Microscope

• 1. Introduction

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  A microscope let a Dutch tradesman see something in 1676 that no human had ever laid eyes on before: creatures so small they were invisible without one. On the 9th of October that year, Antonie van Leeuwenhoek reported his discovery of living microorganisms, using a lens he had ground and polished by hand. That discovery was the payoff of decades of experimentation with curved glass, competing claims to a single invention, and breakthroughs that kept pushing the limit of what people could see. From simple lenses to beams of electrons and probes fine enough to feel individual atoms, each new kind of microscope answered the same basic question: how much smaller can an image still be made clear?

• 2. Invention Of The Compound Microscope

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  Objects resembling lenses date back roughly 4,000 years, and Greek writers described the optical properties of water-filled glass spheres as early as the 5th century BC. Still, the earliest simple microscopes, really just magnifying glasses, did not spread widely until lenses became common in eyeglasses during the 13th century. The first compound microscopes, which paired an objective lens near the specimen with a separate eyepiece, appeared in Europe around 1620, though nobody knows for certain who built the first one.

  Several rival claims point to the same cluster of spectacle-making workshops in the Netherlands. One credits Zacharias Janssen, based on a claim his son made decades later; another credits Janssen's father, Hans Martens; a third names their neighbor and rival lensmaker Hans Lippershey, who had already applied for the first telescope patent in 1608. A fourth claim belongs to Cornelis Drebbel, an expatriate noted to have a working microscope in London by 1619.

  Galileo Galilei, sometimes credited as the compound microscope's inventor in his own right, found sometime after 1610 that he could refocus his telescope to examine small objects up close. After watching a compound microscope built by Drebbel on display in Rome in 1624, Galileo built an improved version of his own and submitted it to the Accademia dei Lincei in 1625. Galileo had called his instrument the occhiolino, meaning little eye, but it was the naturalist Giovanni Faber who coined the name microscope for it. A dozen years later, in 1637, Rene Descartes described a different design in his book Dioptrique, using a concave mirror to focus light onto the specimen from the side facing the lens.

  It would take almost two more decades before anyone published a detailed picture of what a microscope could actually reveal inside living tissue.

• 3. Seeing Living Tissue

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  In 1644, Giambattista Odierna published the first detailed account of microscopic tissue anatomy in a short work called L'occhio della mosca, or The Fly's Eye. The microscope stayed something of a novelty for another two decades, until naturalists in Italy, the Netherlands and England began using it seriously to study biology in the 1660s and 1670s. Marcello Malpighi, later called the father of histology by some historians, began that work by analyzing the structure of the lungs. Robert Hooke's 1665 book Micrographia had an even bigger impact thanks to its detailed illustrations, drawn from tiny lenses Hooke made himself by fusing the ends of spun glass threads.

  Antonie van Leeuwenhoek pushed the technology further still, reaching up to 300 times magnification with a single small glass ball lens sandwiched between two riveted metal plates and an adjustable screw-mounted needle for holding the specimen. Building on Jan Swammerdam's earlier work, Leeuwenhoek rediscovered red blood cells and identified spermatozoa, helping popularize microscopy as a tool for studying living structures rather than curiosities.

  The performance of a compound light microscope depends heavily on how evenly the sample is lit, a problem that was not fully solved until the late 19th and early 20th centuries, once electric lamps became available as light sources. In 1893, August Kohler developed Kohler illumination, a lighting method still central to reaching the light microscope's theoretical resolution limit. Two later refinements addressed the same problem differently: Frits Zernike's discovery of phase contrast in 1953 and Georges Nomarski's differential interference contrast technique in 1955 both made it possible to image transparent, unstained samples for the first time.

  None of these advances, though, solved the fundamental limit on how small a detail ordinary light could resolve.

• 4. Pushing Optical Limits

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  Typical light microscopes reach magnifications up to 1,250 times, limited by a theoretical resolution around 250 nanometers, which caps practical magnification at roughly 1,500 times no matter how good the optics are. Switching to shorter wavelengths of light, such as ultraviolet, is one way researchers push past that limit, alongside devices like the near-field scanning optical microscope. Near-infrared light serves a narrower but useful purpose too: because silicon is transparent at that wavelength, it lets engineers see circuitry embedded inside bonded silicon devices without cutting them open.

  Sarfus, a more recent optical technique, boosts a standard microscope's sensitivity enough to directly visualize nanometric films as thin as 0.3 nanometers and isolated nano-objects as small as 2 nanometers across, using non-reflecting substrates under cross-polarized reflected light. Phase-contrast microscopy takes a different route, converting the small phase shifts that occur when light passes through a transparent specimen into visible changes in contrast; because it needs no staining, the technique made it possible to study the cell cycle in living cells for the first time.

  The traditional optical microscope has since evolved into the digital microscope, which uses a CMOS or charge-coupled device sensor, the same technology found in digital cameras, to capture an image and display it on a monitor instead of relying only on eyepieces. At the extreme low end of light exposure, some researchers now pair sensitive photon-counting cameras with entangled pairs of photons, illuminating fragile biological samples with infrared light while an entangled visible-light partner is captured by the camera, a technique meant to minimize damage to the most light-sensitive specimens.

  None of these refinements to light itself, however, could match what happened once one team decided to replace photons with a completely different kind of particle.

• 5. Switching To Electrons

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  German physicist Ernst Ruska built the first working transmission electron microscope in 1931, working alongside electrical engineer Max Knoll. A transmission electron microscope, or TEM, follows the same basic principle as an optical microscope but replaces glass lenses with electromagnets and light with a beam of electrons, allowing for far higher resolution. Just four years later, in 1935, Knoll developed a second design, the scanning electron microscope, or SEM, though it would not reach the commercial market until 1965.

  TEMs saw research use before the Second World War and became popular afterward. Ruska, working at Siemens, built the first commercial transmission electron microscope, and major scientific conferences devoted to electron microscopy began in the 1950s. The first commercial scanning electron microscope followed in 1965, developed by Professor Sir Charles Oatley and his postgraduate student Gary Stewart and marketed by the Cambridge Instrument Company under the name Stereoscan.

  One of the technology's more recent applications is virus identification. Because an electron microscope produces a clear, direct image of tiny cell structures, researchers can spot a virus or harmful cell without needing chemical reagents at all, making pathogen detection considerably more efficient.

  Electrons could resolve detail that light never could, but they still required a beam passing through or bouncing off the sample. A different approach, developed decades later at a computer company in Switzerland, would abandon beams altogether.

• 6. Feeling The Surface

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  Between 1981 and 1983, physicists Gerd Binnig and Heinrich Rohrer worked at IBM's laboratory in Zurich, Switzerland, studying a phenomenon called quantum tunnelling. Their work produced the first scanning probe microscope, an instrument that reads the tiny forces exchanged between a probe and a sample's surface as the probe passes so close that electrons flow continuously between the two, generating a measurable current. The instrument was not well received at first, since the underlying quantum theory was difficult for many scientists to follow.

  That changed in 1984, when Jerry Tersoff and D.R. Hamann, working at AT&T's Bell Laboratories in Murray Hill, New Jersey, began publishing papers that connected the theory to the instrument's experimental results. Commercial scanning tunnelling microscopes followed in 1985, and in 1986 Binnig, Calvin Quate, and Christoph Gerber invented a related instrument, the atomic force microscope. That same year, Binnig and Rohrer shared the Nobel Prize in Physics for their invention of the scanning probe microscope.

  New scanning probe designs have kept appearing since, each limited by how precisely engineers can machine an ever finer tip.

• 7. Making Cells Glow

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  During the final decades of the 20th century, researchers developed DAPI, a chemical compound that binds to DNA, to make specific structures inside cells visible under a microscope. They also learned to attach fluorescent reporters to antibodies, a technique called immunofluorescence, and to insert naturally fluorescent proteins such as green fluorescent protein directly into living cells, letting different fluorophores reveal cell structure at a molecular level in both live and fixed samples.

  This rise of fluorescence microscopy drove development of the confocal microscope, an idea Marvin Minsky patented in 1957, though the laser technology needed to make it practical did not yet exist. It took until 1978 for Thomas and Christoph Cremer to build the first working confocal laser scanning microscope, after which the technique spread quickly through the 1980s.

  More recent research has focused on pushing fluorescence imaging past the diffraction limit that constrains ordinary light. Structured illumination can sharpen resolution by roughly two to four times, and a technique called stimulated emission depletion, or STED, microscopy now approaches the resolving power of electron microscopes. Stefan Hell won the 2014 Nobel Prize in Chemistry for developing STED, sharing the award with Eric Betzig and William Moerner, who separately adapted fluorescence microscopy to visualize single molecules.

  Fluorescence solved the problem of contrast in transparent samples, but some materials still needed a completely different kind of wave to see through them at all.

• 8. Beyond Light And Electrons

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  In the early 1970s, better X-ray lens optics finally made X-ray microscopy a practical imaging technique, using electromagnetic radiation in the soft X-ray band to see inside objects. These instruments are often used in tomography to build three dimensional images of materials, including biological samples that have never been chemically fixed, and researchers are now working to improve optics for hard X-rays, which penetrate matter even more deeply.

  Scanning acoustic microscopes work on a completely different principle, similar to sonar, measuring variations in acoustic impedance to detect defects buried inside materials such as integrated circuits. On the 4th of February 2013, Australian engineers unveiled a quantum microscope capable of unprecedented precision.

  The most recent addition to the microscope family lives in a pocket: mobile apps can turn a phone's camera into a basic optical microscope simply by switching on its flashlight. These improvised microscopes are harder to use than dedicated instruments, hampered by visual noise, typically capped around 40 times magnification, and limited by the resolution of the phone's own camera lens, but they put a four-century-old idea into the hands of anyone carrying a smartphone.