Velazquez painted so few pictures it is rightly said that he measured his genius in thimblefuls. And until recently it seemed that no new paintings would be found. But they are gradually turning up all over the world, even in the Metropolitan Museum in New York in 2010, hanging on the walls in plain sight. In this lecture, Laura Cumming, Observer art critic, asks whether something about the way that Velazquez painted has led to these mistakes and revelations, looking deep into the mystery and modest of his art for answers.