What to do & observe
You can see a monster in the neighbouring image, which pulsates, just waiting to be grabbed with your mouse (or touched) and moved to the left bottom. [Make sure you still see it in full, just touching the edge.] You know they are of identical size – but do they look so?
The two monsterlets look different in size, although they are totally identical. 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, the author of these drawings, 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