The north and south semi-domes are each pierced by three large windows, which on the interior cut through the curved surface of the domes, and on the exterior appear as dormers in the roof above the cornice. Accordingly they are double glazed, with one glazed frame on the inside corresponding to the curved dome surface, and a second upright glazed frame on the outside. The roofs are covered with lead. The central dome is circular inside, with a high drum pierced by eight windows. On the outside it is octagonal, with a window on each side. These have circular arched heads, but have no moulding, shaft, or inset to either arches or sides. The dome is crowned by a moulded stone cornice of the same type as that of the other walls. In attempting to reconstruct the original form of the church we may first note those features which are evidently Turkish. None of the exterior masonry is Byzantine, as the use of polished ashlar with fine joints, of pointed arches, and of moulded stone cornices clearly proves. The absence of shafts at the angles of the dome drums and the unrecessed windows are additional proofs of this fact, and we may conclude that the entire exterior was refaced in Turkish times. The diagonal arches under the north and south semi-domes are peculiar. Furthermore, in lobed Byzantine churches the lateral apses project beyond the square outer walls. Here they are contained within the walls.[177] Nor are the semi-domes themselves Byzantine in character. The large windows in the dome surface and the lead-covered dormers placed above the flat moulded cornice betray a Turkish hand; for windows in the dome are universal in the great Turkish mosques, and the method of protecting them on the exterior with wooden dormers is quite foreign to Byzantine ideas. The form of the drums and cornices should be compared with the minor domes of the mosque of Sultan Bayazid. A careful examination of the building has led to the following conclusions. The lateral semi-domes with their supporting arches are a Turkish addition. The central dome, including the drum, is probably entirely Turkish, and takes the place of an original ribbed dome. The two easternmost domes in the north 'aisle' and those over the inner narthex and the prothesis are also Turkish, and, as already stated, the exterior of the entire building. On the other hand, the eastern apse, the dome arches, the arcade, and the windows above it on the west side of the dome, the inner narthex with the ground vault to the south of it, and the entire outer narthex, are parts of the original building, dating probably from the sixth or seventh century. It should be particularly noticed that the windows over the western dome arcade are circular-headed inside, though they have been provided with pointed heads on the outside in the process of refacing.