Optogram

An optogram was believed to be image of the last thing that a person saw was retained in their eye after death. Both Madame Vastra and the Eleventh Doctor dismissed the notion as "silly superstition", at least in relation to naturally occurring optograms in humans. The Doctor particularly ascribed the notion to the Romany people. Nevertheless, the Doctor allowed for the possibility, if the body's chemistry were disturbed. The presence of red leech toxin was sufficient disturbance to allow for optograms, as evidenced by the fact that the image of the Doctor was indeed retained by Edmund's eyes. (TV: The Crimson Horror)

The Fourth Doctor referred to this belief on the Nerva Beacon while constructing a device which would enable him to tap into the Wirrn hive mind and learn how the Wirrn Queen died. He said that gypsies used to believe it. (TV: The Ark in Space)