 The outer perimeter of the cupola measures exactly 144 Carolingian feet whereas that of the heavenly Jerusalem, ideal city drawn by angels, is of 144 cubits.The mosaic of the cupola, hidden today behind a 19th-century restoration, showed Christ in Majesty with the 24 elders of the Apocalypse.Other mosaics, on the vaults of the aisle, takes up this subject by representing the heavenly Jerusalem.