MASONRY IMPLICATIONS FOR THE DESIGN OF THE OCTAGON SUITE
Besides the dome itself, the most important architectural features of the Octagon Suite are the vault haunch clerestory windows over the extrados of the dome and the groin vaults in Rooms 123 and 125. The groin vaults are the earliest known examples of this motif in Roman concrete, discussed in greater detail in Section 2 of this chapter, but the vault haunch clerestory windows are more informative as far as the masonry chronology of the Esquiline Wing is concerned. They are integral to the whole design concept of the Octagon Suite, both because they bond to all surrounding masonry and because they play an important rôle in the lighting system. They not only light the radiating rooms, but also help lighten the appearance of the dome when viewed from inside Room 128 by letting light stream into the radiating rooms above the vault. They give the viewer in Room 128 the sense that there is nothing above the vault at all, disguising the fact that that is where the structural system for the vault actually is.
Except for the groin vaults, most major features of the Octagon Suite have to do with lighting. The visual effect of a feather-light dome, appearing to hover in a sea of light all around it, is almost mystical. It is similar in visual impact to trompe l'oeuil painting, clever and wonderful.