In the 19th century, many painters used lead-based paints for priming canvases, and when tubed paints became readily available (and more convenient for the growing trend of painting “en plein air”), they too were lead-based, leading many painters to develop lead poisoning and the accompanying symptoms.  One of these symptoms, believed to be most famously exhibited in the works of Vincent Van Gogh, is a swelling of the retinas.  This swelling supposedly creates the effect of glowing halos around light sources.  Some attribute Van Gogh’s unique color and movement perspectives to his mental illness, but in fact, documentation (mainly in the form of his journals and letters) has proved that he was nearly unable to paint when going through particularly difficult lapses in lucidity, and when he did the resulting tableaux were quite awful and lacking in his signature genius.