Fig. 12.—In this scroll the left half may at first seem concave and the right convex, it then seems to roll or advance like a wave, and the left seems convex and the right concave, as though the trough of the wave had become the crest, and vice versa.
Figs. 13, 13a, and 13b.—The two methods of viewing Fig. 13 are described in the text. Figs. 13a and 13b are added to make clearer the two methods of viewing Fig. 13. The heavier lines seem to represent the nearer surface. Fig. 13a more naturally suggests the nearer surface of the box in a position downward and to the left, and Fig. 13b 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.
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.
Figs. 15, 15a, and 15b.—The two views of Fig. 15 described in the text are brought out more clearly in Figs. 15a and 15b. The shaded portion tends to be regarded as the nearer face. Fig. 15a is more apt to suggest the steps seen as we ascend them. Fig. 15b 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.
