michelangelo pistoletto's project plexiglass, where, for instance, double ladder... is composed of two plexiglass sheets bearing the photographs of the two sections that make up the ladder. see more projects here. thanks to charlie for the link.
[all michelangelo pistoletto. from top to bottom: electric wire hanging on the wall. double ladder leaning against the wall. stack of records. all from plexiglass, 1964.]