As a matter of fact, the visual impression produced by a concave form with the light falling on one side, very closely resembles that produced by a corresponding convex form with the light falling on the other side.

At the same time, it is found that the opposite mode of conversion, that is to say, the transformation of the raised into the depressed form, though occurring occasionally, is much less frequent.
Now, it may be asked, why should we tend to transform the concave into the convex, rather than the convex into the concave?
The reader may easily anticipate the answer from what has been said about the deeply fixed tendency of the eye to solidify a plane surface.

We are rendered much more familiar, both by nature and by art, with raised (_cameo_) design than with depressed design (_intaglio_), and we instinctively interpret the less familiar form by the more familiar.

This explanation appears to be borne out by the fact emphasized by Schroeder that the illusion is much more powerful if the design is that of some well-known object, as the human head or figure, or an animal form, or leaves.[43] Another illustration of this kind of illusion recently occurred in my own experience.

Nearly opposite to my window came a narrow space between two detached houses.