e are two thicknesses of
walling. The outer thickness has arched recesses at intervals along its
length, corresponding to openings in the inner thickness, and thus while
buttressing the latter also enlarges slightly the area of the church.
The length of the rectangular enclosure from west to east is 101 feet,
with an average breadth of 772-1/2 feet from north to south, excluding
the recesses in the latter direction.

All the windows of the church have been altered by Turkish hands, and
are rectangular instead of showing semicircular heads.

The passage intervening between the rectangular enclosure and the
octagon is divided into two stories, thus providing the church with an
ambulatory below and a gallery above.

The domed octagon which forms the core of the building stands at a
distance of some 18-1/2 feet from the rectangle within which it is
placed. It measures 53-1/2 feet by 50-1/2 feet. The eight piers at its
angles rise to a height of 33-1/2 feet from the floor to the springing
of the dome arches. The archways thus formed, except the bema arch, are
filled in with two pairs of columns in two stories set on the outer
plane of the piers. The lower colonnade is surmounted, after the classic
fashion, by a horizontal entablature profusely carved while the upper
columns are bound by arches, thus making seven sides of the octagon a
beautiful open screen of fourteen columns and as many triple arcades,
resplendent with marbles of various hues and rich with carved work. The
mass of the piers is relieved by their polygonal form, a fluted cymatium
along their summit, and a repeating design of a flower between two broad
leaves below the entablature. Though the flower points upwards it has
been mistaken for a cluster of grapes.[108] At the four diagonal points
the sides of the octagon are semicircular, forming exhedrae, an
arrangement which gives variety to the lines of the figure, widens the
central area, secures more frontage for the gallery, and helps to
buttress the dome. The same feature appears in S. Sophia, whereas in San
Vitale all the sides of the octagon, excepting the eastern side, are
semicircular. The extension of the interior area of a building (square
or octagonal) by means of niches at the angles or in the sides, or both
at the angles and in the sides, was a common practice.[109]

There is considerable difference in the size of the piers and the dome
arches. The eastern piers stand farther apart than their com