After four years of construction, it’s finally happening: The impressive new Princeton University Art Museum will open Oct. 31 right in the heart of campus. Did you know that the new museum has concrete walls that were cast in place? Do you know why the Roman mosaics were installed under glass floors, or how much of the collection resides in the museum itself? Or how much admission costs? (Spoiler alert: It’s free.) Visit paw.princeton.edu for a sneak peak of photos and 10 things to know about t