Nicene and Post-Nicene Fathers, Ser. II, Vol I:
The Life of Constantine with Orations of Constantine and Eusebius.: Chapter XXXVIII

Chapter XXXVIII.—Description of the Hemisphere, the Twelve Columns, and their Bowls.

Opposite these gates the crowning part of the whole was the hemisphere, 3270 which rose to the very summit of the church. This was encircled by twelve columns (according to the number of the apostles of our Saviour), having their capitals embellished with silver bowls of great size, which the emperor himself presented as a splendid offering to his God.

Footnotes

[Apparently the altar, which was of a hemispherical, or rather hemicylindrical form—Bag.] Also a much-discussed question. Compare Heinichen, vol. 3, p. 521–522.