entire spaces between the piers, the now growing art of painting on glass led to the universal practice of making them so. Vast and resplendent colour-designs in glass, softening the light and affording a grateful warmth of effect, thus became the leading mode of enrichment of the interior by colour.
The tracery by which these great openings were divided was constructively necessary to support the expanses of enclosing glass against the force of winds; and the greater the area of the opening the larger was the number of tracery bars required to afford this support. Hence the extensively subdivided tracery of the vast apertures of fully developed Gothic buildings—like the Cathedral of Amiens—which, by its very nature, afforded a beautiful mode of enrichment and adornment of the windows.
The enlargement of the clerestory opening, to the extent of doing away with the wall entirely beneath the vault rib, resulted in an important simplification of the construction, of which we have, I think, the first instance in the clerestory of the nave of Amiens, where the longitudinal rib and the archivolt of the opening, as has been pointed out by M. Viollet-le-Duc, become one and the same member, while the longitudinal rib shaft becomes a member of the window jamb.
As before noticed, the clerestory and triforium in the nave of Amiens are united into one grand composition by the descent of the longitudinal rib shafts and the shaft of the central mullion to the level of the triforium string. These three members divide each bay of the triforium into two bays, in each of which is a pointed arch encompassing a sub-order of three pointed arched openings. The same arrangement is repeated, with minor variations, in the nave of St. Denis. In this case each of the three mullions of the clerestory window descends through the triforium, dividing it into four bays—an arrangement which gives to the triforium a somewhat unpleasant effect by making its main divisions to consist of a series of upright rectangles.
The triforium passage in France, when it is not a vaulted gallery, is always enclosed by a thin screen of masonry, hiding the open space between the aisle vaults and the timber roof over them. Nothing but masonry is, therefore, to be seen in
