It is the contrast between the light and the dark parts of a picture. It is employed, in the first place, to make the objects in the picture look more real. If you fix your eyes on any object in the room or out of doors, you will observe that some parts of it are light and some dark, and that there are various degrees of lightness and darkness. It is the light on an object that enables us to see it. If there were no light on it--if it were in complete darkness, that is to say--nothing would be visible. And, while it is the light that enables us to see the object, it is the degree of light on some parts of it and the various degrees of darkness on others that enable us to realize the shape of it. In other words, the contrast of light and dark, received by the eyes, communicates to our brain the sense of form and bulk.