Eye of the Beholder: Johannes Vermeer, Antoni van Leeuwenhoek, and the Reinvention of Seeing

by Laura J. Snyder

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  Leeuwenhoek later claimed that he was making bead lenses as early as 1659—two or three years before anyone else in the Netherlands is known to have been using them. In some ways it is not surprising that Leeuwenhoek, once he decided to make lenses, would have begun with bead lenses, since they were the simplest to make: nothing was required but very thin glass rods and a candle flame. The flames of wax candles are surprisingly hot: their temperature can reach 2,600 degrees Fahrenheit, while typical glass melts at around 1,500 degrees. However, in order to create usable bead lenses, one would need great patience, and the time to make large numbers of them, as so many would be useless for observations because they were too sooty or bubble filled. Later, the Dutch natural philosopher Jan Swammerdam would say he could make forty bead lenses in an hour, but that they varied greatly in quality. Other makers of the bead lenses said that only one in a hundred was perfect. In Leiden, Johan van Musschenbroek sold the bead lenses at the rate of forty for one guilder (about twelve dollars, roughly a day’s wages for a skilled laborer in the Dutch Republic). Natural philosophers would buy them by the scoopful, test each one, and discard all but the very best.

  It is unclear from whom Leeuwenhoek would have learned the technique for making bead lenses, since he did not know Latin well, according to all accounts, and the books reporting the method for producing them before 1659 were exclusively in that language. The earliest written documentation of the use of bead lenses in the Dutch language was not published until the early 1660s. Leeuwenhoek may have discovered the method for making the lenses spontaneously, while toying with a thread of glass in a candle flame, much as the natural philosopher Nicolaas Hartsoeker would later claim to have done in 1674. Or Leeuwenhoek may have learned about bead lenses from one of his compatriots. Johan Hudde, the Amsterdam mathematician (and future burgomaster) had begun making such beads by around 1660; Christiaan Huygens said Hudde’s excellent bead lenses were the size of small peas. Leeuwenhoek may also have heard of these lenses from his friend and neighbor, the Delft city anatomist Cornelis ’s Gravesande. Another possible source of the method of producing bead lenses may have come to Leeuwenhoek from Constantijn Huygens the elder. Although we do not have evidence that the two men were acquainted by 1659, they may well have met through Leeuwenhoek’s position as an employee of the Delft city government. Huygens was quite familiar with lens making by the 1650s; he was friendly with Descartes, and had encouraged him to publish his book on optics, the Dioptrics (1637)—a work that contained illustrations of two different kinds of single-lens microscopes, one of which Leeuwenhoek either adopted or independently invented. At around the same time that Leeuwenhoek probably began making his beadlenses, Hooke was developing his own method for making them in England, although he disclosed this to the world only in 1665, with the publication of his book Micrographia.

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  However it came to be that Leeuwenhoek started to make lenses, he would later claim to have made “hundreds and hundreds” of microscopes with them. Although he began with bead lenses, he would soon move on to the other methods for making lenses: grinding and blowing glass. Some estimates have put his production at 566 microscopes by the time of his death. Only nine specimens remain today; one of them is missing its lens, so only eight complete Leeuwenhoek microscopes remain extant—or, more accurately, are known to remain extant, because who knows how many might remain in boxes and drawers of old metal objects, thought to have no historical importance? One single-lens microscope of historical importance turned up not long ago in a Dorset garage sale!

  Leeuwenhoek’s microscopes shared a basic structure, being constructed of a single bead or tiny lens of glass set into a flat metal holder. These holders were rectangles about one inch wide and one and a half inches long, made of thin sheets of brass or silver that Leeuwenhoek said he had produced entirely by himself—he even claimed to have made the metal by melting the ore he took from the rocks. (This seems overly exacting, because metal sheets were already being manufactured in Delft at the time and could easily have been purchased by Leeuwenhoek and then cut to the size he required.) Each plate had a concavity rounded outward, which had been punched or ground out into its center. A minute hole was pricked into the center of this socket; the hole was slightly smaller on the object side than on the viewing side. The lens was fitted into the sockets, between the plates, and then clamped into place with four homemade screws. To focus on the object being observed, the lens itself was not moved; rather, the object was fastened to a specimen pin with a screw-cut thread in front of the lens. By the use of screws the user could bring the object into focus by moving the specimen pin up and down and back and forth. As Leeuwenhoek himself put it, “You then hold the microscope towards the open sky, within doors, and out of the sunshine, as though you had a telescope and were trying to look at the stars in the sky through it.” The best way to make observations with the Leeuwenhoek microscopes was to put the microscope very close to the eye, and face upward to a narrow beam of sunlight coming in from the window; by adjusting the shutters, one could get this kind of beam and aim it toward the microscope. In certain cases, though, Leeuwenhoek found it best to observe by relying on the light of a candle, sometimes using a mirror to amplify its brightness.

  In recent years the eight remaining Leeuwenhoek lenses have been examined closely. The examination was conducted by constructing a special microscope with which to view the lenses in situ, without removing the lenses from their metal holders—the devices are far too important as historical artifacts to risk damage by taking them apart. The tube of the testing microscope was fitted with a crossbar, to which were attached four tiny incandescent lamps. When a curved lens is viewed with the microscope, the lamps are reflected in its upper surface, and the distance between their reflections can be measured. These distances can be used to calculate the radius of curvature of the lens. By repeating the measurement on the other side of the lens, the lower surface curvature can also be calculated. From these two figures both the thickness of the lens and its refractive index (the precise way that light travels through the lens) can be computed. The focal length of the lens can also be determined, and from the focal length the magnification of the lens can be gauged.

  The analysis of the eight remaining lenses showed that the magnification ranged from 69 times, for the weakest lens, to 266 times, for the strongest. It was also determined that all the lenses, with the exception of the strongest, were ground and polished; the strongest lens was blown. (None of the eight seem to have been bead lenses.) This is known because all but the one lens exhibited a characteristic property of lenses that are ground and polished: an “orange peel texture” on the surface, meaning that when lit in a certain way, one can see shallow pits with rounded edges—a result of polishing glass on a soft, resilient material like felt. But since the pits in these lenses are quite small, relative to those in other lenses from the seventeenth century, it means that Leeuwenhoek polished only for a short time, which would result in a good shape to the surface of the lens. He most likely began by taking a tiny shard of glass, quite possibly from a mirror, and ground and polished it with sand that he had first crushed to a fine, smooth powder. By the late seventeenth century, the grinding technique for large, flat mirrors had improved greatly, and so mirrors would have provided excellent glass that was easy to obtain. Leeuwenhoek was also an accomplished polisher. It was found during the examination of his microscopes that the most skillfully polished surfaces of the surviving lenses are very nearly acceptable even on modern standards of microscopy; and even the least successfully polished lenses are markedly better than two remaining telescope lenses made by the Huygens brothers—considered among the finest lens makers of their time—in 1655 and 1686.

  The eighth lens, the one in the “Utrecht microscope” (named for its current location, in the museum of the University of Utrecht), does not exhibit the orange peel texture, and it contains numerous minute bubbles, leading to the conclusion that Leeuwenhoek
made this lens by blowing it. The lens is also aspherical, the radii of curvature increasing to the margin of the lens. It is nearly impossible to make such a lens by grinding, but this is the shape that would be expected for a blown lens. The magnification of this lens, at 266 times, is such that a bluebottle fly viewed with it would appear one meter in length. And a bacterium would be clearly visible as a dot the size of the period at the end of this sentence. This may not even have been the highest magnification achieved by Leeuwenhoek; in his letters there is evidence that he had used lenses that magnified up to 480 times, and contemporaries believed that he had instruments capable of 500 times. As a point of comparison, the telescope with which Galileo made his most famous discoveries in 1609 had a magnification of only 20 times.

  Leeuwenhoek probably began grinding his lenses by hand, but later he began using a lathe, of a type similar to that used by jewelers. He described the setup of the lathe in his study:

  My Study stands toward the North east, in my Antichamber, and is very close joyned together with Wainscot, having no other opening than one hole of an inch and a half broad, and 8 inches long, through which the wooden spring of my lathe passes towards the street furnisht with 4 windows, of which the two lowermost open inwards, and by night are closed with two wooden Shut[ter]s.…

  This type of “pole-lathe” would have had a cord fixed to a wooden spring, fastened to his ceiling; the other end was attached to a pedal moved by one foot. The cord was wrapped two or three times around the spindle, which, when the pedal was worked, would have a to-and-fro rotation. The spring was fitted outside the room and put through a horizontal slit, and therefore could work only in a horizontal plane, which suggests that the spindle itself was vertical. This would not be practical for turning wood or metal—and so this layout would not be used by woodworkers or metalworkers—but is the best positioning for grinding lenses. A rotating lathe would speed up the grinding process. But since the outer parts of the tool have a greater velocity than the inner, the resulting lens would be worn more at the center, and thus have a flatter curvature; this corresponds to the results of the examination of the surviving lenses, strongly suggesting that Leeuwenhoek did use the lathe in grinding the lenses, as he reported.

  Leeuwenhoek’s use of the lathe may have been similar to the manner a lathe was employed by Baruch Spinoza, the philosopher expelled from the Jewish community in Amsterdam. Born a few weeks after Leeuwenhoek and Vermeer, Spinoza began making lenses as a way of earning a living when he was thrown out of his family’s importing business after his excommunication. Spinoza had an elementary lathe with which he ground his lenses; he then polished the lenses by hand. Like Leeuwenhoek, Spinoza made very small lenses, which he fitted into single-lens microscopes. Leeuwenhoek did not mention when he first acquired the lathe, but it is notable that in the summer of 1665 he and Spinoza were living only four miles apart, both were friends of members of the Huygens family, both had similar backgrounds, as coming from families of merchants, both were practical opticians rather than highly mathematical ones. And, whether or not Leeuwenhoek was Catholic, both men were citizens of the republic who were not formally affiliated with the Dutch Reformed Church. It is not at all unlikely that by the mid-1660s they were acquainted, and that Spinoza may have inspired Leeuwenhoek to purchase the lathe. This is another historical possibility that must remain tantalizingly speculative.

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  Lens making may have begun as a part-time occupation for Leeuwenhoek, but it soon became an obsession, upon which he was able to concentrate by 1660. He probably ceased work as a clothier around that year, because at that time he was appointed to a post in civic government. He was made chamberlain (camerbewaarder) to the sheriffs of Delft, a position he held for thirty-nine years; he continued, even after that period, to draw a salary from the position until his death. It would seem that Leeuwenhoek’s early training by his uncle Cornelis Jacobsz. van den Berch, the sheriff in Benthuizen, qualified him for this position. At first he was paid 314 florins per year (about $5,600 today); eventually this rose to 450 florins ($9,827). The average, well-paid Dutch glassworker would have received around 270 florins a year, so 314 was a good income, 450 a quite good one.*5 In the town records we find his appointment, and the work he was expected to perform:

  Their Worships the Burgomasters and Magistrates of the Town of Delft have appointed and do hereby charge Antony Leeuwenhoek to look after the Chamber wherein the Chief Judge the Sheriffs and the Law Officers of this Town do assemble: to open and to shut the foresaid Chamber at both ordinary and extraordinary assemblies of the foresaid Gentlemen … to show towards these Gentlemen all respect honour and reverence and diligently to perform and faithfully to execute all charges which may be laid upon him and to keep to himself whatever he may over hear in the Chamber: to clean the foresaid Chamber properly and to keep it neat and tidy: to lay the fire at such times as it may be required and at his own convenience and carefully to preserve for his own profit what coals may remain unconsumed and to see to it that no mischance befall thereby nor from the light of the candles.…

  The room of the sheriffs was on the north side of the second story of the Town Hall, a quick walk from Leeuwenhoek’s house on the Hippolytusbuurt. It may have been expected that the chamberlain would hire a servant girl to perform the more menial tasks involved, paying her from his salary. It is noteworthy that part of his remuneration included the leftover coals—he would have brought these home for Barbara to burn in their own fireplaces. Since coal burns hotter than wax or an oil lamp, Leeuwenhoek could have used the coal to burn in a furnace that would be hot enough to melt large quantities of glass in order to obtain molten glass for blowing.

  Since Leeuwenhoek was explicitly charged to keep silent about matters discussed by the council, he must have been present during their conversations. Leeuwenhoek would, then, have been privy to council discussions of issues pertaining to the civic government, such as the elections that took place every October 18 (St. Luke’s Day) for the head of the St. Luke’s Guild—a position to which Vermeer would be elected for the first time two years after Leeuwenhoek’s appointment as chamberlain. Most likely, once he was appointed to this civic position, Leeuwenhoek began to devote himself to making lenses in earnest, employing all of the time he was not employed by his work for the government.

  Once he made his first devices, Leeuwenhoek began to peer at whatever he could find around him. He first trained his new instruments on the little creatures he found crawling and flying around his study and in his small garden, entranced by flies, mites, worms, and moths, just like the other natural philosophers, the artists, and ordinary people with their flea glasses. Leeuwenhoek was irresistibly drawn to the eyes of the insects, more than to other parts of their tiny bodies. Indeed, for most of the new microscopists, eyes were a continual source of fascination—as if they were using this new optical instrument to understand sight itself, and the way that the instrument worked, by examining natural optical systems. Federico Cesi, in the first published microscopic examination in the 1620s, had turned the new instrument on the eye of the bee, Borel had paid especial attention to the eye of the mite, Odierna had carefully studied the compound eye of the fly, concluding that the insect eye both receives and perceives the “multitudinous images of the outside world”—that visual perception in insects occurs outside the brain. Over the next sixty years Leeuwenhoek would return again and again to eyes: the eye of the bee, described in his very first letter to the Royal Society of London; the eye of a cow—obtained from a helpful butcher at the vleeshal—its “aqueous humor” in the anterior chamber of the eye, its optic nerve, which was springy and filled with filaments that seemed to come from the brain, its iris, cornea, and retina; and the eye of a whale, pickled in brandy, brought to him by the captain of a whaling ship. In one notable frenzy of dissection and examination, the eyes of pigs, sheep, dogs, cats, rabbits, hares, fishes, and birds were all discussed in a single lengthy letter to the Royal Society.

&nb
sp; It would be another decade before Leeuwenhoek realized that he had begun to see objects that could not be seen at all with the naked eye. But from the end of the 1650s until the 1660s—the period of his microscopic apprenticeship, as it were—he began to hone his technique for using his devices to see more than anyone else had before him, even in macroscopic structures like the eye of a bee. He was, in a sense, learning to see.

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  The idea that one must “learn to see” was, perhaps inevitably, part of the revolution in the new way of seeing that came about in the seventeenth century. Kepler’s claim that the retinal image was inverted introduced a new element into discussions about vision and the relation between object, light, sensation, and perception. If the image from a viewed object that is projected onto the retina is inverted, what explains the fact that we see it as upright? Something happens in the process that makes the figure appear to us that way. Is that process an innate mechanism, or must we learn to see this way? Kepler had admitted the existence of this problem but demurred to answer it, claiming that how the image is formed is an optical problem but that how it is perceived by the mind is not, and was therefore not his concern. “All this,” Kepler said, “I leave to be disputed by [others]. For the armament of opticians does not take them beyond this first opaque wall encountered within the eye.”