“The second factor, equally important, is that of good continuation and good shape. The things which we see have a better shape, are bounded by better contours, than the holes which we might see but do not. Therefore, when in exceptional circumstances these conditions are reversed, we see the hole and not the things, as the shape of a gap between two projecting rocks with sharp profiles, which may look like a face, an animal, or some other object, while the shape of the rock disappears.”