allienus, A.D. 366) is the next dated example. The Nymphaeum was
decagonal on plan, so that small pendentives were required to carry the
brick dome.

The domed Laconicon of the Thermae of Diocletian (A.D. 302) still exists
as the vestibule of the church of Santa Maria degli Angeli. Of
Constantine's time there are two small domed examples in the tomb of S.
Costanza and the Baptistery of the Lateran, both in Rome, and one in the
tomb of Galla Placidia at Ravenna (c. A.D. 450). From these we pass to
the Sassanian domes at Serbistan and Firuzabad, of the 4th and 5th
centuries respectively. These were built in brick and rested on square
pendentives. In section they were ovoid. In Syria, the dome over the
octagonal church at Esra, built in stone and dated A.D. 515, is also
ovoid, its height being equal to its diameter, i.e. 28 ft. This, as well
as the Sassanian domes, was built without centring. The next example is
that of the church of Sta Sophia at Constantinople, the finest example
existing, both in its conception and execution. It was built by
Justinian (537-552) from the designs of Anthemius of Tralles and
Isidorus of Miletus. The dome is 104 ft. in diameter, and is carried on
pendentives over a square area. The construction is of brick and stone
in alternate courses, and the lower part of the dome is pierced with
forty windows, which give it an extraordinary lightness. The height from
the pavement of the church to the soffit of the dome is 179 ft. No dome
of similar dimensions was ever again attempted by the Byzantine
architects, and the principal difference in later examples was the
raising of the dome on a circular drum pierced with windows.

In order to lighten the dome erected over the church of San Vitale, at
Ravenna, it was constructed with hollow cylindrical jars, fitted, the
end of one into the mouth of the other; a similar contrivance was
adopted in the tomb of the empress Helena (the Torre Pignatiara), the
vaults of the Circus of Maxentius on the Via Appia, and the outer aisles
of San Stefano, all at Rome, thus dispensing with the buttresses of Sta
Sophia.

The domes of the earlier mosques in Cairo were built on the model of Sta
Sophia, with windows pierced round the base of the dome and external
buttresses between them; these domes were all built in brick coated over
with cement or stucco. At a later date, and when built in stone, the
upper portion was raised in height and terminated with a point on which