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.
You see this little baby?
This is a Cooke Troughton and Simms M2000 with not one but two different dark ground condensers!
One dark ground condenser, the long skinny one, is an M1396 which has a collar that allows you to adjust it for slides of different thicknesses.
The other condenser is the more simple M1399 which is adjusted for slides of 1.25mm thickness.
The Cooke Troughton and Simms dark ground condensers
M1396 above, M1399 below
Dark ground condensers can be used with objectives that have numerical apertures up to 0.95. To create the dark background of dark field illumination the centre of the cone of light that enters the objective is blocked out, either by a patch stop or by a special dark ground condenser. Only light from around the edge of the cone of light passes through the specimen. This means that if the numerical aperture of the objective is higher than that of the condenser, the light will miss the objective.
CTS suggest fluorite oil immersion objectives as the best choice in their catalogue but almost any 2mm oil immersion objective can be used in combination with a funnel stop which cuts down the numerical aperture.
Patch stops can produce excellent dark ground if they are optimised, but only with lower power objectives. it gets tricky using a patch stop with objectives over about 20x magnification and near impossible over 40x. You really need a funnel stop (or an objective with an iris) and a nice dark ground condenser for the higher magnifications.
I’m off to have a fight with my Meopta now. Wish me luck. I desire pretty pictures.