MW4-H4 biological University microscope has an iris diaphragm that controls the light that passes through the stage. An iris diaphragm has a lever on the side that is simply moved to open or close the iris, allowing more or less light to flood the specimen.
A disc diaphragm typically has five or six holes of different sizes drilled into a circular disc that is attached beneath the stage. When the disc is rotated, varying amounts of light will pass through the stage.
The disc diaphragm is used to vary both the intensity and size of the cone of light that is projected upward into the prepared slide. There is no set rule regarding which setting should be used for a particular objective lens magnification. Rather, the setting is a function of the transparency of the specimen on the stage and the amount of contrast required to view the specimen.