The part called the diaphragm. It could be a plastic disk with various size holes from small to large. It could be an iris type with a lever to gradually open or close it.
Diaphragm or Iris: Many microscopes have a rotating disk under the stage. This diaphragm has different sized holes and is used to vary the intensity and size of the cone of light that is projected upward into the slide.
A microscope is used to view small objects or features by magnifying them. A compound microscope, commonly found in laboratories, consists of an eyepiece (one or two lenses, for peering down), an objective (lens closer to the sample), a source of illumination (for illuminating the sample), and a stage (to hold the sample on a slide), among other parts. A number of light sources may be used, however a standard compound microscope uses an external source of light placed underneath the sample. The amount of light passing through the sample can be adjusted by using the adjustable diaphragm. By opening or closing the diaphragm, we can allow or prevent the illumination and by changing the opening size, we can adjust the level of illumination.
Such an arrangement is also available in other devices, such as a camera, where the level of illumination is controlled by the same mechanism.
