This dissertation critiques and updates the theoretical frameworks for understanding encyclopedic and diagrammatic images as presented in the scholarship of Lucy Freeman Sandler, Barbara Maria Stafford, John Bender, and Michael Marrinan. It offers a new model for examining the cognitive role of images by studying an important medieval encyclopedia, On the Properties of Things, originally written in Latin by Bartholomaeus Anglicus in the thirteenth century. Bartholomaeus’ text was the most popular encyclopedia of the later middle ages and four vernacular translations were produced and circulated between the fourteenth and sixteenth centuries. Significantly, the French translation of the compendium, coming out of the vernacularization movement of King Charles V but radiating out to other production centres, involved the design of an elaborate and novel illustrative program. The present project examines two exceptional fifteenth-century French copies of this encyclopedia (BnF fr. 9141 and BnF fr. 135/6), and interprets them in light of the shifting intellectual culture and evolving reading practices of late-medieval lay audiences. The information-rich and highly aestheticized miniatures found in such encyclopedic manuscripts have traditionally been defined, by Sandler and others, as having an explanatory function and the capacity to elevate the content of the text through displays of material luxury. My model expands the significance of such images by highlighting their capacity to promote thought. I argue that images in didactic compendia can (i) encourage the reader to actively engage with the text through representations of aristocratic readers performing their understanding of the book socially, and (ii) facilitate visual thinking by aesthetically reflecting the structure of the encyclopedic text through the diagrammatic strategies of the collection, compression, and division of fragmented information. Though the images in my two manuscript case studies take distinct approaches to reader engagement and the mediation of knowledge, in both cases the power of these visualizations rests in the cognitive acts and range of mental associations they provoke. This dissertation demonstrates that epistemically-dense images, in addition to merely reflecting a text, could shape knowledge as it was being formed in the minds of active viewers, readers, writers, and artists, in an intellectually rich period in late-medieval France. / Graduate