XV. The entire front is composed of an upper and lower series of arches, enclosing spaces of wall decorated with mosaic, and supported on ranges of shafts of which, in the lower series of arches, there is an upper range superimposed on a lower. Thus we have five vertical divisions of the façade; _i.e._ two tiers of shafts, and the arched wall they bear, below; one tier of shafts, and the arched wall they bear, above. In order, however, to bind the two main divisions together, the central lower arch (the main entrance) rises above the level of the gallery and balustrade which crown the lateral arches. The proportioning of the columns and walls of the lower story is so lovely and so varied, that it would need pages of description before it could be fully understood; but it may be generally stated thus: The height of the lower shafts, upper shafts, and wall, being severally expressed by _a_, _b_, and _c_, then _a_:_c_::_c_:_b_ (_a_ being the highest); and the diameter of shaft _b_ is generally to the diameter of shaft _a_ as height _b_ is to height _a_, or something less, allowing for the large plinth which diminishes the apparent height of the upper shaft: and when this is their proportion of width, one shaft above is put above one below, with sometimes another upper shaft interposed: but in the extreme arches a single under shaft bears two upper, proportioned as truly as the boughs of a tree; that is to say, the diameter of each upper = 2/3 of lower. There being thus the three terms of proportion gained in the lower story, the upper, while it is only divided into two main members, in order that the whole height may not be divided into an even number, has the third term added in its pinnacles. So far of the vertical division. The lateral is still more subtle. There are seven arches in the lower story; and, calling the central arch _a_, and counting to the extremity, they diminish in the alternate order _a_, _c_, _b_, _d_. The upper story has five arches, and two added pinnacles; and these diminish in _regular_ order, the central being the largest, and the outermost the least. Hence, while one proportion ascends, another descends, like parts in music; and yet the pyramidal form is secured for the whole, and, which was another great point of attention, none of the shafts of the upper arches stand over those of the lower.