Anthony McCall has been making art with light for decades, and he is enjoying a recent revival. An exhibit at the Hamburger Bahnhof features a large dark room full of lights that beam out of walls or down from the high ceiling. The room is also filled with smoke, which catches the light, creating conical forms that viewers can penetrate and explore. And that's the most interesting part. We all know how to view conventional paintings and sculptures, but the McCall exhibit requires the spontaneous generation of new viewing strategies, and each of invented mode of interaction is also an intervention, changing the form of the piece for others to see.