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Focus on the specimen using a low power lens like a 10x air. Fully close the field diaphragm and adjust the condenser focus so the image of the iris is in sharp focus. Centre the image of the iris using the two centring screws. Open the field diaphragm until it is just outside the field of view.
Remove Remove an eyepiece and look into the empty opening. Adjust the iris aperture until it is about 70% open and then replace the eyepiece. The microscope is now adjusted for Köhler illumination.illuminatio