This is illustrated in a curious observation of Sinsteden.

One evening, on approaching a windmill obliquely from one side, which under these circumstances he saw only as a dark silhouette against a bright sky, he noticed that the sails appeared to go, now in one direction, now in another, according as he imagined himself looking at the front or at the back of the windmill.[48] In the interpretation of geometrical drawings, as those of crystals, there is, as I have observed, a general tendency to view the flat delineation as answering to a raised object, or a body in relief, according to the common run of our experience.

Yet there are cases where experience is less decided, and where, consequently, we may regard any particular line as advancing or receding.

And it is found that when we vividly imagine that the drawing is that of a convex or concave surface, we see it to be so, with all the force of a complete perception.

The least disposition to see it in the other way will suffice to reverse the interpretation.