The trouble with the National Gallery's exhibition, "Radical Light", is that it is dominated by Gauguin and Signac, Seurat, Millais and Courbet. Not that any of these artists is actually in the show. Instead, we are given approximate work by a group of painters with names like Pellizza, Morbelli and Segantini, of whom, unless late 19th-century Italian painting happens to be your bag, you may never have heard, and who it is unlikely you will want to hear of again.