Page:Popular Science Monthly Volume 54.djvu/322

306 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

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