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Page:Popular Science Monthly Volume 54.djvu/325

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side nearest to me, seems directed downward and to the left; again, it has shifted its position and is no longer the front, and the side which appears to be the front seems directed upward and to the right. The presence of the diagonal line makes the change more striking: in one position it runs from the left-hand rear upper corner to the right-hand front lower corner; while in the other it connects the left-hand front upper corner with the right-hand rear lower corner.

Fig. 15 will probably seem at first glimpse to be the view of a flight of steps which one is about to ascend from right to left. Imagine

Fig. 15a. Fig. 15b.
PSM V54 D325 Optical illusion with staircases.png
Fig. 15.

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.

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.

The blocks in Fig. 16 are subject to a marked fluctuation. Now