A careful survey of the building shows clearly that the domical character of the chapel is not original, and that the structure when first erected was a simple hall covered with a wooden roof. Both the shallow wall arches and the deep transverse arches under the dome are insertions in the walls of an older fabric. They are not supported on pilasters, as is the practice elsewhere, but rest on corbels, and, in order to accommodate these corbels, the lateral niches, originally of the same height as the central window, have been reduced in height. A fragment of the original arch still remains, cut into by the wall arch of the dome. The flat secondary arches crossing the chapel at each end are similarly supported on corbels.
This view is confirmed by the examination of the plaster left upon the walls. That plaster has four distinct coats or layers, upon all of which eikons in tempera are painted.[499] The innermost coat is laid between the transverse dome arches and the walls against which they are built. Those arches, therefore, could not have formed parts of the building when the first coat of plaster was laid, but must be later additions.
In keeping with this fact, the second coat of painted plaster is found laid both on the arches and on those portions of the old work which the arches did not cover.
The secondary arches under the transverse arches at each end belong to a yet later period, for where they have separated from the arches above them, decorated plaster, which at one time formed part of the general ornamentation of the building, is exposed to view. At this stage in the history of the chapel the third coat of plaster was spread over the walls, thus giving three coats on the oldest parts where unaltered--two coats on the first alterations, and one coat on the second alterations. The fourth coat of plaster is still later, marking some less serious repair of the chapel.
The _voussoirs_ of the lateral dome arches should be noticed. They do not radiate to the centre, but are laid flatter and radiate to a point above the centre. This form of construction, occurring frequently in Byzantine arches, is regarded by some authorities as a method of forming an arch without centering. But in the case of the lateral wall arches before us it occurs where centering could never have been required; while the apse arch, where centering would have had structural value, is formed with true radiating _voussoirs_. The failure of the _voussoirs_ to radiate to the centre therefore seems to be simply the result of using untapered _voussoirs_ in which the arch form must be obtained by wedge-shaped joints. For if these joints are carelessly formed, the crown may very well be reached before the requisite amount of radiation has been obtained. On the other hand, if full centering had been used, we should expect to find marks of the centering boards on the mortar in the enormously thick joints. But neither here nor in any instance where the jointing was visible have such marks been found. Still, when we consider the large amount of mortar employed in Byzantine work, it seems impossible that greater distortions than we actually meet with in Byzantine edifices would not have occurred, even during the building, had no support whatever been given. It seems, therefore, safe to assume the use of at any rate light scaffolding and centering to all Byzantine arches.[500]