[Illustration: FIGS. 13, 13_a_, and 13_b_.--The two methods of viewing Fig. 13 are described in the text. Figs. 13_a_ and 13_b_ are added to make clearer the two methods of viewing Fig. 13. The heavier lines seem to represent the nearer surface. Fig. 13_a_ more naturally suggests the nearer surface of the box in a position downward and to the left, and Fig. 13_b_ makes the nearer side seem to be upward and to the right. But in spite of the heavier outlines of the one surface, it may be made to shift positions from foreground to background, although not so readily as in Fig. 13.] [Illustration: FIG. 14.--Each member of this frieze represents a relief ornament, applied upon the background, which in cross-section would be an isosceles triangle with a large obtuse angle, or a space of similar shape hollowed out of the solid wood or stone. In running the eye along the pattern, it is interesting to observe how variously the patterns fluctuate from one of these aspects to the other.] [Illustration: FIGS. 15, 15_a_, and 15_b_.--The two views of Fig. 15 described in the text are brought out more clearly in Figs. 15_a_ and 15_b_. The shaded portion tends to be regarded as the nearer face. Fig. 15_a_ is more apt to suggest the steps seen as we ascend them. Fig. 15_b_ seems to represent the hollowed-out structure underneath the steps. But even with the shading the dual interpretation is possible, although less obvious.] Fig. 15 will probably seen at first glimpse to be the view of a flight of steps which one is about to ascend from right to left. Imagine it, however, to be a view of the under side of a series of steps; the view representing the structure of overhanging solid masonwork seen from underneath. At first it may be difficult to see it thus, because the view of steps which we are about to mount is a more natural and frequent experience than the other; but by staring at it with the intention of seeing it differently the transition will come, and often quite unexpectedly. [Illustration: FIG. 16.--This interesting figure (which is reproduced with modifications from Scripture--The New Psychology) is subject in a striking way to interchanges between foreground and background. Most persons find it difficult to maintain for any considerable time either aspect of the blocks (these aspects are described in the text); some can change them at will, others must accept the changes as they happen to come.] [Illustration: Fig. 17_a_.] [Illustration: Fig. 17_b_.] [Illustration: Fig. 17.