Think of what limitations are put on us by the
restrictions of our own eyesight: things too far away, or
too small, are beyond our ability to observe without some sort of
devices to help. Just as the telescope was a critical device
for astronomers, the microscope was critical to the
progress of biology.
Although magnifying spectacles (glasses, sort of) have been in
widespread use since the 1300's, the use of lenses to see very tiny
objects was a slowly-developing technology. First of all,
the tinier an object is, the less light reflects off it, and
seeing anything really tiny requires a decent illumination system.
Secondly, magnifying lenses used in early microscopes were not
particularly even or clear, and tended to split light like a
prism, which affected their resolution limits.
Resolution can be thought of as the clarity of focus;
technically, it is the limit at which two tiny objects which are
close together stop being visibly separate. The resolution
of early microscopes was very limited by the glass used in the
Resolution in microscopes.
A guide to resolution in digital cameras.
Some important work was done with early
microscopes, although they were primarily more of a toy than a
scientific instrument. Like telescopes that were being
developed through that same period, early microscopes used lenses
in sequence (making them multi-lens or compound microscopes)
to magnify an image. In 1660, Italian Marcello
Malpighi was able to use a microscope to see blood capillaries in
the tail of a live fish, providing powerful supporting evidence that
blood circulates in the body (prior to this, since no one knew of
how blood runs from arteries through capillaries to veins, it was
thought that the blood flow was one-way from production in the
intestines to consumption in the body tissues). In 1665,
Englishman Robert Hooke found that cork was full of tiny chambers,
which he called cells (there were no actual living structures that we now
call cells in the dead cork, but our modern term comes from his label).
In the 1670's, Dutchman Antony van
Leeuwenhoek, using a special,
especially pure single lens (a simple microscope) placed
in a holder and held up very close to the eye, was the first
person to write extensively about a world of tiny independent
creatures, which he called animalicules, which existed all around
but were too small to see by eye. Imagine what an odd idea
that must have been to the people of the day!
A page about
A page about Hooke.
A page about Leewenhoek.
A description of Leewenhoek's "microscope."
It wasn't until the 1800's, through a interesting
period including instances of thievery, plagiarism and questionable patent
ethics, that many of the distortions of the lenses were corrected.
With the technological limitations for microscopes mostly solved,
by the end of the century microscopes had begun to hit resolution
limits set by physics: to oversimplify, light beams
themselves have a physical size, which when a gap is at or below about 0.2
micrometers can no longer fit through that gap - you can't see the gap,
just a smudgy blob where the light has been blocked. Objects below that size just cannot be resolved
as long as light is used in the imaging system.
microscopes (because they use light as an imaging
system) would remain
useful, but most of the objects
discussed in the next section on cell structures were invisible to
them. Although there have been some
recent techniques in improving light microscopes' abilities, they
are still limited.
More about light microscopes.
Many different ways to use light microscopes.
light microscope tech.
Through the middle of the 1900's, a new system
was developed that could use a beam of electrons, which have an
adjustable beam size but at their smallest are below atomic size.
The beam is focused with magnets and the final image converted to
light in a way similar to how television screens used to work.
Recent versions of electron microscopes have been
used to produce images of
microscopes are more expensive and complicated than light
microscopes, and the beam needs to travel through a vacuum to
avoid scattering off air atoms. It is extremely difficult to use
them on living cells. They are very useful in
research but not going to appear in teaching classroom labs any time soon.
of an electron microscope facility.
Image of DNA.
Most microscopes can be divided into another two groups,
depending upon how the imaging beam interacts with the specimen. In scanning
microscopes, the beam reflects off the surface of the specimen -
this includes the electron microscopes that produce the impressive (and
frequently creepy) "3-D"
images, and from which the name comes
(the electron beam scans across the surface) as well as the
microscopes used in many labs.
send the beam through the specimen. For a specimen to be useful in a
transmission scope, the beam needs to be able to pass
through it and reveal its internal details. Specimens may need to be
meaning colored or darkened with
dyes, to produce clear details. Colored dyes are used in
light microscopy (the use of microscopes is called
electron microscope stains have heavy
metals in them (the
images are black-and-white,
sometimes photoshop-colorized for publication, so the stain
just needs to produce variations in absorption). Obviously,
large / thick specimens or structures are not going to let the beams
through them; to see large specimens, the original object
must be sectioned, converted into very thin slices, then stained for detail. Objects are embedded in a
material that will hold them together (commonly wax for light
microscopy, plastic for electron microscopy) while being thinly
sliced. The ability to translate the two-dimensional
information of sections into three-dimensional concepts is
important in microscopy.
Many scanning electron images.
Image through a dissecting microscope.
How a transmission EM works.
Tissue staining for light microscopes.
List of common stains, with their uses.
Staining for EM.
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