An apparent medieval stereogram incorporating a symbol for optical illusion

Stereograms mark a threshold in understanding visual perception. Modern study of stereopsis began with Wheatstone's invention of the stereogram and stereoscope (~ 1832), important tools in vision research and technical imagery ever since. Stereoscopic images formed with frieze and wallpaper patterns in illuminated Insular manuscripts such as the Book of Durrow (~ 680 CE), Lindisfarne Gospels (~ 700-720), and Book of Kells (~ 800) show that, long before spectacle-quality magnifying lenses (~ 1286), illuminators somehow copied multicolored, microscopically detailed designs _freehand_ with an accuracy unsurpassed in scientific instruments until the Renaissance (but well within the power of normally sighted humans' stereoscopic discrimination). If the artists accomplished this feat by free fusion using the unaided eyes as a magnifying stereocomparator, as suggested, they should have been able to create autostereograms. Did they? Here I report two examples of an apparent stereopair from the Book of Durrow, which gives a sharp, strongly three-dimensional image that includes, among other symbols, an eye-shaped sign identified with mirages (Fig. 1). Apparently, precocious empirical knowledge of stereopsis played more than a technical role in the creation of some of the world's more famous graphic art