The domical arrangement of both these churches may have been originally derived from the =Pantheon=, modified by the example in Rome, of what is called the =Temple of Minerva Medica=, though it was probably a nymphæum. This building is decagonal with niches projecting from nine of the sides, while the tenth provides the entrance. The dome, of concrete ribbed with tiles, is built over an inner decagon of ten piers carrying ten arches. These in turn support a decagonal drum, pierced with windows, the angles at the top being filled in with rudimentary pendentives. The same principle of construction reappears in both =S. Sergius= and =S. Vitale=; the dome of the latter being composed, for the sake of lightness, of earthenware, amphora-shaped pots, the bottom of one being fixed in the lip of another. It is sheathed on the outside with a wooden roof. This =Church of S. Vitale= became the model on which Charlemagne based his domical church at =Aix-la-Chapelle=, which was built as a royal tomb, <small>A.D.</small> 796-814, and was afterward used as the crowning-place of the Emperors of the West. =S. Sophia.=--Finally, the pendentive system was fully developed in Justinian’s church in Constantinople dedicated to the =Holy Wisdom--Hagia Sophia=, called, though erroneously, =S. Sophia=. It marks the highest development of the Byzantine genius for domical construction. The architects were Anthemius of Tralles and Isidorus of Miletus, who began the work in 532 and finished it in 537. The plan shows four mighty piers, 25 feet square, set at the angles of a square of 107 feet. These support four arches and intermediate pendentives of noble height, the apex of the dome being 175 feet from the pavement. For the original dome, having collapsed in 555, was replaced by a higher one, lighted by the introduction of forty circular-headed windows around the spring of the curve; an arrangement not only excellent in admitting light to the interior, but also as wonderfully impressive in its way as the single eye of the =Pantheon=. Rows of small circular headed windows are also pierced in the screens which fill in the north and south arches. Abutting on the east and west arches of this central mass are semi-domes, supported upon the central piers and two others. And from these project, as in =S. Sergius= and =S. Vitale=, small semicircular domes, sustained by an upper and lower story of arcades. Thus was created a vast oval-ended hall, 267 feet long by 107, from every part of which the summit of the dome is visible.