“Sitting down, in a museum, can be an almost radical act: a refusal to flow along with the distracted crowd, idly passing by art as if it was just one more stream of visual enticement in a visually saturated world. A good sit is all about committing to the depth, not the breadth, of the art itself, seeing more by deciding to see less.” Philip Kennicott picks the finest spots in greater Washington to do just that.