The Retimager is a device invented by Foaly allowing anyone who uses it to see what someone else has seen. It resembles two suction cups with leads coming out the back that connect to a computer. The suction cups cover the person or fairy's eyes (knocking them out with sedative in the seals) and magnifying the scratches on a person's retina caused by photons striking it. The computer then takes over comparing images to other images in its database. If over 200 matches are found by the computer then the person (or eyeball) has seen that object or image before.The book says that even if an individual was mind-wiped, the Retimager would still be able to detect it, unless their retinas had been scrubbed.