Dome System of Hagia Sophia
"Dome-System of the Church of Sta. Sophia at Constantinople. The Byzantine style of this first period reached its highest example and splendor in the church of Sta. Sophia at Constantinople. After this church, which was originally built by Constantine, had been burnt to the ground, it was rebuilt, with the utmost care, and at great expense, by Justinian. On each of he open sides of the dome-covered square there abuts a semicircle with semi-domes of somewhat inferior height to the main dome." This diagram shows how the semi-domes are penetrated on each side by three smaller vaults.
A. Rosengarten, W. Collett-Sandars A Handbook of Architectural Styles (New York, NY: Charles Scribner's Sons, 1895)
Courtesy the private collection of Roy Winkelman