What part of the microscope allows you to adjust the intensity of the light?

In order to see a sample on a slide using a compound light microscope, light must pass from the source, through the sample, and through the objective and ocular lenses to your eyes.  Samples must be thin to allow light to pass through.  At times, you need to adjust the intensity of light that reaches your eye.  This can be done several ways.  First, many compound light microscopes will include a knob that allows you to directly adjust the intensity of the light leaving the light source below your sample.  Another way to adjust the intensity of the light directly is to open or close the iris diaphragm.  This structure is located between the light source and your sample in a typical compound microscope and can be closed (tab moved to the right) to decrease the amount of light reaching your sample or opened (tab moved to the left) to increase the amount of light reaching your sample.  At times it may be necessary to close the iris diaphragm to increase contrast and to view specimens that are not stained.  Finally, adjusting the position of the condenser (which narrows the beam of light coming from the light source and through your specimen) can make the light brighter.  Typically, moving the condenser closer to your specimen can increase the brightness of the light.