Leeuwenhoek's one-way microscope takes the human eye into new microfields

Source: Science and Technology Daily

Leeuwenhoek's single microscope is very different from our common duplex microscope, which is less than 10 centimeters long, only two or three centimeters wide, and about 1 centimeter thick, like a small copper plate that can be rubbed in the hand.

The image material was provided by Visual China, and the Leeuwenhoek single-type microscope replica was taken by Liu Niankai

A few days ago, the Science Museum of Tsinghua University exhibited a replica of an Antoni van Leeuwenhoek (1632-1723) single-type microscope in the treasure cabinet, which was handmade by the Boerhav Museum restoration team in Leiden, the Netherlands, which is less than 10 centimeters long, only two or three centimeters wide and about 1 centimeter thick. This delicate and compact scientific instrument aroused the curiosity of many people, and the audience's first reaction to it was often: How can this be a microscope? Indeed, compared to the duplex microscope that most viewers use in their nature or biology classes during their studies, this small copper plate that can be rubbed in the hand inevitably looks a bit "maverick."

So how can Leeuwenhoek's monomodal microscope be used, and how many times can the specimen be magnified? What's the interesting history behind it?

Leeuwenhoek with Robert Hooke

One cannot talk about Leeuwenhoek without mentioning his contemporary, Robert Hooke (1635-1703). In 1662, the young Hooke served as the experimental administrator of the newly founded Royal Society, and he used a modified microscope to observe various things, and found that cork under the microscope appeared one by one chamber, similar to the individual rooms of the monastery, so he called it the "monastery single cell". This is the "cell" we know. In 1858, the missionaries Alexander Williamson (1829-1890), Joseph Edkins (1823-1905) and mathematician Li Shanlan (1811-1882) compiled The Botany, in which "the polycytos are the cells that gather countless cells into one, and the cells are bonded" is considered by posterity to be the first cell of the English cell to be translated as Chinese. Although in the decades after this, the word cell did not become a common translation, cell was also translated as beads, chambers, small concaves, etc., until after the Sino-Japanese War, with the introduction of Japanese translation technology terminology, cells became the mainstream usage, which has been followed to this day.

Hooke used a compound microscope that magnified the specimen more than 30 times. His first discovery, in 1665, the year of the Great Plague in London, was synthesized into the book Micrographia, which became the first of the Royal Society's most influential works. Leeuwenhoek, who was 3 years older than Hooke, was a cloth merchant in Delft, the Netherlands, when he was young. He didn't have much education, spoke only Dutch, and didn't know English or Latin. Leeuwenhoek visited London in 1668, most likely during this time, when he came into contact with Hooke's Microscopic Atlas. Perhaps it was the huge flea illustration depicted in the book that touched the cloth merchant, who became interested in microscopes, and when he returned to Holland, he developed lenses and began a career of monomodal microscopy manufacturing and observation that lasted more than half a century.

The microscope made by Leeuwenhoek, like the instrument displayed in the treasure cabinet of the Science Museum of Tsinghua University, is extremely simple in structure. A very small glass bead is mounted as a single lens in a small hole in the brass plate, and the specimen is placed on a protruding tip in front of the lens, and the position and focal length can be adjusted by turning two screws. Close to the lens, the enlarged specimen can be observed. Leeuwenhoek's microscope, which can enlarge specimens up to 275 times, also made Hooke curious and puzzled throughout his life about how Leeuwenhoek made such lenses.

Enter a new world full of vitality

Through a friend's introduction, Leeuwenhoek began to write letters to the Royal Society in 1673, and his discoveries were gradually made public. Until his death in 1823, Leeuwenhoek maintained close correspondence with the Royal Society and its members, and about 200 letters are still preserved in the Royal Society. Through these letters, we can see Leeuwenhoek's account of the vitality and vitality of the new world into which he entered.

Among them, a letter dated October 9, 1676, left a passage written by Leeuwenhoek after observing the rain for four consecutive days in the spring: "I judge that even if a hundred of these small animals were to be stretched out and placed together, they would not exceed the length of a coarse sand; if this is true, then a million of these living creatures would not be enough for the volume of a coarse grain of sand." The "small animals" in this text are the bacteria we now know as well, and this is also the first time that humans have discovered bacteria.

In another letter from 1683, he wrote: "There is something embedded in it between my front teeth and molars, or growing there... It is extremely surprising that I always see many very small living microorganisms in these substances, moving very cutely. It seems that the world is full of life, in much more number than previously thought. There are living things in the water droplets, and there are also living things in the small particles stored between people's teeth! ”

He also made repeated observations of two elderly people who had never cleaned their teeth and found on the samples taken "an incredible group of live animals that swim faster than any animal I've seen so far." ”

With one new discovery after another—including the first observations of sperm and blood cells—spread from Delft to London, Leeuwenhoek's talent in microscopy and observation began to be recognized in Europe, and he was elected a full member of the Royal Society in 1680, and his home in Delft became a "punching ground" for celebrities, including Peter the Great of Russia, James II of England, and Frederick II of Prussia.

Like an explorer of our world's first entry into a new unknown, Leeuwenhoek went from the everyday scale known to mankind to an unimaginable, dreamlike tiny scale. After returning from the "Otherworld", he shared with joy what he saw, changing man's perception of nature.

In 1716, at the age of 84, Leeuwenhoek, still insisting on observing the world with a microscope, wrote in a letter: "I have done a long time of work, not to obtain the praise that I now enjoy, but mainly from a thirst for knowledge, which I have noticed is greater than that of others." So whenever I discover anything extraordinary, I feel it's my responsibility to write my findings down on paper so that all creative people know about it. ”

It has been a world leader for 150 years

It wasn't until the advent of achromatic microscopy lenses in the 1830s that Leeuwenhoek's lenses were surpassed, that is, his microscope performance was among the leading in the world for 150 years.

During Leeuwenhoek's lifetime, his contemporaries, including Hooke, became interested in how he could make lenses with such high magnifications, but Leeuwenhoek kept his methods of making lenses strictly confidential, and even when guests visited them, they only showed them medium-quality microscopes. In 1745, with the death of Leeuwenhoek's daughter Maria, most of his microscopes were snapped up at an auction held in 1747.

Historians of science believe that Leeuwenhoek made a total of 559 microscopes – meaning that in his 50-year career in microscopy and specimen observation, one was made on average every month, giving a glimpse of Leeuwenhoek's diligence. According to auction records, the buyers appear to have been predominantly local residents of Delft, with notaries, doctors, surgeons, magistrates, coppersmiths, but at least a third of the lots were purchased by anonymous persons by giving intermediaries commissions. Predictably, however, most new owners' ability to use a microscope to view specimens is simply not comparable to That of Leeuwenhoek. In disappointment, these tiny microscopes were set aside, culminating in a massive loss – only about 10 originals in the world have survived.

Due to Leeuwenhoek's strict secrecy of the lens production method during his lifetime, the original microscope that remains so far is extremely precious that it is inconvenient to open the microscope to directly observe the internal structure, so the historians of science have been helpless in solving the mystery of how Leeuwenhoek made a high-magnification lens. In 2021, researchers in Leiden, the Netherlands, used neutron tomography to conduct nondestructive testing of Leeuwenhoek's two microscopes, revealing the complete structure of the lens, one of which was lentil-shaped in the form of a medium magnification lens, and the other high-magnification lens was a spherical glass bead with a rod, which showed that Leeuwenhoek used the lens production method published by Hooke in 1678. He keeps his production methods secret, and may have had this in mind.

Leeuwenhoek used his microscope to "open the door to a new world," and scientists in various fields continue to expand the new world, constantly pushing the limits of what can be observed. With this in mind, the Museum of Science at Tsinghua University plans to collaborate with harvard's Historical Science Instrument Collection (CHSI) to host an online exhibition on the theme of historical microscopes, from Leeuwenhoek's single-style microscope to the latest modern microscopes, a journey to a new world of ever-expanding scales.

(The author is a postdoctoral fellow in the Department of History of Science, Tsinghua University)