Dan Flavin's art is amazingly beautiful - but is it anything more? Certainly his installations are ecstasy for the eyes. Moving among these arrays of dazzling colour, these sheaves of glowing poles, this radiance drifting out across the gallery, light contained and then set lyrically free, you wonder how it is possible that so much pleasure could emit from such a dismal source: the cold fluorescent tubes of strip lighting. Now showing at the Hayward Gallery, London.