Astronomer by Candlelight, Gerrit Dou
On Google Art Project; downloadable high-res file on Wikimedia Commons; original is in the Getty Museum.
A small gem (roughly 13×8″, 33x20cm) of 17th century chiaroscuro by an under-appreciated Dutch master.
For more, see this page on Essential Vermeer, and my previous Eye Candy post on a Gerrit Dou genre scene.
Link: Astronomer by Candlelight, Google Art Project