204 COMPARATIVE ARCHITECTURE, Some bear the monogram of Justinian, and on a column to the south exedra on entering is the date 534. A variation of the dosseret block is in general used on the lines of the Classical abacus. The lighting is partly effected by forty small windows piercing the dome at its base. Additional light is introduced through twelve windows in each of the spandrel walls, north and south, under the great arches which support the dome. The bases of the domes of the smaller exedra^ are also provided with windows. Many of the windows are small and spanned by semicircular arches ; others are more elaborate, as in those to the " Gynaeceum," or women's gallery, reached from the exterior by four gently sloping ascents, one at each corner of the building, and from the interior by stone staircases, in which large semicircular headed openings are divided into six by columns in two heights, the lighting area being filled with lattice work of marble 3 inches thick, pierced with openings about 7 inches square, filled with glass. Externally the walls are faced with brick and stone in alternate courses. The vaulting of the domes and semi-domes is visible, being covered with lead ^-inch thick, resting on wooden battens placed immediately on the brick vaults. The immense buttresses already referred to make imposing external features, as also the two great spandrel walls between them, deeply recessed from their face, and provided with windows lighting the central area. The plainness of the exterior causes the building to depend for effect entirely on the massiveness of its form and the general symmetry of its proportions. S. Sophia is the masterpiece of Byzantine architecture as the Parthenon is of Greek, or the Pantheon of Roman ; but neither in plan nor treatment does it seem to have been largely imitated, especially in respect of the abutting semicircular domes. S. Irene, Constantinople, originally constructed by Con- stantine and several times destroyed and rebuilt, finally about A.D. 740, is interesting as preserving the Basilican plan of nave and two aisles with Eastern apse and Western atrium. It has a dome which is believed to be the earliest example, resting on a high drum pierced wnth windows to light the interior. The Theotokos Church, Constantinople, dating from the ninth to the twelfth century, is a small but perfect example, having a double narthex crowned with three domes, and a central dome over the church itself. The Church of the Chora, Constantinople, is an interest- ing example, dating originally from the fourth century, but subse- quently much altered. It has a central area crowned with a dome resting on a drum 26 feet in diameter, pierced by windows, and has semicircular windows on three sides, and an apse on the fourth. It has an inner and outer narthex, ornamented with
