Perhaps you will grasp better what this means if, when you next go to the theater, you carefully observe the scenery, representing some outdoor effect. On each side of the stage, very likely representing tree trunks, there is a series of “wings,” one behind another at a distance of say five feet, while across the stage, hanging down from the “flies,” is a series of cut cloths, representing foliage, that correspond with the wings and seem to be branches of the tree trunks. Well, these cloths and their wings correspond to the “successive planes” of a picture. They lead gradually back and you can actually walk in and out of them. But, when you reach the back cloth, you are stopped, so far as your legs are concerned. If you are sitting in the auditorium, however, your eye goes traveling on and on a long distance, for the back cloth is itself a picture, in which there is an _illusion_ of successive planes.