Taking photos through a
microscope (photomicrography) is tough enough in a bright field
but when you add polarizing filters you limit the amount of
light available to photograph and it gets even tougher. To
optimize the results, the microscope must be in correct
alignment.
Aren't most microscopes aligned
properly? John Delly the author of Photography Through the
Microscope states, “The necessity for proper illumination cannot
be over emphasized. All too often, a $25,000 microscope is
reduced to the level of a hand magnifier by improper
illumination and alignment.” He further notes that in his
examination of university microscopes he finds the majority are
not in proper alignment.
The simplest and most straight
forward method of microscope alignment is known as the Köhler
illumination technique. The Köhler illumination technique was
first introduced in 1893 by August Köhler of the Carl Zeiss
corporation as a method of providing the optimum specimen
illumination. This technique is recommended by all manufacturers
of modern laboratory microscopes because it can produce specimen
illumination that is uniformly bright and free from glare, thus
allowing the user to realize the microscope's full potential. My
method? I simply place a piece of white paper over the field
diaphragm lens (where the light comes out at the base of the
microscope) stop down the field diaphragm all the way and adjust
the light bulb/lamp housing or what ever is necessary until I
can see the bulb filament clearly CENTERED and CRISP on the
paper. Then I open the field diaphragm just enough to fill the
image in the eyepiece. After that, all other microscope
adjustments such as the condenser diaphragm and focus really
seem to fall into place.
A complete
discussion of the Köhler illumination technique would be too
much to handle here but if you are interested, there is a
fantastic web site on the subject.
http://microscopy.fsu.edu/primer/anatomy/illuminationhome.html
We all want more expensive
equipment and lot's of it, that's part of the hobby, but high
end equipment is not necessary to getting professional looking
results. The aus Jena Fluoval microscope that I am using is a
great microscope but it was found with out a power supply or
working bulb. These both proved to be made out of 100% pure
unobtainium. Even if I wanted to spend the money, I couldn't
find them. All the photos in this cross polarized light gallery
were taken on that microscope which is illuminated by a $15
halogen landscape lamp that I bought at the hardware store and
mounted into the existing lamp house. I used basic Köhler
techniques and a piece of paper to adjust it. If you really want
to spend some money, buy at least one good objective.
Polarizing microscopes have many advanced features that ordinary
scopes do not such as excess light capacity, revolving
polarizing analyzers, compensators, retardation plates, rotating
specimen holders and incident (reflected) lighting. Although all
of these features are useful, most are not absolutely necessary
to great photomicrographs.
Since a quality thin section is critical to good
photomicrographs, select the thin section carefully. I have to
say, I have never been disappointed with any of the thin
sections I have purchased from ‘reputable meteorite dealers’. I
have however, bought some ‘bargains’ that are completely
unusable. They might be good for a micro mount collection but
they should go nowhere near a microscope. It drives me crazy
that I, (just a hobbyist) would put a better polish on a large
hand sample than I have seen on some samples sold as ‘thin
sections’.
Thin section quality can vary greatly and you need to decide on
covered or not. Covered is when a glass cover slip is epoxied
over the sample. A good covered thin section will last a
lifetime but they aren't very useful to view in cross polarized
transmitted (pass through) light combined with incident
(reflected) light. Uncovered thin sections are, but they are
more delicate and vulnerable to humidity. Most problems with
thin sections can't be ascertained until it is on a microscope
so just be aware that if you are having problems, it may not be
you or your microscope. Try a new supplier until you get the
quality you need.
Salvaging poor thin sections is seldom possible as every thin
has two sides and one is glued face down.
Making you're own thin sections is easy in principle but very
difficult to do well. It is only a 30 micron piece of meteorite
glued onto a glass slide. Since an understanding of what goes
into making a thin is both interesting and helpful in evaluation
of thins you may be considering to buy, I'll go over the basics,
but this is an overview, not a guide! I have made a few myself
but I am neither an expert or even that good.
First the sample is polished. Taken down in steps, allowing
enough polish time to remove the sub-surface damage caused by
the previous grit. After the first few grinding steps epoxy is
applied to the face of the sample and allowed to set before the
finish polish steps are started. This helps reduce crystal
fragment "plucking". The polished sample is epoxied onto a glass
slide. (A vacuum chamber mounting press is handy but a clamp
will often work fine.) After setting, the excess is cut off
using a diamond saw. Trimming the sample thin is essential to
avoid excess grinding time, however, to avoid damage to the
sample structure, it should be trimmed to no less than 500
microns.
The greatest challenge is getting the sample to a uniform
correct thickness. the tendency is to round down the edges.
Thick in the middle and worn to nothing on the edges. A good
target thickness for a finished thin section is 30 microns, but
that is 30 microns of sample that has not been destroyed by
sub-surface damage due to shattering of the mineral grains with
rough grit. To help accomplish this, the sample is again coated
in epoxy and allowed to set. It is then ground on diamond laps
to a thickness of approximately 200 microns, recoated with epoxy
and finally taken down to the final thickness using various
lapping compounds and finished on a cloth lap with a diamond
slurry.
There is equipment that is designed specifically for each of
these tasks. I have an older Struers setup. A Discoplan to trim
and rough grind the slide after the sample is attached. A
polisher to grind/polish the sample on and a PdM-Force head to
hold the samples at the appropriate pressure against the
grinding medium while keeping them flat.
It's no wonder why bargain thin sections are often disappointing
and good thin sections are never cheep. Even if the sample
material was free, they are costly to produce. Speaking of
sample material, the minimum amount used in the production of a
standard thin section is approximately 1 gr. So keep that in
mind when you are eyeing those exotic thins.
Weather or not you are interested in taking photomicrographs,
viewing meteorites in Xpol is something that every meteorite
collector should experience.