Here you see the specimen stage. Note the scale around the perimeter of the stage. It is marked in degrees. The clips may be used to hold a microscope slide in place. The screw marked A is used to tighten or loosen the ability of the stage to rotate. The screw marked B, along with its mate on the other side, is used to center the stage. Proper centering of the stage is essential to the techniques based on stage rotation.