Final answer:
The amount of light hitting the slide on a microscope is regulated by the illumination system, which includes a high-intensity bulb, a condenser lens, and a diaphragm. The brightness can be adjusted to maximize illumination.
Step-by-step explanation:
The amount of light that will hit the slide on a microscope is regulated by the illumination system of the microscope. In a brightfield microscope, a high-intensity bulb located below the stage provides the light. The light passes through a condenser lens, which focuses the light rays on the specimen to maximize illumination. The brightness can be adjusted by opening or closing a diaphragm between the condenser and the specimen, or by using a dimmer switch called a rheostat to control the intensity of the illuminator.