he centre of the apse, the side and mullions are placed parallel to
the axis of the church, thus obviating all difficulty. Generally the
centre to which the mullions radiate is considerably beyond the apse, so
that any necessary little adjustment of the arch could easily be made.

Triple windows supported on circular columns are not infrequent in the
north and south cross arms. Sometimes the central light is larger than
the lateral lights, at other times, as in the Pantepoptes, the three
lights are equal. The lower part of these windows was probably filled in
with a breastwork of carved slabs, as in S. Sophia, while the upper part
was filled by a pierced grille. At present the existing examples of
these windows have been built up to the abaci of the capitals, but in
the church of S. Mary Diaconissa (p. 186) the columns still show the
original form on the inside.

_Vaulting._--All Byzantine churches of any importance are vaulted in
brick. The only exception to this rule in Constantinople is the little
church known as Monastir Mesjedi (p. 264). The different systems of
Byzantine vaulting have been so fully treated by Choisy and other
authorities, that in the absence of any large amount of new material it
is not necessary to give here more than a few notes on the application
of these systems in Constantinople. It should always be kept in view
that, as these vaults were constructed with the lightest of centering,
the surfaces and curves must have been largely determined by the mason
as he built, and would not necessarily follow any definite geometrical
development. "Il serait illusoire," remarks Choisy, "d'attribuer a
toutes les voutes byzantines un trace geometrique rigoureusement
defini."[30]

[Illustration: PLATE II.
(1) THE MYRELAION (SINCE IT WAS BURNED), FROM THE NORTH-WEST.
(2) THE MYRELAION (SINCE IT WAS BURNED), FROM THE SOUTH-EAST.
(By kind permission of H. M. Dwight, Esq.)
_To face page 20._]

The vaults commonly found are the barrel vault, the cross-groined vault,
and the dome-vault. The first is frequently used over the cross arms and
the bema, and sometimes over the narthex in conjunction with the groined
vault (Diaconissa). It is the simplest method of covering an oblong
space, but it does not easily admit of side windows above the springing.

A very beautiful form of cross-groined vault is found in S. Sophia and
in SS. Sergius and Bacchus, in which the crown is considerably domed,
and the gro