Library Eusebius of Caesarea Life of Constantine
Book 3
Chapter 38: Description of the Hemisphere, the Twelve Columns, and their Bowls
Opposite these gates the crowning part of the whole was the hemisphere, 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.
English translation by Ernest Cushing Richardson, in Nicene and Post-Nicene Fathers, Series II, Vol. I (eds. Philip Schaff and Henry Wace; Buffalo: Christian Literature Publishing Co., 1890). Public domain. Digital text via New Advent Church Fathers.