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individual cells from the mucous membrane in your mouth come loose and
stick to the cotton swab. You should rub with plenty of pressure but not too
hard; you don't want to hurt yourself! Rub the cotton swab in the water drop
on the prepared slide. Now just lay a cover slip on top of it and put it under
the microscope!
Microscopy Technique — How to Put an
Object under the Microscope
Any object that you would like to observe under the microscope needs to be
in a liquid (usually water). The optics of your microscope is set up for objects
that are surrounded with a liquid medium. This has to do with the fact that
the light refraction in the air is completely different than in liquid. So, in gen-
eral, dry objects cannot be observed under the microscope, except in magnify-
ing glass mode.
Consequently, slide preparation usually begins with the placement of a drop
of water in the middle of a slide. Either you can use the pipette for this
purpose, or you can simply dip your finger into a container of water (e.g. a
beaker) and dab the drop of water hanging on your finger when you pull it
out in the middle of the slide. Next, the object is placed into the water drop.
Now a cover slip still needs to be placed on top of it. When doing so, make
sure that as few air bubbles as possible remain under the cover slip. They are a
nuisance when working with a microscope because they appear black at their
edges and can sometimes obscure the object you are trying to examine. It's
best to proceed as follows.
Take a cover slip by its edges using two fingers and place one of its edges
right next to the drop of liquid on the slide. Then slowly slide the cover slip
up to the drop. As soon as the cover slip touches the drop, the lower edge
gets wet with the liquid immediately. Now you can slowly lower the cover slip
down onto the slide. Tweezers can be helpful here.
If some air bubbles should get under the cover slip, simply lift it up with the
tweezers on one side of the slide and try it again. With a little practice, it will
become easy for you.
What do you do if there is too little or too much water under the cover slip?
The space between the cover slip and the slide should be completely filled
with liquid. It's possible that the drop that you dabbed onto the slide initially
was too small, so that there is no water under the cover slip at the edges.
That's undesirable, since parts of your object could dry out and then not pro-
duce a useful image under the microscope. You can resolve this problem quite
easily by dabbing a drop of water at the edge of the cover slip under which
the liquid is located.
The liquid is sucked up under the cover slip as if by magic. However, there
should not be too much liquid under the cover slip, either. If there is too
much liquid under the cover slip, you will notice the liquid swelling out from
under the cover slip, or there may even be droplets next to the cover slip. You
can simply absorb the excess water using a strip of blotting paper or a paper
towel. This is important above all because in this way you prevent liquid from
getting onto your microscope during use, which can soil or even damage it.
The mucous membrane cells from the mouth all look about the same. Shapeless
formations with pockets and a sphere on the inside. This sphere, which generally
cannot be seen with the microscope without staining the object, is the famous cell
nucleus. Perhaps you've already heard that the police take so-called "DNA swabs"
in their search for a murderer. If we were to compare the cell to a computer, DNA,
which is the genetic material, is the cell's hard drive. It is packaged up nicely in
the nucleus of every cell. There, all of the information is stored that our body —
and hence every single one of the 70 billion cells of our body — needs in order
to function properly. While some of the information is the same in every human
being — for example, that five toes grow on our feet while we are developing in
our mother's stomach — DNA changes from person to person in certain places. Fo-
rensic police take advantage of this fact in order to compare traces of DNA found
Here's how a water drop can be applied to the
slide even without a pipette.
Steps in preparing a slide
Cells from the mucous membrane of a
human mouth
15
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