The Turbine Hall of Tate Modern is plunged into deep black gloom. At its east end, like the stained glass window of a cathedral, is a giant vertical screen. It is framed at the edges with sprocket holes, so we feel we are looking at a vast reel of film. In the centre, an ever-changing series of images: a snail on a wind-wobbled leaf, the powerful spume of a fountain, a chimney loosing trails of vapour. Sometimes the image is of the back wall of the Turbine Hall itself, but with its gridded form coloured in red, yellow and blue so it resembles a Mondrian. Or with a giant egg apparently floating from ceiling to floor.