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Rh A nave arcade, for instance, was often planned with a more or less square pier and then a column alternately. In some German churches square piers alternately wider and narrower may be seen.

The pointed arch has been known from time immemorial. It was generally adopted by Saracen builders from the seventh century, and it became well known in the West from the eleventh. It proved especially useful in adjusting the many difficulties which arose in applying vaulting to compartments of various sizes and shapes. And further, it was used as a strong structural form before it was generally admitted into the architectural code. Thus, as ever, the aesthetic delight of one century was found in the structural device of an earlier one.

The cusping of arches fell in with the general tendency toward subordination and grouping. The cusped arch had a distant origin in the shell forms carved in the arched heads of the niches, which were common in Hellenistic architecture. Byzantine and Arab builders simplified the scalloped edge of this shell into a series of small lobes set within the containing arch. Such cusped arches were passed on to the North-West by the Moors. The special centre for their distribution seems to have been the south-east of France, where the delight in cusped arches is very noticeable at Clermont, Vienne and Le Puy. The forms of trefoiled arches appear in the North as early as the tenth century in the ornaments of illuminated books, and probably they were handed on in this pictorial form long before they entered into real structures. Architecture and sculpture often followed where painting led. Circular windows had been used by the Romans and are frequently found in Romanesque work. Both circular and quatrefoil openings were probably known in the West from Carolingian days. The quatrefoil became popular as a form of cross. Ordinary windows, when grouped into pairs with a circle above, formed the point of departure for the development of the traceried window.

From the early days of Christian art glazing of various colours arranged in patterns had been used. Doubtless the beautifully patterned casements of Arab art were, like so much else, taken over from the Byzantine school. The jewelled lattices of Romance must have been suggested by the use of coloured glass. At some time in the great Carolingian era, which we are only now beginning to appreciate, painting was added to the morsels of coloured glass, and they were joined together by thin strips of lead rather than by some ruder means. These two steps of development brought into being the stained glass window proper. From this time windows were conceived as vast translucent enamels of which the leads formed the divisions. The agreement of style between the earliest known stained glass windows and Romanesque enamels is so close that we may not doubt the near kindred of the two arts. The earliest windows still extant, like those of St Denis (c. 1140-50), were probably designed by some enameller.

For long the style of German Byzantine enamels may be traced in