This article deals with the ‘paper work’ of the seventeenth-century microscopist Jan Swammerdam: drawing as part of experimental practice rather than mere documentation. In the “Biblia Naturae”-corpus of the Nachlass, Swammerdam’s raison d’recherche is the position he takes against the theory of spontaneous generation in larval transmutation. Drawing what he sees beneath the microscope Swammerdam adds certain elements to the image of the specimen in ‘layers.’ Nevertheless, this is not the old chestnut of a preponderance of theory overruling ‘realistic’ representation. Rather, microscopical observation must rely on experimental cognition as an intermediary between plain sensual input and ‘how to see.’ It turns out that this kind of experimental cognition depends crucially on the ‘paper work’ of the microscopist rather than on her skill handling the technical device. Surprisingly, this is equally true for later stages of microscopy, as this article demonstrates for the 20th-century neuroanatomists Ramòn y Cajal and James Papez.