During the second world war, the collection of the National Gallery had to be hidden in a mountain in Wales to prevent bomb damage. Its director, Kenneth Clark, eventually realised, however, that this was bad for morale, and so made a single but striking exception. Starting with Rembrandt’s ‘Portrait of Margaretha Trip’, which the gallery had just acquired, he ensured that each month one famous painting would be on display in an alcove at the top of the main staircase.