The only remaining members whose profiles call for examination are mullions and tracery. These are very simple in outline. The earliest form of mullion—a, Fig. 139, from St. Leu d'Esserent[1]—is a plain column of stone simply bevelled both outside and inside. This form is very appropriate in connection with the plain pierced tympanum of St. Leu; but in connection with true tracery, and as a part of an open framework, whose function is to support the glass of an opening through which as much light as possible must be allowed to pass, it is not a good form. The strain upon a mullion is in one direction only—that which is exerted by
FIG. 139.
the force of winds pressing inwards,—it accordingly requires to be deep in this direction. But in order that it may as little as possible obstruct the passage of light, it ought to be as narrow as it consistently may. In fact, the conditions which it has to meet are substantially the same as those which have determined the forms of the members of an ordinary window sash. These conditions were fully recognised by the designer of the tracery of the apsidal openings of the Cathedral of Reims, and the mullion section b, Fig. 139, from those openings,[2] established the typical Gothic form, the principal modifications of which were such only as the exigencies of more complex tracery called for. The
