Most of the great bas-reliefs have but one plane, and to this they owe the simplicity that gives them a certain nobility in spite of their monotonous design (see Vol. I. Figs. 4, 5, 6, 8, 13, 22, 23, 24, &c.). Examples of two planes, in which the figures are grouped in couples, the nearest to the spectator in each couple covering a large part of his companion, are by no means rare (see Fig. 62); we may say the same of those in which a background of trees is introduced beyond the figures (Fig. 63). This arrangement is especially frequent in the more complicated pictures, where the figures are small and numerous; but even in the last century of the Assyrian monarchy, when the sculptor showed an ever-increasing desire to draw attention and excite interest by the introduction of these picturesque details, he never quits his hold of a right instinct for the true conditions of the bas-relief. Unlike the Roman sculptors, and even those of the Renaissance, he shows no hankering after those effects that seem to get rid of the bed; he never destroys the clarity of his conception by unduly multiplying the planes. He did not understand how to put objects in perspective or manage foreshortening, and this ignorance served him well; it preserved him from the temptation into which more skilful artists are so prone to fall; it prevented him from forgetting “that the design best suited to the bas-relief is purely geometrical in its essentials.”[147] The relief, of course, becomes higher as the size of the figures increases. It is as much as from eight to ten inches in the winged genii (Figs. 27 and 34) that accompany and divide the bulls on the decorated façades and in the gateways. Even when it is highest the salience does not go beyond what is called _mezzo-relievo_; that is to say, no part of the principal or accessory figure, of the genius himself or of the lion, stands out from the wall in the round, as, for instance, do the heads and limbs in the metopes of the Parthenon. [Illustration: TWO CHALDÆAN HEADS J. Bourgoin, del. Ramus sc. Imp. Ch. Chardon [Illustration: FIG. 62.--Pair of warriors. Louvre. Drawn by Saint-Elme Gautier.] The same remark holds good of the colossal figures of lions (Plate VIII.) and winged bulls (Plate IX.) which acted as guardians of the palace. We have already explained the ideas attached to these monsters by the Assyrians,[148] we shall here dwell upon some peculiarities of their execution. In these images there is a compromise between “the round” and the bas-relief of a very original and peculiar character.