Larry Legg's Learner Projects
Project 2- Let there be Light.

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The right way!
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Dark Ground Illumination
One of the most spectacular methods of improving visibility and resolution of many types of specimens is by using dark ground, or dark field, illumination. This technique produces brightly lit objects on a dark (black) background - greatly increasing contrast.
Ideally suited for microscopes using an Abbe Illuminator, dark ground is best used with the lower power objectives, and achieves stunning results with pond life subjects.

The Technique
What we need to do is produce a hollow cone of light instead of a solid one. By excluding all the direct light in the cone, we end up seeing only reflected light from the  surfaces of the specimen. The technique therefore produces the best result when yer specimen has the greatest mis-match between the refractive indices of its two surfaces.

You can purchase patch stops but really there is no need - they're easy to make!

We need to make a small disc (disk) wide enough to cover the objective lens and about 10% more. It must be placed below the stage close to the underside of the substage condenser - which, itself - is normally raised to adjust it for shorter focus of the light rays in this method.

We call the small disc (disk) a patch stop.

Making a Patch Stop
This must be one of the cheapest accessories yer'll (you will) ever have for yer microscope. Almost any black material will do to make the stop - providing light don't show through it. I made mine from a black piece of thick paper.

Yer can see me (my) cutting wasn't very good 'cos (because) I didn't mark out the circle first one the paper before cutting. Its best to try and get a nice round disk not one like mine here. I'm gonna (going to) use this one though just to show you the best you can get with the worst disk you make. Yours will be better - it only takes a few minutes to make one.

How big?
What size should the disk be? Well - experimenting will tell you what size for yer low power objectives but for my microscope and me 10x objective, I used a disk that was 
about 13 mm in diameter - give or take me bad cutting!


Where do yer put it?
I reckon me picture below will show you where to stick yer disk. My microscope has a filter carrier and if I took a little time, I could have stuck me disk onto a disk of clear transparent plastic - such that the entire thing sat perfectly centred in the filter holder.

I didn't do this though because I wanted to test off several size patch stops quickly until I found the right size. So I just stuck it on with sellotape, judging and adjusting how centred it was, by swinging the filter holder back into place and looking at a specimen under the microscope. I re-cut the disk and re-centred it several times until I got one to achieve the dark ground effect.

You could do this too until you get the perfect patch stop to then stick onto transparent plastic for a permanent solution.

Once yer manage to get yer disk centred up, don't forget to swing yer filter holder back into position. If yer don't have a filter holder - yer should try sticking the disk somewhere else in the light cone before the light passes through the condenser. Yer will have to vary the size of yer disk until yer get it to work. Remember it must be big enough to block the light at the objective plus 10% more (wider)!
Now's yer moment to find out how useful dark ground illumination is. Take a few of yer slides or a drop of pond water and take a look with and without yer patch stop. Here's a couple I looked at with mine!

Hippuric Acid (normally a good slide for trying out polarized light but good for testing dark field (dark ground) illumination too!

Normal Illumination Dark Ground Illumination

Plant Hairs (normally another specimen which benefits from viewing in Polarized light but a good subject for dark ground illumination too - a seen below).

Normal Illumination Dark Ground Illumination
Mite Didn't get this one quite right so dark ground image on right still shows a background that's too bright. Mind you - the technique shows up all the dust and debris in me slide :))
Normal Illumination Dark Ground Illumination

Mixing Techniques
Don't forget that yer can also use dark ground illumination and overhead lighting together- that is: try using over lighting to pick up additional detail not revealed by dark ground. I've taken some good piccies using a high powered halogen torch as my over light to brighten up detail missing in a dark ground lit dustmite!

Right... shall we move on to polarized lighting techniques?

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