but it often happens that works on architecture are of little use, because they afford the reader no means of judging of the effective intention of the arrangements which they state. No person, looking at an architectural drawing of the richly foliaged cusps and intervals of Or San Michele, would understand that all this sculpture was extraneous, was a mere added grace, and had nothing to do with the real anatomy of the work, and that by a few bold cuttings through a slab of stone he might reach the main effect of it all, at once. I have, therefore, in the plate of the design of Giotto, endeavoured especially to mark these points of purpose; there, as in every other instance, black shadows of a graceful form lying on the white surface of the stone, like dark leaves laid upon snow. Hence, as before observed, the universal name of foil applied to such ornaments.
XIX. In order to the obtaining their full effect, it is evident that much caution is necessary in the management of the glass. In the finest instances, the traceries are open lights, either in towers, as in this design of Giotto's, or in external arcades like that of the Campo Santo at Pisa or the Doge's Palace at Venice; and it is thus only that their full beauty is shown. In domestic buildings, or in windows of churches necessarily glazed, the glass was usually withdrawn entirely behind the traceries. Those of the Cathedral of Florence stand quite clear of it, casting their shadows in well detached lines, so as in most lights to give the appearance of a double tracery. In those few instances in which the glass was set in the tracery itself, as in Or San Michele, the effect of the latter is half destroyed: perhaps the especial attention paid by Orgagna to his surface ornament, was connected with the intention of so glazing them. It is singular to see, in late architecture, the glass, which tormented the bolder architects, considered as a valuable means of making the lines of tracery more slender; as in the smallest inter-
