Final answer:
The iris diaphragm on a microscope controls the amount of light that is allowed to reach the specimen. It can be adjusted by opening or closing, and together with the rheostat, it ensures optimal illumination for clear viewing of the specimen.
Step-by-step explanation:
The part of the microscope that allows you to adjust the amount of light allowed inside is known as the iris diaphragm. Located beneath the stage, the iris diaphragm can be opened or closed to regulate the amount of light reaching the specimen. Additionally, the light intensity can be modified using the microscope's illuminator and the rheostat, which acts as a dimmer switch. It is important to adjust these settings, especially when changing the magnification levels with different objective lenses, to ensure the image is neither too dark nor overly bright, resulting in the best image possible.