§ 25. This abstract conclusion the great thirteenth century artists were the first to arrive at; and whereas, before their time, ornament had been constantly refined into intricate and subdivided symmetries, they were content with this simple form as the termination of its most important features. Fig. 3., which is a scroll out of a Psalter executed in the latter half of the thirteenth century, is a sufficient example of a practice at that time absolutely universal.
§ 26. The second great discovery of the Middle Ages in floral ornament, was that, in order completely to express the law of subordination among the leaf-ribs, two ribs were necessary, and no more, on each side of the leaf, forming a series of three with the central one, because proportion is between three terms at least.
That is to say, when they had only three ribs altogether, as a, Fig. 4., no law of relation was discernible between the ribs, or the leaflets they bore; but by the addition of a third on each side as at b, proportion instantly was expressible, whether arithmetical or geometrical, or of any other kind. Hence the adoption of forms more or less approximating to that at c (young ivy), or d (wild geranium), as the favorite elements of their floral ornament, those leaves being in their disposition of masses, the simplest which can express a perfect law of proportion, just as the outline Fig. 7. Plate 8. is the simplest which can express a perfect law of growth.
Plate 9. opposite gives, in rude outline, the arrangement of the border of one of the pages of a missal in my own possession, executed for the Countess Yolande of Flanders,[72] in the latter half of the fourteenth century, and furnishing, in exhaustless variety, the most graceful examples I have ever seen of the favorite decoration at the period, commonly now known as the "Ivy leaf" pattern.