Centuries before the invention of optical lenses, a small number of monastic artists in the early medieval world produced manuscript pages of such microscopic precision that modern scholars have long struggled to explain them. A Cornell researcher now proposes that these illuminators may have turned inward for their instrument — using the human eye's own capacity for stereoscopic depth perception as both a copying device and an error-detection system. The theory suggests that what appeared to be a mystery of divine patience may in fact be a lost technology of embodied vision, one so elegant it