When it was unveiled in 1533, the double portrait The Ambassadors by the German and Swiss artist Hans Holbein the Younger must have struck observers as a road-bump for the soul. Looked at straight-on from the front, the huge oil-on-oak painting is enigmatic enough, presenting to the observer a pair of distinguished diplomats caught in a clutter of worldly amusements: musical instruments and scientific whatnots scattered about the shelves on which they lean.