PETER OF LIMOGES AND THE TRACTATUS MORALIS DE OCULO

Towards the very end of the Perspectiva, his influential treatise on vision, the thirteenth-century Franciscan natural philosopher Roger Bacon proclaims the wonders and powers of mirrors. They can make one man appear to be many men and one army to appear to be many armies. Some people, he adds, believe that demons can transform the air into a kind of mirror with which they reveal and betray hidden military encampments. Indeed, legend has it that the philosopher Socrates used mirrors to locate an evil dragon within its mountainous hiding places, a beast whose foul breath had poisoned both animals and men. “Anything,” Bacon concludes, “that has been hidden away in some concealed place in cities, armies and the like can be brought to light through reflected vision.” As it turns out, mirrors can even reveal secrets hidden in the pages of sacred scripture.

Imagine that an eye is placed in the centre of a spherical, concave mirror. The natural properties of this mirror are such that wherever that eye looks, it will see only itself. Now imagine that another eye, placed somewhere else, anywhere else but the mirror's centre, looks at the mirror. It will never see the reflected image of that other eye. One eye sees itself everywhere while remaining entirely invisible to the other – ubiquitous, yet hidden. These are peculiar phenomena, but well enough known during the second half of the thirteenth century.