Proper Set Up for Kohler Illumination

Insert and center the recommended light source, covering it with a blue filter to to absorb excess yellow light from the tungsten light source.
Focus the specimen with a lower power objective (fig. 1).
With the specimen in focus, move the condenser with its rack and pinion focus on the sub-stage up toward the top of its range and, closing the field diaphragm, move it slightly up and down until a sharp image of the field iris diaphragm appears (fig. 2).
Center the circle of light from the field iris diaphragm using the condenser centering screws (fig. 3).
Open the field iris diaphragm until the dark edges are just outside the field of view, illuminating the entire field (fig. 4).
Remove one eyepiece and looking into the tube, adjust the iris diaphragm (aperture diaphragm) so that it covers 2/3 to 3/4 of the circular illuminated area that represents the objective aperture.
Replace the eyepiece. Proper Koehler illumination has been attained.