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
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.