[Illustration: FIG. 21.--VIEW OF THE EXTERIOR OF THE DOME OF SS. SERGIUS AND BACCHUS.] On the other hand the eight compartments directly above the angles of the octagon are narrower than the preceding compartments; they have no windows, and, what is of special importance to note, they are deeply concave.[115] Such marked hollowness is found in later domes as a decorative feature, but here it is primarily and supremely a constructive device. By its means the concave compartments are set slightly back from the octagon's inner face, leaving, at the springing line, portions of the wall-head to appear as little flat ledges on each side of the angles. This is a most skilful expedient, and compares favourably with the methods employed elsewhere to apply the dome to the octagon.[116] In the octagonal church of S. Lorenzo at Milan the octagon is turned into the circle by the introduction of squinches. In San Vitale a considerable walling is built between the line of the octagon and the springing line of the dome, while the bed for the dome is formed by introducing, in the space over the angles of the octagon, niches which are worked above to the circle on plan. On the other hand, it is interesting to compare with these methods the method employed in the baptistery of S. Sophia, now a Sultan's Turbé, near the southern entrance to the inner narthex. Although the walls of the building describe a square on the exterior, they form an octagon on the interior with semicircular bays at the diagonals, as in SS. Sergius and Bacchus. But in the application of the dome the true pendentive is used. The baptistery was erected shortly before S. Sophia, and in view of the erection of the great church. The curvature of the dome of SS. Sergius and Bacchus has three zones, which have respectively a radius of m. 8, (drawn from the centre of the octagon), m. 3-1/4, and m. 9-1/2, (centre about m. 2, below the springing of the dome). The first extends to a point a little above the heads of the dome windows; the second about m. 2 higher; the third to the crown of the dome. The groins stop short a little below the dome's apex, where they are arched into one another, leaving a saucer-shaped crown now capped by a Turkish finial. The dome is covered with lead, and presents an undulating surface owing to the protuberance of its eight concave compartments.[117] The system of weighting and buttressing the dome displays great skill, and will be best understood by studying Mr. Henderson's geometrical and constructive sections of the systems (Figs. 28, 29). [Illustration: FIG. 22.--BRICK STAMPS IN SS. SERGIUS AND BACCHUS. (From rubbings by Mr. A. E. Henderson.) _At east end of south aisle._ _In the gallery._] [Illustration: FIG. 23.] [Illustration: FIG. 24.] [Illustration: FIGS. 25, 26, AND 27.] [Illustration: FIG. 28.] [Illustration: FIG. 29.] [Illustration: FIG. 30.] [81] Procop. _De aed._ i. c. 4; Banduri, iii. p. 45. [82] _De top._ ii. c. 14.