I wasn’t prepared to spend a lot of money – not least, because I had no idea if it was even possible with simple components. Also, I spend too much on meteorites as it is – if I become addicted to thin sections, it’s not going to get any cheaper!
| I found a UK supplier of inexpensive polarising film – Middlesex University.
They do a lot of science kits and components, and are well worth a look
for all home hackers. They sell 48mm (about 2”) square film for just over £1. It’s cheap, and it’s not remotely optical grade, but for a proof of concept it would be fine. I ordered a few sheets. As I’d long since sold my Dhofar TS, I also needed to get my hands on a specimen to study. I spotted a nice piece of teschenite on eBay – good Scottish rock! – and it was delivered within 24 hours. |  |
I took out the two most badly scratched pieces of polarising film – they’re quite soft and easily damaged – and put the others away for safe keeping.
The first thing I tried was my QX5 USB digital microscope. It’s cheap and cheerful, but I’ve taken some nice pictures of chondritic material with it, and thought it might be suitable for the TS. Unfortunately, with even a single piece of film in place, the output was essentially black. I dismantled the camera head to remove the filter before the CCD, but this didn’t help – it was probably just an IR blocking filter. It’s possible there’s some polarising going on in the optical train; alternatively, it may just be that the illumination just isn’t sufficient to overcome the inherent loss of light through the polarising film.
Although disappointed I couldn’t use the ‘all in one’ microscope with camera, I thought perhaps I could do something with the little Bresser. I placed a slide between two pieces of film at right angles to each other, and mounted it on the stage. Even before I went to the eyepiece, I could see little sparkles of colour on the surface of the slide! Peering my eye down to the scope, I focused up, and was delighted to see something that looked a lot like the pictures I’d been admiring on the web!
I got quite lost panning around the section; long, thin white crystals were set at various angles, like miniature steel girders. Ruby reds and turquoises, yellows, smooth beads and ragged crystals – lovely! The problem with this was as I was moving the slide and polarisers around on the stage, the film was getting quite badly scratched. Worse than that, it was starting to delaminate (I mentioned it wasn’t optical grade, right?) Now that I had the principle, I decided to make a ‘fair’ attempt.
| The Bresser has a little filter wheel built in to the underside of the stage, containing red, blue and yellow filters, as well as 3 aperture stop holes. One of these was very close to full aperture. |  |
| I carefully cut a piece of undamaged polarising film to size, and glued it over the hole. The piece was aligned so that it would be parallel to the slide when the filter was in position. |  |
| Next, I next cut another piece of film, slightly narrower and
smaller than a microscope slide, and glued this to a clean slide. I cut
it so that in position it would be at right-angles to the film under
the specimen – ie, to maximise the relative angle of polarisation.
|  |
| Professional setups have a rotating plane to allow the polarisation to
be continuously varied, but 90 degrees was fine for my purposes. Finally, I placed the filter slide on top of the thin section – film face down against the specimen – mounted the two slides on the stage, and focused in. |  |
I’m pleased to say it worked a treat! The added advantage with this setup is that it’s easy to switch between normal and XPL illumination, by just moving the filter wheel between ‘clear’ and ‘filter’ positions.
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