Set your compound microscope to the ideal illumination setting.
- First, gently close the iris diaphragm as far as it will go.
- Turn the condenser knob until the condenser is at its uppermost position.
- Next, turn the nosepiece to the intermediate (10x) objective and lower the lens to within 1 cm of the stage, watching from the side.
- Then turn the rheostat (light dimmer switch) to the lowest setting at which you can see the light.
- Center on your stage a slide (a letter e or thread slide works best). Bring the slide into focus with the coarse and fine adjustment knobs.
- Now, lower the condenser slowly until you see granularity come into focus superimposed on the specimen slide. You are seeing the granularity of the ground glass diffuser covering the light source.
- Now raise the condenser slightly until the granularity of the diffuser just becomes no longer visible. This will be your optimal condenser setting which is about 0.5 mm from the top of the stage.
- Finally, open and close the iris diaphragm slowly, watching the changing quality of the image of the specimen.
- When the iris is full closed, contrast is maximum and the chance for “false lines” is the greatest, but the “washout” of the image takes over, due to imperfections in the condenser.
- Find a position, slightly opened, where the image is clear, bright, detailed and sharp. This is your optimal iris setting at 10x (usually the best setting is right in the middle of the iris diaphragm’s range.)
REMEMBER: You will have to change the iris diaphragm setting with different magnifications and with the various specimens you view. You have now achieved “critical illumination.”