Lighting Problems Connected with the Construction of Half Domed Apses
It is a characteristic of the half dome that it exerts a large amount of downward pressure and but little outward thrust, particularly if it be of pointed section. For this reason, such a vault requires a firm support but only a slight amount of buttressing. As long, therefore, as the half dome rested directly upon comparatively low exterior walls, it had plenty of support, and it was even possible to pierce the walls with windows without endangering its stability. But with the increase in height of the more developed Romanesque churches and the introduction of ambulatories, it became difficult to light the sanctuary and still retain the half dome.
Two methods were evolved for overcoming this difficulty. The first, which may be seen in the abbey church at Cunault (Maine-et-Loire) (second half of twelfth century),[339] consisted in the construction of a lofty ambulatory opening into the apse through arches rising to the impost of the half dome, or even slightly above it, and resting upon piers of as slender proportions as possible, so that, although the sanctuary was deprived of all direct light, a certain amount was obtained from windows in the outer wall of the ambulatory or from the radiating chapels, while, at the same time, the vault of this ambulatory aided in the support of the apse and vice versa. Such a system, though structurally correct, was not entirely satisfactory. The sanctuary and choir were the portions of the church most in need of lighting, since they contained the altar and the seats for the monks or clergy by whom the services were chanted, and indirect light was bound to be insufficient.
The second method, which may be seen in the church of Saint Savin (Vienne) (eleventh century),[340] consisted in reducing the height of the ambulatory, even when this involved making it lower than the side aisles, and then placing a clerestory above the ambulatory arches beneath the springing of the half dome. This may be considered as the best type of apsidal termination developed during the purely Romanesque period. It was only when the half dome was discarded that a satisfactory solution was finally reached in the development of the chevet vault. There were, however, two important series of ribbed half domes, the second of which, at least, may have had some bearing upon the evolution of the chevet type.