, a single-lens microscope, was not available in a store. Van Leeuwenhoek had to design and make his own, right down to the screws and rivets.
Early in his career, van Leeuwenhoek settled on a "good enough" design for the device to hold his tiny spherical lenses. It had to solve these problems:
 how to bring the specimen into focus
how to hold both dry specimens and liquids
how to illuminate the specimen
He always referred to it as a Vergroot-glas, magnifying glass.
Focus
With most microscope designs, as with most telescope designs (see left column), the specimen stays still and the lens moves closer or farther. If the specimen is always fixed and flat, as on a modern glass slide, essentially making it two dimensional, then the one dimension is controlled by hand when placing the specimen, and the other dimension is controlled by moving the lens closer or farther.
Van Leeuwenhoek, however, especially early in his career, was often observing three-dimensional objects that either were moving or could be rotated on an axis. It made more sense to keep the lens fixed and to move the specimen. His microscope had to accomplish three tasks: magnification, resolution (separating the details), and visibility to the human eye. In 1699, he wrote:
But to mount such small glasses well, requireth a far greater judgment, than to make them.
The parts (see pop-ups in right-hand column): lens, two plates with matching holes, rivets, pin, block, screws.
Each van Leeuwenhoek microscope is a one-off, a unique product.
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Van Leeuwenhoek mounted his lenses between palm-sized plates. The widest of the surviving plates is 28 millimeters or 1.1 inches, and the longest is 47 millimeters or 1.9 inches. The matching plates, riveted together, had matching holes slightly smaller than the lens that they held.
Behind the plates was a pin on a block controlled by three screws, one for each dimension. The pin itself could be turned to rotate the specimen around its vertical axis. These parts are not interchangeable. Van Leeuwenhoek made them all by hand out of brass, copper, silver, and even gold.
Martin Folkes, vice-president of the Royal Society, in the year after van Leeuwenhoek died, described the microscopes that he bequeathed to the Society:
... a very small double Convex-Glass, let into a Socket, between two Silver Plates rivetted together, and pierc'd with a small Hole:
The Object is placed on a Silver Point, or Needle, which, by Means of Screws of the same Metal, provided for that Purpose, may be turn'd about, rais'd, or depress'd, and brought nearer or put farther from the Glass, as the Eye of the Observer, the Nature of the Object, and the convenient Examination of its several Parts may require.
The pin was fine for dry or sliced specimens, but not for liquids. Van Leeuwenhoek drew liquids into a thin glass tube, which had to be held or clamped directly behind the lens, or broken into a small enough section to glue to the pin.
 According to the description of the lot of hundreds of microscopes sold after the death of van Leeuwenhoek's daughter, a few of the microscopes were wider and had two or three single lenses mounted side by side. One of these, with three lenses, is clearly visible in the 1686 portrait by Verkolje. It looks as though it has only one specimen pin. Perhaps it was for wide specimens.
Short focal length
The greater the magnification, the smaller the lens and thus the closer the focal length, that is, the distance from the lens where the object will be in perfect focus. The focal length of a spherical lens is just a little more than its radius but much less than its diameter.
Thus, for van Leeuwenhoek's strongest lenses, the specimen had to be .9 millimeters, less than 4 hundredths of an inch, away from the lens. That led to Van Leeuwenhoek's final design problem: how to light the specimens enough to make the microscope useful, as discussed on the Using page.
The Aalkijker - his showcase experiment
"More delightful than any mine eyes had ever beheld."
The pin also did not work for large living specimens. Van Leeuwenhoek's showcase for visitors was a demonstration of the blood circulating through the capillaries in the tail of an eel. The eel had to be living if the blood were to circulate and a living eel would not stay still on the edge of a pin. So van Leeuwenhoek modified the design of his microscope and referred to it as an aalkijker, or eel viewer, diagram left, and photo right of the Boerhaave Museum's.
Van Leeuwenhoek wrote about a similar observation of tadpoles on September 7, 1688:
A sight presented itself more delightful than any mine eyes had ever beheld; for here I discovered more than fifty circulations of the blood, in different places, while the animal lay quiet in the water, and I could bring it before my microscope to my wish.
For I saw not only that in many places the blood was conveyed through exceedingly minute vessels, from the middle of the tail towards the edges, but that each of the vessels had a curve or turning, and carried the blood back towards the middle of the tail, in order to be again conveyed to the heart.
For the curious who came to visit, this observation was easy to explain and, finally proving Harvey's famous conjectures, it was an important observation. In addition, it could be easily seen with a relatively low-powered lens without the patience and rigor needed for viewing protozoa. People could relate it to their own bodies without having to accept anything too unusual or too numerous.
Van Leeuwenhoek wrote what he may well have told his visitors:
If we now plainly perceive, that the passage of the blood from the arteries into the veins of the tadpole, is not performed in any other than those vessels, which are so minute as only to admit the passage of a single globule at a time, we may conclude that the same is performed in like manner in our own bodies, and in those of other animals.
Even though he wasn't seeing a capillary as a separate structure, he understood the function of transporting the same blood:
Hereby it plainly appeared to me that the blood-vessels which I now saw in the animal, and which bear the names of arteries and veins, are, in fact, one and the same; that is to say, that they are properly termed arteries so long as they convey the blood to the furtherest extremities of its vessels, and veins when they bring it back to the heart. And thus it appears that an artery and a vein are one and the same vessel prolonged or extended.
Faced with the uncertainties of publication by the Royal Society and the fact that most of his letters were not getting published by them anymore, van Leeuwenhoek began in 1684 to publish his own letters, first as pamphlets with several letters, then beginning in 1688 collecting 165 of them, most of them never published in Philosophical Transactions. Details on the Publications page.
He had only only three letters printed as their own self-contained piece. The last of them was his letter to the Royal Society on September 7, 1688, quoted above and titled Den Waaragtigen Omloop des Bloeds, On the True Circulation of the Blood. The images accompanying it are on the right, illustrating the experiment that the aalkijker demonstrated for van Leeuwenhoek's visitors.
This letter was never published by the Royal Society, and we do not know how many of them van Leeuwenhoek printed and gave out to visitors.
Dead-end design
Now that van Leeuwenhoek had the instrument, he had to learn how to use it. Using it was so difficult that his design was never used by anyone else to make important discoveries. In the history of the microscope, it was a dead end.
But it let van Leeuwenhoek see things that it took until the mid-1800's for the double-lens microscope to reveal to Pasteur and Lister. Not until these scientists developed the germ theory of disease did they begin to understand the significance of the bacteria and protozoa discovered two centuries previously by Antony van Leeuwenhoek.
On June 9, 1699, midway through his career with them, van Leeuwenhoek wrote to the Royal Society:
As to what concerns my Magnifying glasses, I will not brag of them, I make them as good as possible I can in my power, and I must say that several Years since, I have not only Ground them still better and better, which is a matter of consequence, but I have also mounted them better from time to time, which is also very Material.
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Van Leeuwenhoek's Vergroot-glas,
magnifying glass
Mayall
drawings of van Leeuwenhoek's microscope
The drawings left and right were made by John Mayall in 1886, probably from the van Leeuwenhoek microscope now at the Zoological Laboratories at the University of Utrecht. Van Leeuwenhoek made hundreds of these little microscopes, and they all shared the same basic design.
The drawing on the left shows the side that the observer looks through. The eye hole is considerably smaller than the concavity holding the lens, a feature that helped to reduce spherical aberration and kept the lens from shifting.
The drawing on the right shows the side where the specimen is mounted on a pin. The lower screw, in addition to acting as a handle, adjusts the vertical position of the pin. The screw going into the stage adjusts how close the pin is to the lens. The little knob or handle lets the specimen be rotated.
The diagram in the middle shows the same four screws as well as the double plates that the lens is mounted between. It does not show the concavity that was punched into each plate for the lens to fit between.
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Parts
of van Leeuwenhoek's microscope
This diagram from Brian J. Ford's book The Leeuwenhoek Legacy
labels all the parts of a generalized van Leeuwenhoek microscope. The nine surviving examples as well as descriptions of lost microscopes indicate that the parts were always the same.
Van Leeuwenhoek made all of his microscopes himself.
As a surveyor, van Leeuwenhoek knew the usefulness of low-power telescopes to see distant landmarks. As a cloth merchant, he knew the usefulness of low-power magnifying glasses to count threads. When he made his microscopes, he did not use the little-telescope design of Galileo and Hooke.
He would use a single lens, so there was no need for a tube to change the relative position of multiple lenses. Instead, he would attain focus by moving the specimen.
Telescope lenses and even early microscope lenses had diameters measured at the scale of inches. Van Leeuwenhoek's tiny lenses were less than 2 millimeters in diameter, less than a tenth of an inch. It did not need sliding cardboard tubes; it needed, more than anything, something that would keep lens and the specimen at an unchanging but adjustable distance from each other. Once he finally got focus, he didn't want to lose it!
After he developed this design and adjusted to the trade-offs inherent in its differences with Hooke's design, he stayed with it for fifty years. He put his energy into improving the lenses.
The shape of the plates and the stage, the angle of the main screw, the composition of the plates, and the number and placement of rivets were a little different each time because each van Leeuwenhoek microscope is a one-off, a unique product.
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Photos
of
replicas of van Leeuwenhoek microscopes
Hooke's microscopes were constructed like short-focus telescopes. Van Leeuwenhoek employed a completely different design that matched what Hooke had described in Micrographia, a popular book that we have only this indirect evidence that van Leeuwenhoek read. However he came to it, he ended up producing instruments that were even shorter-focus magnifying glasses.
While he gave some as gifts, van Leeuwenhoek's microscopes were not for sale. They did not need fine workmanship or any decoration. They just had to work. Van Leeuwenhoek put more effort into the production of his lenses, where workmanship made all the difference.
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How tiny are van Leeuwenhoek's microscopes?
To give a sense of scale, these photos show van Leeuwenhoek microscopes in use by Hans Loncke and in hand (insets). Loncke's eye should probably be closer because the eye point of a van Leeuwenhoek microscope is shorter than most people's eyelashes.
Having used a replica, I found that holding it off to the side let my eye get even closer. In his letters, Van Leeuwenhoek frequently mentions "sticking" and "gluing" specimens to the pin. As long as the specimen was securely fastened, it would not fall off.
In any event, we have no evidence of which position van Leeuwenhoek used.
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Three lenses
In 1686, Jan Verkolje made a mezzotint portrait of van Leeuwenhoek holding a triplet.
Omloop des bloeds / Circulation of the Blood
The three images below (rotated 90-degrees right for display) illustrated the pamphlet Omloop des bloeds, self-published by van Leeuwenhoek in 1688.
The first two are tadpoles and the third is an unspecified fish.
They illustrated the showcase experiment with the aalkijker that van Leeuwenhoek presented to visitors.
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