Over the square center of the Greek cross a round dome is elevated on a high drum, which, in turn, rests, at the intersection of the cross arms, on four arches and pendentives. Gaulli likewise painted the cupola, in which is represented the Vision of the Glory of Heaven. This included three great concentric rings, the lowest filled chiefly with Jesuit saints and martyrs, the next devoted to an angelic concert, and the last composed of cherubs miraculously bearing aloft the lantern, in which appears the dove of the Holy Ghost.

(Another photo of the dome fresco here)