For a general view of the vaulted system of S. Sophia we would especially refer to Choisy, whose remarks on the construction of these vaults are most interesting. He clearly shows how the large flat bricks made possible the construction of vaults without centring. The extrados of the arches from which the vaults spring being splayed to a skew back, the large surfaces of the thin light bricks allowed them to be stuck up against this skew back, or any part already done, much as if they were square sheets of cardboard (see left side of Fig. 40). Indeed the bricks seem sometimes to have been placed quite vertically, but the better plan seems to have been to incline the beds, the vaults were thus built in _sections_ rather than in layers. To take the simplest instance, a cylindrical vault, the arching would begin at one end against the vertical wall, the rings of large thin bricks being placed “on edge” in planes of say 60° right down the vault. In other words, in a longitudinal section of such a vault the joints instead of being horizontal might be vertical, or a mean between the two. This method was known in ancient Egypt and at Khorsabad, and the immense vault at Ctesiphon is built in this way. Although the mosaic covers most of the vaults at S. Sophia, a vast number are exposed in the contemporary cisterns, and Choisy seems to have found a cylindrical vault uncovered in a chamber in one of the buttress masses (Plate ii.), he also shows the construction of the aisle and narthex vaults (Plates ix. and xi.), but he does not say if he had any authority for these. We agree with him that the vaults of S. Sophia owe much of their exceptional beauty to the fact that arches do not break up the curving expanse of the vaulting to any appreciable degree; in the narthex the arches become one with the vault, see Fig. 41. [Illustration: FIG. 41.--Section of Narthex and Gallery over showing Royal Doors. Scale twelve and a half feet to an inch (1/125).] [Illustration: FIG. 42.--Dome Construction.]