Contemporary with this was the Church of the Apostles, also erected by Justinian, but now destroyed. It showed, however, according to the description of it by Procopius, an immense advance upon that last named; for, while in one the dome was carried by an octagonal wall, thus showing no practical advance upon the antique form of dome, in the latter a vast cruciform building was covered by five domes, which is just the advantage which the pendentive system affords; for, when the base of a dome is cut into a square by four arches, those arches may aid in the support of other domes beyond, and thus any space may be covered over by a series or a group of domes. This last church, then, was the true type of advanced domical structure. The great glory, however, of this age, and of domical structure of this class (for it has never again been equalled), is the Church of St. Sophia, or of Sacred Wisdom, erected by the same Emperor as the Metropolitan or Patriarchal Cathedral of the Eastern Church. The plan of this church differs in ideal, and yet more in fact, from the contemporary church last referred to. The ideal of each is a cross with a central dome. The difference is that in the Church of the Apostles the limbs of the cross were each covered by a complete dome of equal dimensions with the central one (Fig. 422), while those of St. Sophia are covered each by a semi-dome only of equal diameter with the arches carrying the central dome (Fig. 423); so that if we consider the latter to spring from the top of its pendentives, which it in effect does, its springing is on a level with the crown of the surrounding semi-domes. [Illustration: Fig. 422. Church of the Apostles.] [Illustration: Fig. 423. St. Sophia.] In reality, however, this idea is not carried out to completion, as only two of the semi-domes have been erected, the other two arches of the central dome being filled in with an arcade in many storeys. This incompleteness, however, is greatly more than compensated--firstly, by the vastness of the scale--the central dome, if measured on the diagonal, being 150 feet in diameter--and, secondly, by other semi-domical projections branching out from the walls which support the great semi-domes, three from each, excepting that on the western side, one is devoted to the entrance, and is not domed. Even these secondary projections are mostly arcaded, so as to allow the eye to pass onward into a yet inner chamber (Fig. 424). So that, simple as is the primary ideal, the actual effect is one of great intricacy and of continuous gradation of parts, from the arcades last alluded to, up to the stupendous dome which hangs with little apparent support, like a vast bubble, over the centre, or, as Procopius, who witnessed its erection, described it, “as if suspended by a chain from heaven.” [Illustration: Fig. 424.--Plan, St. Sophia, Constantinople. (_From Fergusson._)]