Historical story

Antoni van Leeuwenhoek drags the superior lenses for his microscopes with which he made many discoveries

In the 17th century, Antoni van Leeuwenhoek made microscopes that were many times better than those of his competitors. It was not clear how he made lenses with which he magnified preparations up to 270 times. Research with intense neutron beams at the Reactor Institute in Delft shows that he achieved this unsurpassed quality with grinding.

The brass device that you clamp between two fingers does not resemble a modern microscope at all. The Dutch trader and microbiologist Antoni van Leeuwenhoek made more than five hundred of these devices in the 17th century, who made several discoveries with them due to their surprisingly good optical performance. He was the first to see red blood cells, moving sperm cells, and structures in various biological tissues. A world opened up with Van Leeuwenhoek's instruments, it made him world famous.

Until now, it was unclear how Van Leeuwenhoek made the minuscule lenses, only a few millimeters in size. Grinding pieces of glass is obvious, but little is known about his methods. He has talked about glass blowing, but whether that is correct is the question. However, it may have been an attempt to mislead competing microscope builders.

Researchers from Rijksmuseum Boerhaave (with four Van Leeuwenhoek microscopes in their possession) and the Reactor Institute Delft are now shining more light on the precise shape of the lenses. They scanned two microscopes with an intense beam of so-called neutrons from the reactor, and with this they looked through the metal plates that clamp the lenses. It provides a rare insight into the production method of Van Leeuwenhoek's lenses and microscopes.

Mysterious

Van Leeuwenhoek's discoveries were a sensation, but he himself was always very secretive about his instruments. He was probably afraid of competition. “It's easy to count on one hand the sources on Van Leeuwenhoek's techniques,” says Tiemen Cocquyt, curator of Rijksmuseum Boerhaave. “Despite requests from, among others, the Royal Society in England – with whom he corresponded about his discoveries – Van Leeuwenhoek kept as much as possible to himself for more information. Van Leeuwenhoek did not even want to sell his microscopes to visiting monarchs.”

Fortunately, we still have the microscopes, which contain information about their production method. Eleven of the more than five hundred microscopes that Van Leeuwenhoek made are still left, but research into the lenses is difficult. “They are sandwiched between two metal plates and the piece that is visible from the outside is usually no more than half a millimeter in size,” says Cocquyt. “Opening is not an option and with many scanning techniques you cannot see through the metal plates.”

Cocquyt was enthusiastic when the Reactor Institute Delft approached the museum with the question whether they wanted to scan objects with the strongest neutron beams from their nuclear reactor. A proven method with which several metal artefacts have already been screened. Neutrons (elementary particles that you normally find in atomic nuclei) from the nuclear reactions of the uranium in the reactor, are bundled for this purpose and fired at an object at thousands of kilometers per hour.

The neutrons are chargeless and – unlike charged protons or X-rays – can hardly be stopped by very dense materials such as metals. They usually fly right through it unless a neutron hits an atomic nucleus. At that moment he changes direction like a small billiard ball. The reflected neutrons can be collected with a detector behind the object. They then tell something about the materials and structure of the scanned object. By illuminating an object from different sides, a detailed three-dimensional image is created with a resolution of up to 0.05 millimeters.

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Grinding versus glass blowing

The shape of the lens inside the microscope can be deduced from the accurate scan. “We see a clear lens shape, with a sharp edge,” says Cocquyt. “It is unlikely that you will get such a shape through glass blowing. Then you expect a more convex lens shape, without edges. I myself visited glass workers to see what they make with different techniques. The uniform curvature that this lens has to the edge convincingly points to grinding work.”

The university in Delft already headlined that a 350-year-old mystery has been solved, but there are still questions. For example, about how Van Leeuwenhoek drags his lenses. In any case, he was extremely adept at it. “The quality varies by lens, but you can say they are surprisingly good across the board. He was a craftsman who, using traditional techniques, managed to create lenses that performed almost to the optical limit. Even a hundred years later, others could not match this quality”, says Cocquyt. “Incidentally, we cannot rule out the possibility that he did blow glass. He didn't do it with this microscope, but he made many more.”

Radioactive showpiece

A good result, but Cocquyt says he was hesitant about the question from the Reactor Institute. “Do you want to expose a historical showpiece to radioactivity, after which it might be radioactive itself?” he says. “No matter how special an object is, you really can't get it back if it's radioactive. In the end, the people at the Reactor Institute Delft were able to guarantee us that the microscopes come out as radioactive as they go in.”

Depending on the composition, the beam of fast neutrons from a nuclear reactor also makes a material radioactive itself. When a neutron hits an atomic nucleus in the material, it can be absorbed. If that nucleus knocks the neutron out again after a while, you are dealing with radioactivity. “Most neutrons leave the material almost immediately, after which the extra radioactivity has also disappeared,” says Lambert van Eijck, researcher at the Reactor Institute Delft that carried out the research. “But the copper in the brass microscope (an alloy of copper and zinc – red. ) can remain radioactive for longer. The only way to get rid of that is to wait. With a so-called half-life of twelve hours, we were no longer able to measure an increased level of radioactivity after five days, or ten half-lives.”

This temporarily introduced radioactivity is useful to determine the precise composition of the microscope material. The researchers focused mainly on the lens. “Using so-called gamma spectroscopy, we were able to find out that there is probably sodium in the microscope that picked up neutrons from the beam,” says Van Eijck. "We have yet to verify, but if this sodium is in the glass of the lens, that says something about the composition of the glass used."

Glass of the gunpowder disaster

Cocquyt would like to know more about the origin of the glass. “He probably just used glass that also served for windows and drinking glasses,” says Cocquyt. “Interesting is where he got it from. In Van Leeuwenhoek's time, a gunpowder storage facility in Delft exploded, flattening part of the city. Delft was probably full of broken glass. Some suspect that this was the source for Van Leeuwenhoek's microscopes. Perhaps our research will provide indications for this in the future.”

This requires more scans, also of glass material for reference. “I'm guessing that I'll have to go to Delft more often in the near future with different types of glass,” concludes Cocquyt.