3 o8 



POPULAR SCIENCE MONTHLY. 



probably suggest at first view a book, or better a book cover, seen 

 with its back toward you and its sides sloping away from you; but 



7V. 



Fisr. 13. 



Fig. 13a. 



Fig. \Zb. 



Figs. 13, 13a, and 13*. — The two methods of viewing Fig. 13 arc described in the text. Figs. 

 13a and 134 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 

 sui*face of the box in a position downward and to the left, and Fig. \Zb makes the nearer 

 side seem to he 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. 



it may also be viewed as a book opened out toward you and present- 

 ing to you an inside view of its contents. Should the change not 

 come readily, it may be facilitated by thinking persistently of the 

 appearance of an open book in this position. The upper portion of 

 Fig. 9 is practically the same as Fig. 8, and if the rest of the figure 

 be covered up, it will change as did the book cover; when, however, 

 the whole figure is viewed as an arrow, a new conception enters, and 

 the apparently solid book cover becomes the flat feathered part of the 



Fig. 14. — Each member of this frieze represents a relief ornament, applied upon the back- 

 ground, which in cross -section would be an isosceles triangle with a huge 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. 



arrow. Look at the next figure (Fig. 10), which represents in out- 

 line a truncated pyramid with a square base. Is the smaller square 

 nearer to you, and are the sides of the pyramid sloping away from 

 you toward the larger square in the rear? Or are you looking into 

 the hollow of a truncated pyramid with the smaller square in the 

 background? Or is it now one and now the other, according as you 

 decide to see it? Here (Fig. 13) is a skeleton box which you may 

 conceive as made of wires outlining the sides. Now the front, or 



