The National Gallery has in recent years made a speciality of examining the hitherto unexamined. Just for starters, a surprise hit some years ago was Spanish Still Lifes, 2007 saw Renoir Landscapes (who knew?), last year there was the ravishing Christen Kobke, star of the Danish Golden Age, and just this spring New York’s Ashcan School, all with committed scholarship throwing light on the internationally disregarded.