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The Story Behind: Light Microscopy
Illuminating the "small world"
The pursuit of knowledge in optical sciences has progressed in two opposite directions since the field was founded. Some researchers look into the stars - the vastness of the cosmos - for insight and explanation. Others look into the "small world," the microcosm, for clues on the inner workings of organisms and to better understand known phenomena in fields such as medicine and chemistry.
Both directions, micro and macro optics, have their fixed and equally important place in the optical sciences: "Where the telescope ends, the microscope begins, and who can say which has the wider vision?" asked the 19th century French dramatist and poet Victor Hugo.
The light microscope - from the Greek "micron" (small) and "skopein" (to look at) - has been a key asset in exploring the microcosm for almost 400 years. Over the centuries, the technology has grown by leaps and bounds, although up until the arrival of the Stimulated Emission Depletion (STED) Micro-scope in 2001, the wavelength of light was a barrier to maximum magnification in light microscopes for more than 130 years.
Humble beginnings
The origins of both the microscope and its far-seeing counterpart, the telescope, can be traced back to Italy. Here, the art of grinding optical lenses was perfected in the 14th century with the aim of improving eyesight. In the early 1600s, a Dutch eyeglass maker by the name of Hans Lippershey together with fellow lens crafter Hans Janssen and his son, Zacharias, created the first working compound micro-scope by arranging two lenses in a tube.
It wasn't long before scientifically-minded Europeans were using microscopes systematically for in-sight into structures too small for the naked eye to detect. In 1665, English scientist Robert Hooke observed a thin slice of cork under a compound microscope, which revealed small, room-like struc-tures he called "cells" - although these structures were far larger than actual cell organisms. Driven by the discovery, Hooke continued to study various objects through his microscope, including the fruit-ing bodies of moulds, and he published the results in his Micrographia.
In the Netherlands, amateur scientist Anton van Leeuwenhoek began pointing his simple, one-lens microscope at blood, rainwater and scrapings from teeth with stunning results. He observed living cells - which he called "animalcules" - for the first time in history, making him the world's first microbiolo-gist. Meticulously recording his findings, the Dutchman also became the first to describe and catalogue cells and bacteria.
Finally: Empirical truth
The microscope filled an important gap in the empirical sciences. Since Roman times, scholars had suspected the existence of small organisms which were the cause of infection and decay. But, as we now know, these organisms are invisible to the naked eye, and therefore without any empirical data to support their reality microorganisms remained in the realm of speculation. This was changed by the arrival of the microscope.
Microscopy continued to advance and, although not widely available, compound microscopes became the norm. However, the different refraction of light in two lenses could create a disturbing chromatic effect, to which Joseph Jackson Lister in 1830 finally found a solution. He introduced a combination of various weaker lenses to achieve the same magnification without risking a blurred image. This made microscopes widely available to - and practical for - the scientific community.
The resulting impact on 19th century science can hardly be overstated. The study of microorganisms advanced into a coherent theory with German botanist Matthias Schleiden observing plant parts in 1838 and German zoologist Theodor Schwann investigating animal parts in 1839, both finding the same thing: These organisms consisted of cells. The actual basic elements of organic life had been made visible under a microscope.
Subsequently, Robert Koch opened up the study of bacteria and viruses in the 1870s as smaller and smaller elements of life became visible through increasingly stronger microscopes. As the Belgian scientist Albert Claude noted: "Small bodies, about half a micron in diameter, and later referred to under the name of ‘mitochondria' were detected under the light microscope as early as 1894."
An insurmountable barrier?
Despite the initial euphoria, the 19th century was also the time when scientists realized that light mi-croscopy has its limits. In 1873, optics pioneer Ernst Abbé proved mathematically that the wavelength of light presented the benchmark for how far light microscopes were able to magnify. And for the next 130 years, this so-called Abbé Limit seemed an insurmountable barrier to light microscopy.
To create more powerful microscopes, scientists began departing from light-based optics, breaking the Abbé Limit with a number of different methods. These Nobel-prize winning systems include the phase-contrast microscope developed by Frits Zernike in 1932, the electron microscope created by Ernst Ruska in 1938 and the Scanning Tunnelling Microscope (STM) invented by Gerd Binnig and Heinrich Rohrer in 1981.
While the STM proved an asset to science by allowing three-dimensional imaging down to the atomic level, it shared with the other systems a crucial shortcoming compared to light-based microscopy: Though more powerful, it could not visualize live tissue such as cells and membranes - the kind of insights that had made the light microscope so valuable to biology and medicine in the first place.
A historic breakthrough
While most of the scientific community accepted the Abbé Limit, Professor Stefan Hell from Goettin-gen set out to take light optics microscopy to the nano level. By using lasers to achieve fluorescence in a marked focus spot, Hell's Stimulated Emission Depletion Microscope (STED) not only broke the Abbé Limit, but achieved resolutions around 20 nanometres - in other words, down to the molecular level.
Subsequently, living cells can now be imaged at a higher resolution than ever, continuing the long legacy of light-based optics in the life sciences, especially in medicine. Groundbreaking applications include the detailed study of protein complexes that play a role in viral infections and cancer. The STED has proven a valuable addition to clinical practice around the globe since Leica Microsystems started shipping the technology in early 2007.
"We expect all well-known universities and research institutes to buy one of these systems in the next few years," said Dr. Martin Haase, Manager of Leica Microsystems CMS.
With those systems, modern scientists will continue the work started by pioneers such as Robert Hooke and Anton van Leeuwenhoek. And despite four centuries of efforts in microbiology, scientists to this day have merely glimpsed into a realm that largely remains unseen: According to a recent esti-mate by leading microbiologists, humans have so far only studied about 1 percent of all of the microbe species on Earth.
Read more about the inventor: Stefan Hell (Germany)