Through the microscope

Dark field, or dark ground microscopy give very pleasing images, specimens appear white on a black background. It is easily achieved at low magnification by popping a patch stop (a clear filter with a central black circle) into the filter tray under the substage condenser.They are simple to make, you just have to make some filters with central stops of different sizes to determine which size stop works best for you.


patch stops of different sizes

Explanations of dark ground are a google search away but basically a patch stop blocks the central portion of light leaving a circle of light around it. This allows light that has been scattered by the specimen to be viewed whilst removing zero order light. This produces the bright, beautiful dark ground images with which we are familiar.


zero order light

At low magnification it is easy, pop in a patch stop and away you go. At high magnification it is much trickier. At higher magnification you need a proper dark ground condenser that can be used with oil and you must also reduce the numerical aperture of the objective using an iris objective or a funnel stop.

The other problem is that is quite hard to centre  a condenser when there is very little light coming through. I find it best to set up in bright field and then switch condenser tops so I know that I am centred.

Below are some pictures of Pleurosigma angulatum and some other test diatoms taken on a Vickers M4000 with a 100X oil objective and a dark ground condenser.


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