A diaphragm on a microscope is the piece that enables the user to adjust the amount of light that is focused under the specimen being observed. A diaphragm is typically found on higher-power microscopes versus less expensive or toy models.
The diaphragm is located directly under the stage or platform where user places the specimen or slide. The diaphragm disc, sometimes called an iris, has tiny holes in it that let varying degrees of light in under the specimen. By opening the diaphragm, an item that at first appears too dark is easier to observe. Adjusting the diaphragm can also create contrast for better viewing transparent specimens.