This coffered dome has a diameter of 142 ft, exactly the same as the height from the floor to the middle of the dome. It is the largest unreinforced masonry dome ever built. The cassettes of the coffering number twenty-eight in each of five concentric circles, obviously getting smaller as they approach the oculus. The oculus, which provides the only natural light and air for the church once the door is shut, is 30 ft in diameter.