In the late 17th century, a Dutch draper and self-taught scientist named Antonie van Leeuwenhoek earned renown for building some of the best microscopes available, at a time when the instrument was just beginning to revolutionize scientific inquiry. He rarely divulged his lens-making methods, however, leading to centuries of speculation as to how he achieved such superior magnifications.
Now neutron tomography has enabled scientists at TU Delft in the Netherlands to peer inside van Leeuwenhoek’s microscopes for the very first time. A new paper published in the journal Science Advances reveals that, far from requiring his own secret lens-crafting method, van Leeuwenhoek was a master craftsman who was able to achieve his extraordinary magnifications by honing and perfecting the typical lens production methods of his era.
It’s not entirely clear who invented the first bona fide microscope, but contenders for the claim include a late 16th century Dutch maker of spectacles named Zacharias Janssen, a neighboring rival spectacle manufacturer named Hans Lippershey, and a Dutch engineer and inventor named Cornelis Drebbel. Galileo noted the basic principle sometime after 1610, and built his own compound microscope after seeing one of Drebbel’s instruments on display in Rome in 1624. He dubbed it the “occhiolino” or “little eye.”
English scientist Robert Hooke was among the first to make significant improvements to the basic design. He was a skilled lens grinder, which resulted in better magnification, and his early training as a draughtsman enabled him to render what he saw under the microscope in drawings of exquisite detail. He published his magnum opus, Micrographia, in January 1665, illustrated with 58 stunning engravings—including his famous depiction of a magnified flea.
Van Leeuwenhoek’s own interest in lensmaking stemmed from his desire to more clearly see the quality of the thread he used in his draper business, and when he learned of the wonders of microscopy—he was a great admirer of Hooke’s Micrographia—he began making his own improvements. He built more than 500 microscopes in his lifetime, although only a handful have survived. A single lens was mounted in a tiny hole in the brass plate making up the body of the instrument, and the specimen was mounted on a sharp point just in front of it. The position and focus could be adjusted by turning two screws. The entire instrument was only three to four inches long.
Van Leeuwenhoek used his microscopes to study protozoans found in pond water, animal and plant tissues, mineral crystals and fossils. He discovered such microscopic creatures as nematodes, as well as blood cells, and was the first to see living sperm cells of animals. By 1683, he had turned the instrument on himself to study the plaque between his teeth, and also observed teeming hordes of bacteria in the mouths of two elderly men who had never cleaned their teeth in their lives—the first observation of living bacteria ever recorded. He even experimented with using the ovum of a cod and the corneas of dragonflies as biologically-derived lenses, succeeding in generating clear images of a candle flame with the latter.
His microscopes were little more than powerful, handheld magnifying glasses, but they were nonetheless considered to be the best of his era. He was able to achieve magnifying power up to 270 times larger than the actual size of the sample, using a single lens, with clearer and brighter images than those achieved by any of his colleagues.
Van Leeuwenhoek was certainly skilled at grinding and polishing lenses, and it’s likely that many of his instruments contained these ground lenses. He also experimented with ball-shaped lenses early in his career, which involved melting and collecting glass on a needle tip. However, van Leeuwenhoek wrote that the glass suffered from contaminations, and that he soon abandoned the method. But he also hinted that he had invented his own advanced method of glass-blowing “with which nonspherical lenses could be produced”—a remark that has led to considerable speculation over the centuries about a “secret technique” lost to posterity.
Eleven of van Leeuwenhoek’s microscopes have survived, but since he encased his lenses between two metal plates secured with rivets, with just one tiny hole about half a millimeter in diameter, one would need to take the microscopes apart in order to access the lenses—and no museum would consider damaging such a priceless artifact in this way. So the TU Delft scientists proposed using a non-invasive imaging technique called neutron tomography, similar in concept to x-ray tomography.
This makes it possible to image the entire shape of the lens, since the neutrons yield a higher contrast between the metal plates and the glass within. The object is rotated 180 degrees in a neutron beam as a camera takes multiple photographs, and the resulting 2D images can then be used to construct a 3D image of the object on the computer.
The 3D image of one of the microscopes in the Rijksmuseum Boerhaave’s collection showed that this medium-powered instrument contained a lentil-shaped lens consistent with the standard grinding and polishing methods of the time. The authors note in their paper that the data also showed the great care and precision van Leeuwenhoek brought to bear when crafting his instruments. “Judging from the preserved copies, each microscope that van Leeuwenhoek produced held a lens with a distinct curvature and magnification,” they wrote. “The tight fitting of the lens that the tomogram shows suggests that the brass plates were specially adapted to hold this specific lens.”
The TU Delft team also imaged the highest-powered known surviving microscope, housed at the Utrecht University Museum. This revealed that the Utrecht microscope does not contain a lentil-shaped ground lens; rather, it holds a ball-shaped lens with a tiny glass thread connected to it—the kind of lens produced by glass-blowing methods. What’s most interesting is that the shape of that lens fits a glass-melting lens “recipe” published by Hooke in 1678, which involved turning the end of a thin glass thread into a ball shape by melting it in a flame, and using the remaining stem as a handle to mount the lens.
It was a variation of a technique Hooke had earlier described in Micrographia, which predated van Leeuwenhoek’s instruments, so the draper would have been familiar with the basic principle. Hooke’s method also produced ball-shaped lenses free of the contaminations that marred earlier glass-blowing methods van Leeuwenhoek said he had experimented with and found wanting. He was always strangely silent about Hooke’s lens-making methods.
“We may now assume that van Leeuwenhoek’s silence was a deliberate choice,” the authors wrote. “Van Leeuwenhoek adopted the very lens-making procedure by Hooke soon after he published it, and brought it to a great success but never told anyone about it. This is ironic, as Hooke always wanted to find out the secret of van Leeuwenhoek’s lenses but never managed to do so.”
Taken together, this research “has offered visually conclusive poof that van Leeuwenhoek did not limit himself to only a single lens type for making his pioneering discoveries, but adopted distinct lens-making procedures that circulated at that time, and integrated these into his microscopes,” the authors wrote. “Van Leeuwenhoek was far from the isolated scholar he is often claimed to be; rather, his secrecy about his lenses was motivated by an attempt to conceal his indebtedness to Hooke.”
Their findings also serve as a testament to his skill at pushing existing methods and designs to their full potential, honed over many years of building hundreds of microscopes. “It was craftsmanship and careful aperture control that made the difference,” the authors concluded. “Van Leeuwenhoek seems to have fully mastered these grinding and framework techniques, to have combined them with appropriate apertures, and to have brought them to perfection, resulting in the superiority of his iconic microscopes, in which all the attention and efforts were guided toward their one essential component: the lens.”