"To judge from the combination of coloured stones still remaining in this edifice, the effect must have been very rich and beautiful. The elaborate capitals and bases of white marble must have formed a fine contrast to the yellow shafts of the pillars and the stripe of porphyry inserted in the architrave. The largest specimen of this coloured mode of decoration has been preserved in the pavement; although here also we must take it for granted that the original arrangement has been disturbed, the sunken bases of the columns sufficing to prove that the pavement has been raised in course of time. This circumstance is not without optical reaction on the proportions of the different masses. The horse-shoe arch over the entrance-door is remarkable. It forms a striking contrast to that of the tribune, where the projecting cornice rests upon two pillars, whereas the architrave, broken through by the doorway, is supported only by pilasters. "The ædiculæ, now converted into altars, are covered in, partly with gables, partly with arches, the former resting upon fluted pillars of yellow marble, the latter upon porphyry pillars. The walls behind are likewise faced with slabs of coloured marble, which, in their original splendour, must have reflected the magnificence of the pillars. "The facing of the door is the only considerable portion still remaining of the rich bronze-work with which this edifice was formerly fitted up. Simple as the decoration of these massive doors now appears, it is yet imposing for such persons as are capable of appreciating pure symmetry and a judicious distribution of the parts in surfaces so extensive. The nails, with heads in the form of rosettes, separating the different panels, are the only ornament. The window above the door is closed by a grating composed of curves placed one above the other, thus admitting both light and air. The destruction of the bronze cross-beams which formed the roof of the vestibule till the time of Urban VIII., is most to be regretted. This was composed of bronze tubes, on precisely the same principle as that on which Stephenson, a few years ago, constructed the bridge over the Menai Straits. "The cupola is nearly seventy feet in height, and rests on the attica, corresponding to the second outer belt. This attica has suffered most severely from modern alterations. The walls behind this afford space for a series of chambers. The massive wall of the third belt, on the other hand, surrounding the cupola to a third of its height, is rendered accessible by a passage running round the whole; and this again is spanned by frequently recurring arches, and lighted by the windows visible on the outside.