ON SEPTEMBER 7, 1674, Antonie Van Leeuwenhoek, a fabric seller living just south of The Hague, Netherlands, burst forth from scientific obscurity with a letter to London’s Royal Society detailing an astonishing discovery. While he was examining algae from a nearby lake through his homemade microscope, a creature “with green and very glittering little scales,” which he estimated to be a thousand times smaller than a mite, had darted across his vision.
Two years later, on October 9, 1676, he followed up with another report so extraordinary that microbiologists today refer to it simply as “Letter 18”: Van Leeuwenhoek (lay-u-when-hoke) had looked everywhere and found what he called animalcules (Latin for “little animals”) in everything.
He found them in the bellies of other animals, his food, his own mouth, and other people’s mouths. When he noticed a set of remarkably rancid teeth, he asked the owner for a sample of his plaque, put it beneath his lens, and witnessed “an inconceivably great number of little animalcules” moving “so nimbly among one another, that the whole stuff seemed alive.” After a particularly uncomfortable evening, which he blamed on a fatty meal of hot smoked beef, he examined his own stool beneath his lens and saw animalcules that were “somewhat longer than broad, and their belly, which was flat-like, furnished with sundry little paws”—a clear description of what we now know as the parasite giardia.
With his observations of these fast, fat, and sundry-pawed creatures, Van Leeuwenhoek became the first person to ever see a microorganism—a discovery of almost incalculable significance to human health and our understanding of life on this planet.
Microorganisms are the second most abundant life-forms on Earth. Two of the types that Van Leeuwenhoek identified—protozoa and bacteria—are by some estimates responsible for more than half the deaths of every human who has ever lived, and yet until he observed them their existence had hardly been seriously postulated, much less proven. Of course, he had no idea about the pivotal role his little animals played, but his revelation provided the foundation for germ theory—the greatest leap forward in the history of medicine. Even more surprising, this monumental discovery was not made by one of the 17th century’s great scientific minds such as Galileo or Isaac Newton. Instead, a secretive, obsessive, self-taught Dutchman of little renown did it by handcrafting a lens 10 times more powerful than anything built before it. His design wouldn’t be bested for another 150 years.
Yet even as scientists steadily unlocked the secrets of Van Leeuwenhoek’s microworld over the past 350 years, one great mystery eluded them: How the hell did he do it? How did a shopkeeper working during his off hours build a microscopic lens that surpassed the world’s greatest by an order of magnitude?
While Leeuwenhoek shared nearly everything he saw through his microscope in exactingly detailed letters, he zealously guarded how he made his revolutionary lens. When asked, he declined or obfuscated. Even as his discoveries made him so famous that the King of England requested to see his animalcules and Peter the Great stopped in Delft to see his lenses, the Dutchman never revealed his secrets.
Van Leeuwenhoek crafted more than 500 microscopes, but only 11 of his instruments survive today—and only one that produces the 270X magnification he used to make his greatest discovery. Because that lens remains sandwiched between brass plates, determining its mode of manufacture would require disassembling the microscope—an affront tantamount to scraping paint off the Mona Lisa to determine the sequence of Leonardo’s brush strokes.
Most of Van Leeuwenhoek’s contemporaries believed he had invented a new glassblowing technique. Clifford Dobell, who wrote the brilliant 1960 biography Antony Van Leeuwenhoek and His Little Animals, postulated that he created his best lenses by simply grinding and polishing them better than anyone else. But in three centuries of speculation, no one could say for sure.
TIEMEN COCQUYT’S INTEREST in Van Leeuwenhoek’s secrets began in the late 2000s, soon after first seeing one of his microscopes, which was then locked away in the basement of the University Museum Utrecht. “How could this toy open up the microworld?” Cocquyt remembers thinking.
Cocquyt is a curator in the National Museum Boerhaave in Leiden, Netherlands, which houses an array of early optical instruments, including several of the microscopes. He has spent much of his career investigating the origins of Europe’s 17th-century optical revolution, when visual instruments suddenly leaped from simple magnifiers to the great telescopes of Galileo and Christiaan Huygens. (That revolution was inadvertently sparked, Cocquyt says, by Italian advances in making ultra-clear glass.)
Over Zoom, Cocquyt shows me a replica of a Van Leeuwenhoek microscope, and it does look like a toy—a doll’s hand mirror, to be exact. It’s barely 3 inches tall, with a thin handle leading to a square brass plate. The lens sits beneath a pinhole in the plate’s center, and on the back side a pin for holding samples is connected to a set of screws for focal adjustment.