Tardigrades and polarized light (I)
From time to time we come across those typical 1970s microscopy boxes lined
with some kind of artifical, typically red velvet. Most of them appear to have been used
not too much, possibly only until the enclosed liquids were dissipated
and the butterfly wing lost. Let's have a look at one of them:
Microscope box, 1970s,
with a rather modest microscope, but with ample accessories: Colorants ,
embedding media and other chemical helpers, specimens including a dead flie and/or a dead spider
and the typical butterfly wing (well, from a dead butterfly, of course.
Apparently no-one was too scrupulous in this era). Micro projection screen,
porcelain mortar with pestle, beaker glass, foam for microtome cutting,
filter paper, a permanent specimen on a slide, pipette, scissors and more.
When judged with the eyes of the truly scientific scientist
all this bonus material might be considered as rubbish or primitive educational toy at best.
But we do not fully agree to this verdict, as we are still remembering our own "childish" period
when we had a lot of fun with this mixed media rubbish. We have taken out and photographed a few items in
order that you will be able to make up your mind, just as you like.
Some selected items from the microscope box.
When looking closer on to the image above you will notice many objects
which might be not easily accessible just for a few bucks, even in times of the internet, Ebay etc.
For example petri dishes, loupe, sample container, scalpel, coverglass positioner for the fabrication
of permanent specimens on slides (metal, the object with the square window), a cardbox spacer
for the preparation of dry specimens without embedding medium, microtome embedding wax, a simple hand microtome
(the black cylinder in position 10 o'clock), a metal needle, a glass stirring staff,
test tube with cork, test tube holder, and even "terribly dangerous" chemicals like pure 100% alcohol,
which might be difficult to buy at a local pharmacy nowadays.
In the petri dish there is an eye-piece cap bearing a polarizing filter. So, as
the kids of the 1970s had it, we might wonder whether we - the grown-ups - might use this kind of
equipment as well, in order to enhance our tardigrade understanding.
Keeping in mind the tiny tardigrade dimensions we will discuss polarizing filter
addition in the normal compound microscope only, for transmitted light.
In principle, polarized light can be used in the dissecting microscope (under incident light)
as well but we will not include this kind of application here.
There is no need for big financial investments. Basically you will only need
a so-called "polarizer" under the microscope table and a second, identical "analyzer" above
the microscope table.
You do not own one of those 1970s red pseudo velvet-lined toy microscopy boxes with polarizing filters?
No problem. Nowadays you will make most dealers of photographic equipment happy when asking
for second hand polarizing filters. As a rule they will have plenty of them and actually few
people will ever ask for this kind of typical grandpa photo equipment. At this point you might choose among
many filters in different sizes and cleverly select the one fitting into your microscope snugly without
any glass cutting artwork. A further possibility to think about are polarizer foils
made of plastic which are readily available via internet. Of couse those plastic filter foils
can be cut to any format and exactly fitted according to your specific needs.
Just take care to buy classical linear polarizing filters ("PL") instead
of the more recent circular polarizing filters ("CPL"). But most vintage filters
will we PL anyway, no reason to bother about it. Once you have acquired your polarizing filters,
you can to check their polarizing ability and integrate them into your microscope.
In many cases glass filter discs will have to be removed from their mounts in order to better fit into your microscope:
Removal of the filter glass from a photographic polarizing filter.
In most old fashioned photographic polarizing filters the glass filter is simply kept in place by a threaded ring
which can be removed by help of a fine screw driver. In some cases a bit of
knocking to the screw-driver head will help to get things going or, alternatively,
scratch or even break the filter glass ...
The integration of the first polarizing filter ("polarizer") under the microscope table
poses no serious problem as this filter can be simply placed upon the lamp or
put in the filter holder. As far as the second polarizing filter ("analyzer")
is concerned, microscope integration might be more cumbersome. But relatively small
filter discs will possibly fit somewhere into your eye-piece or photo tube without any tinkering.
Demo experiment to check the polarizing
filter equipment. In this example we used a filter foil and a glass polarizing filter.
The polarizing foil was placed on a lightbox, the test specimen (a plastic filter box cover) directly above,
and the glass polarizing filter on top. When turning the glas filter clockwise- or counterclockwise
there will be moments with most vivid colour and deep black illustrating the polarized light effects.
The same demo experiment as above,
shown in top view. Good polarizing foils and glass polarizing filters will reveal
rich colours and a clean black when positioned above each other in the "full extinction" angles.
In case the polarizing filters should reveal a clean, deep black
in extinction overlay position they can be considered as suitable for microscope integration.
As a starting point, before going on to the tardigrades, we should check our polarizing filters
by means of everday objects like the following:
Test object 1: wrinkled food wrapping film (measuring ca.
20 µm in thickness) as seen under polarized light. One filter was placed
in the filter holder below the specimen, the other one in the photographic tube above the specimen.
Test object 2: two scratches on adhesive tape,
as seen under the microscope in polarized light
Don't worry, polarized light tardigrade stuff will follow.
© Text, images and video clips by
Martin Mach (firstname.lastname@example.org).
Water Bear web base is a licensed and revised version of
the German language monthly magazine
Style and grammar amendments by native speakers are warmly welcomed.