When the middle distance is hidden by a rising foreground, an illusion may be created by the far distance alone if this be of considerable depth. Since the fifteenth century it has been a frequent practice to conceal the middle distance, though mostly in pictures of figure subjects.[64] The Dutch artists of the seventeenth century who painted open-air scenes of human and animal life, as Paul Potter, Wouverman, and Albert Cuyp, avoided the middle distance whenever possible, but often managed to secure a fair illusion. In pure landscape the system is less often practised, and never by great artists. The only means available to the painter of land views for creating an illusion of motion, apart from that of opening distance, is by the representation of flowing water so that a series of successive events in the flow, each connected with, but varying in character from, the preceding one, can be exhibited. Thus, a volume of water from a fall proceeds rapidly over a flat surface to a ledge, and thence perhaps to another ledge of a different depth, from which it passes over or round irregular rocks and boulders, and thence over smaller stones or into a stream, creating in its passage every kind of eddy and current.[a] Here is a series of progressive natural actions in which the progression is regular and continuous, while the separate actions cover such time and space that they may be readily separated by the eye. If, therefore, the whole series be properly represented, an illusion of motion will result.[65] Obviously the canvas must be of considerable size, and the breaks in the flow of water as varied in character and as numerous as possible. Everdingen and Jacob Ruysdael seem to have been the first artists to recognize the significance of this progression, but Ruysdael far surpassed his master in the exhibition of it. He examined the problem in all its variations, solved it in a hundred ways, and at his death left little for succeeding painters to learn regarding it. Very rarely, one meets with a landscape where the double illusion of motion of water and opening distance is provided, and needless to say the effect is superb.[b] [a] See Plate 19. [b] For examples see S. Bough's Borrowdale, and Thoma's View of Laufenburg.