The Microscope
Lighting attitudes after A. Koehler
Condenser
narrow the luminous-field diaphragm
This is where Köhler illumination really begins: we narrow the luminous-field diaphragm and move the condenser carefully up and down via the condenser drive until we see a sharp image of the luminous-field diaphragm, or at least a piece of it in the edge. If this does not work at once, we set different diameters of the luminous-field diaphragm and move the condenser again until a piece of the edge of the diaphragm appears in focus somewhere. In most cases, this search will take some time.

picture sharppiece of the edge of the diaphragm
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Luminous source
Luminous-field diaphragm
Aperture diaphragm
Condenser height
Interpupillary distance
Sharpness
Condenser
Centering screws
Contrast
Change eyepiece