The BnF was able to acquire a very complete file assembled by the photographer Pierre Petit about the invention of photography by artificial light, a crucial commercial issue. In it, he requests Nadar's testimony, because his competitor, Liébert, is suing him to try to keep him from using what each of them beieves to be their own invention. By emphatically stating that the invention was in fact both his and already quite old, Nadar rendered the quarrel moot. To back up his assertions, he provided these precious annotated prints.