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 à toutes les voutes byzantines un trace géométrique rigoureusement défini."[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 groins, accordingly, lose themselves in the vaulting surface. This form is found in Greek churches of late date, but does not occur in the later churches of Constantinople. A full description of the form and construction is given by Choisy[31] and by Lethaby and Swainson.[32] The cross-groined vault as found in the Myrelaion and many other churches of the city is level in the crown, with clearly marked groins. It is sometimes used with transverse arches resting on pilasters, or without these adjuncts. One of the most interesting of the vault forms is the dome-vault, a shallow dome with continuous pendentives. It is distinguished in appearance from the groined vault, as found in S. Sophia, by the absence of any groin line, and is completely different in construction.