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Monday, 21 November 2011

Making wet mounts

A note for those coming in part-way through:

The material that is going into my writing blog (that's this one) is currently being drawn from edited draft material that I wrote for Australian Backyard Naturalist.  This book (ISBN 9780642277428) will be published by the National Library of Australia in May 2012, and because we were shaping the concept as we went, I wrote rather more than twice as much material as we were able to fit into what will, in fact, still be quite a large book.

I wanted to add microscopy, but it would be somewhat beyond the reach of part of the target audience, and it tended to involve sharp blades, worrisome stains and a fair amount of expense.

I still believe in the idea of introducing young people to microscopy, so I have taken about 11,000 words of draft, and I am slowly feeding it in here.  Remember that you really need to read the entries in reverse to get the sequences and sometimes the logic.

Right now, we are up to wet mounts: putting something on a slide and covering it with a cover slip.

Now read on . . .

* * * * * * * * *

It is possible just to look at a drop of water on a slide, but you can see more through a flat surface, and that means using a cover slip of very thin glass to flatten the water out. You will almost certainly break a few cover slips and cut yourself at least once. When you are starting out, wear safety goggles to protect your eyes, and practise very hard at being gentle with the cover slips.

By now, I hope you will have tried making the odd thin section, cut using a home-made microtome. If you want to skip past that and look at tiny life forms, I will come to some ideas in my next entry. For now, think this sections, placed on a microscope slide, spread flat with a brush, a needle or forceps, but needing a cover slip to be added, without trapping any air bubbles.

Here's how you do it, first as a simulation:

Making wet mounts: a simulation using a knitting needle and a microscopy plate, about 10 cm across. The change of scale meant the operator had to hold the "cover slip" to stop it slipping. Notice how the needle always touches the "slide" and comes out gradually.

And now, the real thing.  There is nothing in the water here, but there could have been a slice of carrot, a piece of onion skin, some blood or skin (if you had been really careless with the razor blade)—the options are endless.

You need to avoid air bubbles, and that means putting the cover slip down so it touches the drop of water on one side, while holding the slip up with a dissecting needle on the other side. Then you slowly pull the needle out, keeping the needle down at an angle of maybe 20°, so that the cover slip comes slowly down on the drop, and the air underneath is pushed sideways.

The surface tension effects stop the cover slip from skipping away as the cover slip is lowered: notice how the water front pushes across. I used too much water in this one, so as to make it show out. If I planned to look at this mount, my next step would be to sop up some of the water, using the corner of a face tissue.

You now have the basics: the next post will be rather longer, and will concentrate on what you can find in "green slime". There's a whole new world waiting for you out there!

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