[f] The Transfiguration, Vatican. [Illustration: PLATE 22 (See page 244) Christ on the Cross, by Van Dyck (_Antwerp Museum_)] The painter has a comparatively easy task in presenting an illusion with several figures presumed to be moving, for he has only to comply with two simple conditions. The first is that the particular step represented in the act of progression of any individual should vary from the steps of the persons immediately behind or in front of him; and secondly that the actions of the different persons be connected with each other so far as possible. With these conditions reasonably fulfilled, illusion of motion necessarily follows. Naturally in such a mechanical matter, the character of the invention depends upon the scale of the design. When the moving figures are presumed to be comparatively near at hand, the position of the limbs must be entirely presented, or the progression will appear broken. The effective illusion presented in Burne-Jones's Golden Staircase is due to his ingenuity in so arranging the numerous figures descending the winding stairs, that all their feet are visible. In the case of a crowd of figures of whom some are supposed to be moving and others standing still, the visibility of the limbs is of less importance than the connection of the various actions. In Menzel's Market in Verona,[a] the illusion, which is remarkable, is entirely produced by the skill in which innumerable instances of action are made dependent upon others. An illusion is created in the same way though in a lesser degree by Gustave Doré in several works.[b] When the motion arises from the actions of the arms of a number of persons, it suffices if the arms are in various positions, as in Menzel's Iron Mill, and Cavalori's Woolworkers[c] where many men are using long tools; but if the limbs are working together, an illusion is impossible. The beauty of Guardi's great picture, Regatta on the Grand Canal, is much diminished by the attitude of the gondoliers, who all hold their poles in the same position. [a] Dresden Gallery. [b] See Samson Slaying the Philistines. [c] Palazzo Vecchio, Florence.