1. A device for deterring unauthorized use of xerographic copying machines of the type having a transparent platen on which a document having a primary image to be copied is registrable, and means for reproducing said primary image thereby creating a copy of said document, said device comprising transparent means rigidly associated with said platen and having a secondary image thereon, said transparent means for involuntarily and concurrently reproducing said primary and secondary images on each one of said document copies reproduced by said xerographic copying machine.