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Mounting crystals, pollen, and other granular objects for viewing with the microscope
It often
happens that an
object is a little too small to view with a stereo inspection
microscope (a
"macroscope"), yet it's a little too large to have a coverslip
dropped over it. Salt crystals and tiny grains of sand are good
examples. 
Above: Cupric chloride dihydrate, viewed at 40x with a
biological microscope. The crystals here are just loose
on the microscope slide. Carefully spraying them with
a couple of light coats of Krylon "Kamar" varnish was
found to work well, however.
There are
also objects
floating in the air which, ideally, are collected on a slide as they
settle and
viewed without a coverslip- a good example is pollen, which is
especially
abundant in the spring and early summertime in most areas.A simple
way to prevent
these uncovered objects from rolling off or being blown away with the
slightest
draft is to coat a portion of the slide with an adhesive substance.
Canada
balsam is often used for this, but you can prepare a very useful
adhesive by
diluting some varnish in acetone or xylene. Mounting medium, a
synthetic resin
similar to varnish, is of course also suitable if it's thinned with
solvent. If
you're not doing research that's going to be published, you needn't
worry about
meticulous cleaning of the slide-- so long as there are no visible
fibers or
dirt particles on it. If you're still concerned about dust, though,
keep a HEPA
air-filtration unit in the room where you prepare the varnish or
mounting
medium.For pollen
collection,
simply carry the clean slide (covered with dust-free cloth or plastic
wrap if
possible) outside on a clear, dry day in the spring or early summer.
Bring with
you a bit of the diluted varnish you've prepared- don't bring the whole
bottle,
just a small vial if possible. Uncover the cleaned slide and apply a
thin
coating of the diluted varnish. Set the prepared slide in an area where
it's
not exposed to direct sunlight (preferably in the shade of a tree that
bears
blossoms). Allow it to sit for an hour or so; before the varnish dries
it
should have time to catch at least a few grains of pollen and other
airborne
materials. You can then take the slide indoors and examine it under the
microscope. If you're using an Outfitter
portable
microscope, bring the 'scope outside with you and enjoy the sunlight!For
viewing crystals,
prepare a clean slide with the thinned varnish in the same manner, but
sprinkle
a few tiny crystals of sodium chloride or other metal salt on it. If
you want a
more permanent slide where the salt won't be readily attacked by
atmospheric
moisture, wait until the crystals are dried to the varnish and then
apply a
thin coat of the diluted varnish on top of them. This will alter the
appearance
of the crystals, of course. Make sure the compound in question isn't
soluble in the varnish you're using! "Kamar" spray varnish, made
by Krylon, works well for some crystals.Experiment
with different varnish types (shellac,
urethane, etc.) until you find one whose index of refraction is
not so
close to that of the crystals that it makes them "disappear"! Sodium
chloride (common salt) crystals have an index of refraction of 1.544.
Sodium
ammonium phosphate (salt of phosphorus) has three indices of
refraction: 1.439,
1.441, and 1.469 depending on which crystal axis you're looking down.
(The
indices of refraction of these and many other crystalline compounds can
be
found in the Handbook of Chemistry and Physics from CRC Press).
If you
really want to give some customer service reps a workout, call the
manufacturer
of your favorite varnish and ask them for its index of refraction (when
dry)...The
advantage to
"salting" a varnished slide over actually growing the crystals on the
slide is that the "salted" crystals have readily-distinguishable
faces and better three-dimensional relief overall. (This can be a
disadvantage in some ways, too; the crystals' thickness makes it
harder to keep everything in the focal plane). Try
mounting and
viewing a
few grains of alum, cupric sulfate, magnesium sulfate (epsom salt),
cupric
chloride, salt of phosphorus, or other metal salts. If you're an avid
mineral
collector, see if you can identify the crystal system of each compound.With
colorless compounds
such as magnesium sulfate, altering the angle of incident light can
help make
the crystals more visible. This is not really possible with a
transmitted-light
(biological) microscope, but you can adjust the condenser to bring out
some
contrast. If the crystals are large enough or if your scope has, say,
30x or
higher capability, you can by all means use a stereo inspection microscope to view the
mounted
crystals.This is a
quick
demonstration that's ideal for showing with the Mini-VID
eyepiece camera - set one in the eyetube of an Observer III or Observer IV microscope,
connect the
camera to a video screen, and a whole classroom can watch.Copyright: Articles, photographs, and other contents of this site are property of CR Scientific except where otherwise noted. They may not be copied or distributed, in whole or in part, without prior written permission (click here for contact info).
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