Thus, in the following drawing, the reader can easily see at will something answering to a truncated pyramid, or to the interior of a cooking vessel.
[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.