The microscope was never really "invented." It's probably more accurate to say that it evolved. And that seems right for what is possibly the most useful and important laboratory tool ever created. The history of the microscope is directly tied into the history of the lens. But it's not completely clear where the first lens came to fruition. Some say the earliest known example is the Nimrud Lens (or the Layard Lens) -- a carved piece of convex crystal that dates back to 750 B.C., Assyria, which historians believe could have been used as a crude magnifying glass (though according to The British Museum, which houses the lens, its convex shape was more likely an accident and the lens is actually just a piece of jewelry). Others say the first lenses weren't created until the 11th century for the purpose of magnifying small text (at the time they were called "reading lenses"). But other historians say the 11th century is late in the lifespan of the lens -- that's because there are references to a "burning-glass" in literature from ancient Greece. One thing we do know for sure is that the first real and true use of eyeglasses can attributed to the Italians around 1260. But before that, well, let's just agree that humans have been using lenses to magnify things for a really, really, really long time.
Given our long history with using lenses to magnify objects and manipulate light, it's kind of surprising that it wasn't until 1590 that a Dutch eyeglass maker and his dad made the world's first actual microscope. Zacharias Janssen and his father Hans made the first sort-of microscope in Middleburg, Holland (Germany and Holland were well-known at the time for being the world's top lens-makers). It looked a bit like a kaleidoscope and consisted of three tubes -- two with a lens on one end and one that was open on either side to hold them together. The lens you looked through was biconvex, meaning it was curved on both sides. The lens that faced the object you wanted to magnify was plano-convex, meaning it was flat on one side and curved on the other. By elongating or shortening the tube, the microscope's user could magnify an image from three to nine times its original size. Kind of like a telescope.
It should be noted that some say Janssen and his father actually didn't invent the first microscope but, rather, their competitor Hans Lippershey, an eyeglass maker who lived nearby actually beat them to the punch. Because even in the 1500s new inventions had intellectual property controversies! Either way, this early device almost immediately set off a flurry of innovations and upgrades.
In 1667, mapmaker and biologist Robert Hooke would be the first researcher to publish the results of using a microscope for scientific purposes. He would also bring its design much closer to the device we use today by redesigning it to meet his specific needs. Hooke's book about using the microscope, called Microphagia, was the first publication of the Royal Society of London and it was also the very first scientific best-seller. As the Royal Society's Curator of Experiments, Hooke designed his own devices by modifying pre-built microscopes. Hooke's designs hacked together the first compound-light microscopes by attaching the standard (for the time) microscope to a flask filled with water set in front of an oil lamp to focus the illumination on the specimen.
The results were revolutionary. In Hooke's Microphagia he illustrated objects that had been magnified by more than 50 times through the microscope lens. One object, a thin slice of cork, he described as being made up of a series of "cells" -- little did he know how accurate that statement was. He was making the very first observation and description of a plant cell. It wasn't until 200 years later, in 1839, when scientists would actually begin to understand what the heck a cell actually was.
After that the microscope went through a series of tweaks and upgrades. Around the same time that Hooke was illustrating corks cells and close-ups of fleas, Anton van Leeuwenhoek, a master lens-maker, crafted a single-lens microscope with so few imperfections that he was able to magnify objects up to 200 times. This allowed him to see microorganisms like bacteria that he dubbed "animalcules." Unfortunately, there are no surviving examples of Leeuwenhoek's microscopes, so no one knows exactly how he managed to create such a perfect lens.
After this the evolution of the microscope gets much more refined. In the 1870s Ernst Abbe created an equation that theorized the maximum possible magnification of a microscope. Called the diffraction barrier, the theory stated that the maximum magnification of an object wasn't based on the quality of the lens but rather the wavelength of the light being used to illuminate the object. So, he believed, humans would never be able to see beyond 200 nanometers (200 times smaller than the width of a human hair). The diffraction theory, along with a standardization of the correct illumination for an object under a microscope devised by August Koehler, helped standardize the production of microscopes.
Once the microscope could be mass marketed, all bets were off. Scientists started developing countless different ways and means to see the smallest objects possible. Most of those included the manipulation of light. Today there are microscopes that utilize ultraviolet light, fluorescence, phase shifts, polarization, beam splitters, frequency wavelength manipulations, and photon excitation. In the early 90s researchers even developed one that relied on a green protein from jellyfish to mark specimens so they'd be easier to spot when magnified.
One of the most recent and revolutionary updates to the microscope earned the inventor a Nobel Prize for Chemistry in 2014. To win the prize, Romanian chemist Stefan Hell demolished Abbe's theory about the maximum possible magnification of an object (an idea that scientists had believed to be true for more than 100 years). Using a technique that combined fluorescence with a sweeping light, Hell and his colleagues were able to see inside a living cell, a view that even electron microscopes hadn't been able to give scientists in the past.
So what's next for the microscope? Seeing even smaller objects, of course. Since Hell broke Abbe's diffraction barrier, the new limit on magnification is based on the very size of molecules themselves. At least, that's what scientists believe for now. The evolution of the microscope will surely continue for the foreseeable future.
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Science could never get done without the right tools. And all that gear has to come from somewhere. Many of the gadgets sitting on laboratory shelves around the world have histories just as interesting as the discoveries they've made. Each month we're telling the stories of how the most important lab tools came to be.