When you turn the coarse adjustment knob either clockwise or counterclockwise (up or down), the distance between the tube and the stage changes. This in turn changes the quality of the image of the specimen. Say for example that you looked into a microscope and you can't see an HD image of the specimen. You will turn the coarse adjustment knob either counterclockwise our clockwise until you see the image clearly. To make the image clearer you'd turn the fine adjustment knob (the little knob just below or above the coarse adjustment knob).
