In an arched, window-like opening, an astronomer works late into the night. Concentrating, he reads an astronomical treatise and measures the distance between two points on a celestial globe. The candle he holds provides the only source of light; it illuminates his face, the book, the precious globe, the beaker of water, and his hourglass. Strong contrasts between light and dark show the painter Gerrit Dou's knowledge of a popular Baroque technique known as chiaroscuro . Carefully rendering minute details such as the handwriting in the book, the texture of the glass flask, and the folds in the astronomer's cloak was Dou's special talent. Collectors treasured these small, detailed paintings for pleasurable observation in their private rooms.