A microscope is an instrument used to see objects that are too small for the naked eye.
The science of investigating small objects using such an instrument is called microscopy.
Microscopic means invisible to the eye unless aided by a microscope.
There are many types of microscopes. The most common is the optical microscope, which uses
light to image the sample. Other major types of microscopes are the electron microscope,
the ultramicroscope, and the various types of scanning probe microscope.
History
The first microscope to be developed was the optical microscope, although the original
inventor is not easy to identify. Evidence points to the first compound microscope appearing
in the Netherlands in the late 1500s, probably an invention of eyeglass makers there: Hans
Lippershey and Zacharias Janssen. There are other claims that the microscope and the telescope
were invented by Roger Bacon in the 1200s, but this is not substantiated. Giovanni Faber
coined the name microscope for Galileo Galilei's compound microscope in 1625 .
Rise of modern light microscopy The first detailed account of the interior
construction of living tissue based on the use of a microscope did not appear until 1644,
in Giambattista Odierna's L'occhio della mosca, or The Fly's Eye.
It was not until the 1660s and 1670s that the microscope was used extensively for research
in Italy, The Netherlands and England. Marcelo Malpighi in Italy began the analysis of biological
structures beginning with the lungs. Robert Hooke's Micrographia had a huge impact, largely
because of its impressive illustrations. The greatest contribution came from Antonie van
Leeuwenhoek who discovered red blood cells and spermatozoa and helped popularise microscopy
as a technique. On 9 October 1676, Van Leeuwenhoek reported the discovery of micro-organisms.
In 1893 August Köhler developed a key technique for sample illumination, Köhler illumination,
which is central to modern light microscopy. This method of sample illumination gives rise
to extremely even lighting and overcomes many limitations of older techniques of sample
illumination. Further developments in sample illumination came from Frits Zernike in 1953
for inventing Phase Contrast, and Georges Nomarski 1955 for his development of differential
interference contrast illumination; both of which allow imaging of unstained, transparent
samples. Electron microscopy
In the early 1900s a significant alternative to light microscopy was developed, using electrons
rather than light to generate the image. Ernst Ruska started development of the first electron
microscope in 1931 which was the transmission electron microscope. The transmission electron
microscope works on the same principle as an optical microscope but uses electrons in
the place of light and electromagnets in the place of glass lenses. Use of electrons instead
of light allows a much higher resolution. Development of the transmission electron microscope
was quickly followed in 1935 by the development of the scanning electron microscope by Max
Knoll. Electron microscopes quickly became popular
following the Second World War. Ernst Ruska, working at Siemens developed the first commercial
transmission electron microscope and major scientific conferences on electron microscopy
started being held in the 1950s. In 1965 the first commercial scanning electron microscope
was developed by Professor Sir Charles Oatley and his postgraduate student Gary Stewart
and marketed by the Cambridge Instrument Company as the "Stereoscan".
Scanning probe microscopy
The 1980s saw the development of the first scanning probe microscopes. The first was
the scanning tunneling microscope in 1981, developed by Gerd Binnig and Heinrich Rohrer.
This was closely followed in 1986 with Gerd Binnig, Quate, and Gerber's invention of the
atomic force microscope. Fluorescence and light microscopy
The most recent developments in light microscope largely centre on the rise of fluorescence
microscopy in biology. During the last decades of the 20th century, particularly in the post-genomic
era, many techniques for fluorescent labeling of cellular structures were developed. The
main groups of techniques are small chemical staining of cellular structures, for example
DAPI to label DNA, use of antibodies conjugated to fluorescent reporters, see immunofluorescence,
and fluorescent proteins, such as green fluorescent protein. These techniques use these different
fluorophores for analysis of cell structure at a molecular level in both live and fixed
samples. The rise of fluorescence microscopy drove
the development of a major modern microscope design, the confocal microscope. The principle
was patented in 1957 by Marvin Minsky, although laser technology limited practical application
of the technique. It was not until 1978 when Thomas and Christoph Cremer developed the
first practical confocal laser scanning microscope and the technique rapidly gained popularity
through the 1980s.
Much current research on optical microscope techniques is focused on development of superresolution
analysis of fluorescently labeled samples. Structured illumination can improve resolution
by around two to four times and techniques like stimulated Emission Depletion microscopy
are approaching the resolution of electron microscopes.
Types
Microscopes can be separated into several different classes. One grouping is based on
what interacts with the sample to generate the image, i.e., light or photons, electrons
or a probe. Alternatively, microscopes can be classed on whether they analyse the sample
via a scanning point or analyse the sample all at once.
Wide field optical microscopes and transmission electron microscopes use the theory of lenses
in order to magnify the image generated by the passage of a wave transmitted through
the sample, or reflected by the sample. The waves used are electromagnetic or electron
beams. Resolution in these microscopes is limited by the wavelength of the radiation
used to image the sample, where shorter wavelengths allow for a higher resolution.
Scanning optical and electron microscopes, like the confocal microscope and scanning
electron microscope, use lenses to focus a spot of light or electrons onto the sample
then analyze the reflected or transmitted waves. The point is then scanned over the
sample to analyze a rectangular region. Magnification of the image is achieved by displaying the
data from scanning a physically small sample area on a relatively large screen. These microscopes
have the same resolution limit as wide field optical, probe, and electron microscopes.
Scanning probe microscopes also analyze a single point in the sample and then scan the
probe over a rectangular sample region to build up an image. As these microscopes do
not use electromagnetic or electron radiation for imaging they are not subject to the same
resolution limit as the optical and electron microscopes described above.
Optical
The most common type of microscope is the optical microscope. This is an optical instrument
containing one or more lenses producing an enlarged image of a sample placed in the focal
plane. Optical microscopes have refractive glass and occasionally of plastic or quartz,
to focus light into the eye or another light detector. Mirror-based optical microscopes
operate in the same manner. Typical magnification of a light microscope, assuming visible range
light, is up to 1250x with a theoretical resolution limit of around 0.250 micrometres or 250 nanometres.
This limits the practical magnification limit to ~1500x. Specialized techniques may exceed
this magnification but the resolution is diffraction limited. The use of shorter wavelengths of
light, such as the ultraviolet, is one way to improve the spatial resolution of the optical
microscope, as are devices such as the near-field scanning optical microscope.Sarfus, a recent
optical technique increases the sensitivity of standard optical microscope to a point
it becomes possible to directly visualize nanometric films and isolated nano-objects.
The technique is based on the use of non-reflecting substrates for cross-polarized reflected light
microscopy.
Ultraviolet light enables the resolution of microscopic features, as well as to image
samples that are transparent to the eye. Near infrared light can be used to visualize circuitry
embedded in bonded silicon devices, since silicon is transparent in this region of wavelengths.
In fluorescence microscopy, many wavelengths of light, ranging from the ultraviolet to
the visible can be used to cause samples to fluoresce to allow viewing by eye or with
the use of specifically sensitive cameras. Phase contrast microscopy is an optical microscopy
illumination technique in which small phase shifts in the light passing through a transparent
specimen are converted into amplitude or contrast changes in the image. The use of phase contrast
does not require staining to view the slide. This microscope technique made it possible
to study the cell cycle in live cells. The traditional optical microscope has more
recently evolved into the digital microscope. In addition to, or instead of, directly viewing
the object through the eyepieces, a type of sensor similar to those used in a digital
camera is used to obtain an image, which is then displayed on a computer monitor. These
sensors may use CMOS or charge-coupled device technology, depending on the application.
Electron
Three major types of electron microscopes exist:
Scanning electron microscope: looks at the surface of bulk objects by scanning the surface
with a fine electron beam. See also environmental scanning electron microscope.
Transmission electron microscope: passes electrons through the sample, analogous to basic optical
microscopy. This requires careful sample preparation, since electrons are scattered so strongly
by most materials.This is a scientific device that allows people to see objects that could
normally not be seen by the naked or unaided eye.
Scanning probe AFM, atomic force microscopy
BEEM, ballistic electron emission microscopy EFM, electrostatic force microscope
ESTM electrochemical scanning tunneling microscope FMM, force modulation microscopy
KPFM, kelvin probe force microscopy MFM, magnetic force microscopy
MRFM, magnetic resonance force microscopy NSOM, near-field scanning optical microscopy
PFM, piezo force microscopy PSTM, photon scanning tunneling microscopy
PTMS, photothermal microspectroscopy/microscopy SAP, scanning atom probe
SCM, scanning capacitance microscopy SECM, scanning electrochemical microscopy
SGM, scanning gate microscopy SICM, scanning ion-conductance microscopy
SPSM spin polarized scanning tunneling microscopy SThM, scanning thermal microscopy
STM, scanning tunneling microscopy SVM, scanning voltage microscopy
SHPM, scanning Hall probe microscopy SSM, Scanning SQUID microscope
RTM, Recurrence tracking microscope Of these techniques AFM and STM are the most
commonly used. Other types
Scanning acoustic microscopes use sound waves to measure variations in acoustic impedance.
Similar to Sonar in principle, they are used for such jobs as detecting defects in the
subsurfaces of materials including those found in integrated circuits. On February 4, 2013,
Australian engineers built a "quantum microscope" which provides unparalleled precision.
See also
Bright field microscopy Confocal microscopy
Dark field microscopy Digital microscope
Electron microscope Fluorescence interference contrast microscopy
Fluorescence microscope Laser capture microdissection
Microscope image processing Microscope slide
Microscopy Multifocal plane microscopy
Phase contrast microscopy Timeline of microscope technology
X-ray microscope References
External links Microscopy laboratory in: A Study Guide to
the Science of Botany at Wikibooks Milestones in Light Microscopy, Nature Publishing
FAQ on Optical Microscopes Nikon MicroscopyU, tutorials from Nikon
Molecular Expressions : Exploring the World of Optics and Microscopy, Florida State University.
Microscopes made from bamboo at Nature.com Audio microscope glossary
Billings Microscope Collection Catalogue, National Museum of Health and Medicine.