At first glance, István Orosz's illustrations look like ordinary, if vaguely cartoonish, scenes of medieval life. But contained in each scene is a picture of a human skull, if only you know how to look.
Orosz frequently places with optical illusions in his work, and his Ship of Fools series, based on Albrecht Dürer's engravings from Sebastian Brant's satire of the same name, plays with framing and perspective to create a perceptual illusion, forcing your brain to switch between the depicted scenes and the skulls.
If you visit the Orosz's online gallery, you'll at first see just the thumbnails, rows upon rows of skulls. As you see the details in the larger images, however, it's more difficult to hold onto your perception of one image or the other.