Recall what the part of the microscope that controls how much light passes up towards the slide is called.

The part of the microscope that controls how much light passes towards the slide is known as the iris diaphragm. The iris diaphragm is an essential component of optical microscopes, located beneath the stage, and its primary function is to regulate the intensity and contrast of the light entering the specimen being viewed.

When adjusting the diaphragm, one can increase or decrease the light that reaches the slide, which is crucial for improving the clarity and visibility of the specimen, depending on its properties. For instance, thicker or more opaque specimens might require a broader opening to allow more light, while thinner specimens may benefit from a smaller opening to enhance their contrast.

Using the iris diaphragm effectively enables users to have better control over the microscopic image, making it vital for both observational and research purposes in biological and material sciences.