Hockney and Falco's theory has already inspired an increase in research regarding the use of optics throughout the history of art. For instance, there was the case of the decade-long research on Rembrandt's works conducted by painter Francis O'Neill. In the published paper he wrote with Sofia Palazzo Corner entitled, "Rembrandt's Self-portraits", O'Neill presented recurring themes in the painter's works that serve as evidence in his use of mirrors, particularly, in his self-portraits. These include the use of chiaroscuro, which is a signature of the lighting conditions necessary for projections as well as Rembrandt's off-center gaze in his self-portraits, which - according to O'Neill - indicated that the artist might have been looking at a projection surface off to the side rather than straight onto a flat mirror.