An illusion of motion is sometimes assisted by the title of the picture. A remarkable example of this is Robert's The Israelites Depart. Although individual action cannot be distinguished owing to the scale of the design, yet when one is acquainted with the title, the imagination is instinctively set to work, and the enormous crowds packing the wide streets seem to be streaming in one direction. Obviously for the title to have this effect, the number of signs must be overwhelming, and there must be no possibility of interpreting the picture in two ways; that is to say, accessory signs must be used to indicate the direction in which the crowd is moving.[71] CHAPTER III ILLUSION OF SUSPENSION AND MOTION IN THE AIR With the assistance of drapery--Of clouds--Of winged figures--Miscellaneous devices. The representation of figures suspended in the air, or moving through it, has never offered much trouble to painters, though necessarily involving an apparent miracle. The very slightest pretended physical assistance suffices for the illusion, and this help is usually rendered in the shape of flying drapery, winged figures, clouds, or artificial devices based upon the contact of two or more figures. The only difficulty met with is in respect of an upward vertical movement. Here, wings or clouds can scarcely be made to differentiate between a rising and a falling movement, and flying drapery is of little service inasmuch as a rush through the air would, if the feat were actually performed, cause the drapery to cling to the figure. The surest remedy for the disabilty is to support the figure directly by winged figures placed at a considerable angle from the vertical, but this plan is only rarely adopted by great masters because of the consequent complications in the design of the group. Since flying drapery is commonly added to the figure presumed to be ascending, and seeing that artists almost invariably insist upon giving their ascending figures upright attitudes, it is seldom that the movement is correctly expressed. Usually the figure appears to be held immovably in suspension, but occasionally, owing to the drapery arrangement, a descending movement is indicated.[a] Without the assistance of winged figures, the illusion of ascension can only be given when the figure is shown directed upwards at an angle of at least fifteen or twenty degrees from the vertical. As a rule the larger the angle, the more easy is the production of an illusion. With a fairly large angle, and an appropriate arrangement of limbs and drapery, heavy figures can be made to appear naturally ascending, as in Rubens's Boreas and Orithyia, both voluptuous forms.[b] [a] As in Murillo's Ascension of Christ, Madrid Academy. [b] Venice Academy.