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Evolution of Microscopy
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The field of Biology owes a lot to Microscopes. It was the invention of this very instrument, which widened the horizons of Life Sciences. From the physical world of plants and animals, the focus was drastically shifted to the world beyond the sight of naked eyes--the Microbiological World (the world of bacteria, fungi, protozoa, algae and though acelular, viruses too!). Here in this article, I would take you through the evolution of microscopy, encompassing the times of single eye lens to the modern day Electron and UV Microscopes!

How it started:
Anton van Leeuwenhoek, one of the most revered scientists in the history of Biology, was the one who did the astonishing invention of world's first "Microscope" between 1670-1700, turning him from an ordinary draper to a pioneer in Biology. Though, his contemporaries as well as the primitives, had tried their hands on making a magnifying lens combination (use of single eye magnifying glass dates back to 13th century; Jansen in 1590, was the first to use a combination of lenses in a tube to get a 10X magnification) Anton's lens tube (holded single lens) was so craftly designed that it gave him a magnification of 270X, outdoing the work of everyone else! Following is the actual sketch and image of Anton van Leeuwenhoek's work:
[Image: Van_Leeuwenhoek%27s_microscopes_by_Henry_Baker.jpg] [Image: 144px-Leeuwenhoek_Microscope.png]
[Note: Images are free to use/share even commercially)
The needle in front of the orifice(holding the finely grinded lens) was used to place the objects. The position of the needle could be finely adjusted to get magnification of desired levels!

It is with this particular work that Leeuwenhoek entered the field of Microbiology. He was the first person to observe motile sperms, bacteria, fungi, microbial flora and fauna of aquatic life and turned his observations into sketches giving him the dignity he still holds today (The Father of Microbiology)!

Invention of Achromatic Lenses: Emergence of Compound Microscopes:
Chester Moore Hall, another non-scientific background turned scientist! He was a lawyer by profession and led to the highly significant invention of achromatic lenses in 1729. Chromatic and spherical aberration of lenses was a big hurdle for coupling two or more lenses together to increase the magnifying power. These effects lead to focusing of lights of different colors in different planes when passed through an ordinary lens. So, whereas a single lens could give a good view of magnification, coupling the lenses would amplify the problem of aberrations, leading to a completely out of focus image.
. Hall's discovery changed the entire prospect of Microscopy and Telescopy, paving the way for compound microscopes and telescopes.
[Image: Chromatic_aberration.png] [Image: Lens6b.png]
First image shows the problem of chromatic aberration in ordinary lens, second image shows the correction through achromatic lens(doublet).
(Images free for reuse)

Birth of Compound Microscopes:
With the widespread realization of characteristics of Achromatic lenses, numerous companies producing Microscopes, started working on making a better version of Microscope, which led to the emergence of Simple Compound Microscopes. (Precise details of who first came up with compound microscope with achromatic lenses aren't available, though Italy is considered the most prolific producer of high end microscopes at those times ie late 18th century)
The compound microscope basically came-up with Achromatic objective and eye piece in the same tube.

Development on Light Source
After the invention of achromatic lenses, the next hurdle was in the use of a light source for illuminating the sample. Earlier version used Critical Illumination, which led to appearance of the light source's image in the final focus (termed Filament Glare/image), apart from the problem of non-uniform illumination of the sample. With the advent of Kohler Illumination (1893), the problem of glare and non-uniformity was well dealt with. Kohler used a set-up in the microscope, involving a Collector Lens, Collector Diaphragm, Condenser Diaphragm and Condenser Lens as a way to make sure that specimen is illuminated uniformily and no glare appears in final image.
This was a major development, and still used today.

Emergence of Fluorescence Microscopy
Fluorescence microscopy added another dimension to the visualization of specimens in a more clear way (as the specimens themselves act as the source of light!). In most simple terms, it involves the labeling of the samples with flourophores, which are excited upon bombarding photons. The fluorescent light is separated from reflected light using filters, and used to create an image of the specimen.
Oskar Heimstädt, a physicist, was the inventor of Fluorescent Microscope, in 1911.

The Limitation of Light Microscopes: Emergence of Electron & UV Microscopes:
A big limitation of Light Microscopes was the size of objects it could magnify. If somebody needed to focus on objects smaller than 350nm, the light microscope could never do it! So, basically, going inside the details of the cells was only an imagination till the advent of next generation microscopes-Electron & UV Microscopes. The very basic reason that light microscopes cannot focus on objects less than 350nm in size is that: "Size range of the object that can be focused by the microscope, depends upon the wavelength range of the source of light being used!" So, visible light cannot be the source of illumination, if target object is smaller than light's own wavelength i.e 350nm. This is where the need for better source of illumination was felt, and that led to the idea of using a beam of electrons or UV light to solve the purpose.
Knoll & Ruska are coined the fathers of electron microscopy (1931). An electron microscope can visualize objects as small as 50 picometer! (almost 100000 times shorter than that by light).

There after, many sources of illumination have been tried ranging from X-ray to laser light to infra-red radiations. All with the purpose of focusing the object of different sizes.

Apart from that, little and progressive developments were made in the way of processing the light reflected by the specimens, leading to variety of methodologies like Phase Contrast, Bright Field, Dark Field, Confocal microscopy etc.

Literature is in abundance, this was just an attempt to make you familiar with the series of events that led to the current era of Microscopy.

Suggested Readings:
http://cbe.ivic.ve/mic250/pdf/microscope.PDF

http://www.celestron.com/c3/images/files...esinfo.pdf

http://micro.magnet.fsu.edu/primer/pdfs/...beyond.pdf

Happy Reading!

Thanks
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