what is the purpose of the diaphragm on a microscope

The diaphragm of a microscope is a mechanical part located beneath the stage of the microscope that controls the amount of light reaching the specimen. Specifically, the diaphragm regulates the amount of light passing through the specimen by controlling the angle of the light rays emerging from the condenser and reaching the specimen from all azimuths. The diaphragm has the same function as the iris in a human eye and regulates the light under a specimen. Closing or opening the diaphragm controls the amount of light that is allowed to pass through the microscope stage and up toward the objective lens and ultimately affects the intensity and size of the cone of light that is projected upward into the prepared slide. The diaphragm is used to vary both the intensity and size of the cone of light that is projected upward into the prepared slide, and there is no set rule regarding which setting should be used for a particular objective lens magnification.