How to Adjust the Light on a Compound Microscope
The light source of a compound microscope can be a light bulb or an indirect source of light that comes from a reflection from a mirror. The quality of the object you are looking at under a microscope depends on both the amount of magnification you have and the amount of contrast you create in the image, which is accomplished by adjusting the light.
Focus the condenser. The condenser is what increases the brightness of the light and is located on the base of the compound microscope.
Close the diaphragm. The diaphragm is controlled with a small lever located beneath the stage of the compound microscope and is what determines the diameter of the beam of light entering the lenses.
Open the diaphragm slightly, which will begin to let light in.
Continue to adjust the diaphragm until you can see the object you are examining under the microscope. The diaphragm also serves to alter the contrast. The more it is closed, the greater the contrast (which is why it is good to begin with it closed). Some objects are better viewed with little light while others require more light.