Final answer:
The flat platform on a microscope that keeps the slide stationary during viewing is known as the stage. It is equipped with clips and knobs for positioning but remains fixed in height while the focus is adjusted with other knobs.
Step-by-step explanation:
The flat platform you are asking about is called the stage of the microscope. This is where the slide, which holds the specimen, is placed and kept stationary for viewing. The stage often has clips to secure the slide in place and is equipped with knobs that allow the slide to be moved precisely over the light source, which helps to position the specimen correctly for viewing through the objective lenses. However, the stage itself does not move up and down; the focusing is done by adjusting the distance between the objective lenses and the stage using the coarse and fine focusing knobs.