This paper presents a recently discovered episode in the intellectual history of urbanism. It concerns theory and practice in the field in the first decades of the twentieth century when it was in the process of establishing itself as an academic discipline and a profession. I will demonstrate how the idea of ambiance as we understand it today can serve as a point of departure for questioning certain ideas and developments of that time. In doing so, I will seek to contribute to current research on urban ambiances by addressing the question of ambiance as a subject of historical inquiry. I will present an archive of iconographic documents, photographed on glass slides under the direction of Marcel Poëte (1860–1950), an archivist and historian working in Paris as of 1906 who soon became a central figure in the emerging discipline of Urbanism. In doing so, l will emphasize the manner in which the visual documents were used to develop dense, historical descriptions of urban life in the spaces of Paris, depicting them with extreme historical accuracy and, at the same time, through a form of empathetic reactivation of their sensual qualities.