[Illustration: FIG. 6.--When this figure is viewed as a black pattern on a white background, the four main vertical lines seem far from parallel; when it is viewed as a white pattern on a black background this illusion disappears (or nearly so), and the black lines as well as the white ones seem parallel.] 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 but an irregular and meaningless assemblage of black and white blotches. At a distance of fifteen to eighteen feet, however, a man's head appears quite clearly. Also observe that after the head has once been realized it becomes possible to obtain suggestions of it at nearer distances. [Illustration: FIG. 7.--This is a highly enlarged reproduction taken from a half-tone process print of Lord Kelvin. It appeared in the Photographic Times.] A much larger class of ambiguous diagrams consists of those which represent by simple outlines familiar geometrical forms or objects. We cultivate such a use of our eyes, as indeed of all our faculties, as will on the whole lead to the most profitable results. As a rule, the particular impression is not so important as what it represents. Sense impressions are simply the symbols or signs of things or ideas, and the thing or the idea is more important than the sign. Accordingly, we are accustomed to interpret lines, whenever we can, as the representations of objects. We are well aware that the canvas or the etching or the photograph before us is a flat surface in two dimensions, but we see the picture as the representation of solid objects in three dimensions. This is the illusion of pictorial art. So strong is this tendency to view lines as the symbols of things that if there is the slightest chance of so viewing them, we invariably do so; for we have a great deal of experience with things that present their contours as lines, and very little with mere lines or surfaces. If we view outlines only, without shading or perspective or anything to definitely suggest what is foreground and what background, it becomes possible for the mind to supply these details and see foreground as background, and _vice versa_. [Illustration: 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.] [Illustration: 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.] [Illustration: 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.]