In Downcast Eyes, Martin Jay often uses words that he borrowed from Vasco Ronchi's Optics: lumen and lux. The aim of this paper is to clarify the features of Jay's thought, comparing Jay's usages of the words lumen and lux with Ronchi's. I will develop my discussion as follows: (1)I will clarify Ronchi's history of optics. He argues that the first fifteen of the last twenty centuries talked only about the subjective lux and that the last five centuries - especially Kepler's seventeenth century - talked about the objective lumen. (2) I will then consider Jay's usage of lumen/lux, comparing it with Ronchi's. Although Ronchi emphasizes that the change occurred during the last Middle Ages, Jay insists on continuity from ancient Greece to the seventeenth century. He refers to the neo-Platonic tradition of speculation with the eye of the mind as lumen, and he insists that the change from lumen to lux occurred in the twentieth century; Ronchi insists that change from lux to lumen was in the seventeenth century. (3) Through this comparison, I will show the three features of Downcast Eyes: (a) Jay discusses the modern scopic regimes as "Cartesian perspectivalism" and "the denigration of vision in twentieth-century French thought," using the difference between lumen and lux; (b) he also discusses the things that are deviated from such schema -three scopic regimes of modern thought, another aspect of Cartesian perspectivalism; and (c) Downcast Eyes attempts to draw portraits of various discourses about lumen from the point of view of vision and language.