Shepard’s “Terror Subterra”
What to observe
Move your mouse pointer over the neighbouring image. A second ‘monsterlet’ appears at the bottom. Compare their sizes. In the context of these pages you probably guess that they are of the same size – but do they look identical in size?
What to do
Click–and–hold your mouse over one of the monsters. Then a duplicate of the monster will travel along until you release the mouse button. Thus you can test whether the two are really the same size in the image plane.
The two monsterlets look different in size although they are identical drawings. Your eyes don’t deceive you, though: while the monsterlets are identical in the image plane, in a ‘real’ 3-dimensional scene they would not be identical at all.
This is a particularly nice example of the classical “Ponzo Illusion”, where the context suggests different depths in the drawing – here by the subterranean catacomb. Assuming size constancy, our visual system estimates the size of any object as follows: retinal size multiplied by the assumed distance. Thus, the two monsters, though identical, look quite different in their differing contexts.
Interestingly, doesn’t the lower left monsterlet, the one being pursued, have a particularly horrid grin on its face, while the soon-to-be-successful pursuing monster, by way of being much more massive, wears a gruesome, satisfied smile?
Roger N. Shepard originated a number of beautiful illusions in addition to the present one. He drew them himself, and many are now “floating around the internet” without proper attribution (e.g., the elephant with the impossible legs). His “Turning the Tables” is also in the present collection.
Shepard RN (1990) Mind Sights: Original Visual Illusions, Ambiguities, and other Anomalies, New York: WH Freeman and Company