and natural view of this we focus the attention upon the black lines and observe the familiar illusion, that the four vertical lines seem far from parallel. That they are parallel can be verified by measurement, or by covering up all of the diagram except the four main
 |
 | |
| Fig. 8.—This drawing may be viewed as the representation of a book standing on its half-opened covers as seen from the back of the book; or as the inside view of an open book showing the pages. | ||
 |
||
| Fig. 10.—The smaller square may be regarded as either the nearer face of a projecting figure or as the more distant face of a hollow figure. | Fig. 9.—When this figure is viewed as an arrow, the upper or feathered end seems flat; when the rest of the arrow is covered, the feathered end may be made to project or recede like the book cover in Fig. 8. |
lines. But if the white part of the diagram is conceived as the design against a black background, then the design is no longer the same, and with this change the illusion disappears, and the four lines seem parallel, as they really are. It may require a little effort to bring about this change, but it is very marked when once realized.
A curious optical effect which in part illustrates the change in appearance under different aspects is reproduced in Fig. 7. In this case the enchantment of distance is necessary to produce the transformation. Viewed at the usual reading distance, we see nothing
