[Illustration:  FIG. 5.]

Similarly, in the accompanying figure of a transparent solid, I can at will select either of the two surfaces which approximately face the eye and regard it as the nearer, the other appearing as the hinder surface looked at through the body.

[Illustration:  FIG. 6.]

Again, in the next drawing, taken from Schroeder, one may, by an effort of will, see the diagonal step-like pattern, either as the view from above of the edge of an advancing piece of wall at a, or as the view from below of the edge of an advancing (overhanging) piece of wall at b.

[Illustration:  FIG. 7.]

These last drawings are not in true perspective on either of the suppositions adopted, wherefore the choice is easier.  But even when an outline form is in perspective, a strenuous effort of imagination may suffice to bring about a conversion of the appearance.  Thus, if the reader will look at the drawing of the box-like solid (Fig. 3, p. 79), he will find that, after a trial or two, he succeeds in seeing it as a concave figure representing the coyer and two sides of a box as looked at from within.[49]

Many of my readers, probably, share in my power of variously interpreting the relative position of bands or stripes on fabrics such as wall-papers, according to wish.  I find that it is possible to view now this stripe or set of stripes as standing out in relief upon the others as a ground, now these others as advancing out of the first as a background.  The difficulty of selecting either interpretation at will becomes greater, of course, in those cases where there is a powerful suggestion of some particular local arrangement, as, for example, the case of patterns much brighter than the ground, and especially of such as represent known objects, as flowers.  Yet even here a strong effort of imagination will often suffice to bring about a conversion of the first appearance.

A somewhat similar choice of interpretation offers itself in looking at elaborate decorative patterns.  When we strongly imagine any number of details to be elements of one figure, they seem to become so; and a given detail positively appears to alter in character according as it is viewed as an element of a more or less complex figure.

These examples show what force belongs to a vivid preconception, if this happens to fit only very roughly the impression of the moment, that is to say, if the interpretative image is one of the possible suggestions of the impression.  The play of imagination takes a wider range in those cases where the impression is very indefinite in character, easily allowing of a considerable variety of imaginative interpretation.