The paintings are sublime of course. But visitors to the exhibition have to keep an  appalling and upsetting fact at the forefront of their mind: that although Leo Stein has been called 'arguably the greatest collector of early modern art in the C20th' (in a very good New York Times review by Andrea Barnet of the 1996 book about Leo and Gertrude, Sister Brother by Brenda Wineapple) and built up the Stein collection between 1903 and 1914: after the rupture between the two of them and the dispersal of the collection, they never spoke again. It is really impossible to look at these incredible paintings without feeling unutterably depressed about that fact. Especially as some of us have very, very mixed feelings about Gertrude's importance as a writer. As Andrea Barnet puts it: 'History, of course, has a way of sorting things out. Leo Stein would be remembered, in the words of Alfred H. Barr Jr., the first director of the Museum of Modern Art, as ''possibly the most discerning connoisseur of 20th-century painting in the world.'' Gertrude, Ms. Wineapple writes, would become ''one of the most well-known literary personages people hadn't read.'' Leo bought Matisse's Femme au chapeau (normally at the San Francisco Moma but temporarily in Paris) in 1905.