Ghost images

As a picture conservator, I am used to finding hidden figures - people that the artist, for whatever reason, has painted over. But, over the years, I have identified a phenomenon whereby I might make out a face or a figure which, on later examination, cannot be seen. For a long time, I put this down to a trick of the light or of the mind but I am now not so sure.

There is a figure in Picasso's Guernica which disappears for years at a time but is then clearly present. And in Bosch's Garden of Earthly Delights, a malevolent demon not only disappears and reappears, but moves around the triptych.