The dormer windows in the outer sheath not only broke the surface of the vault, but also served to light this passage to the lantern.

Vasari's description squares with the model, now preserved in a chamber of the Vatican basilica, and also with the present fabric.
It would not have been necessary to dwell at greater length upon the vaulting here but for difficulties which still surround the criticism of this salient feature of S.Peter's.

Gotti published two plans of the cupola, which were made for him, he says, from accurate measurements of the model taken by Cavaliere Cesare Castelli, Lieut.-Col.

of Engineers.

The section drawing shows three shells instead of two, the innermost or lowest being flattened out like the vault of the Pantheon.