The true functions of stained glass were never more admirably understood than in the twelfth century. The artists of that day had a perfect comprehension of those colour-harmonies, the subdued splendour of which best accorded with the simple and vigorous forms of Romanesque architecture. Upon his glass of various tints the painter first outlined his figure or ornament in black. This outline he supported with a flat half-tint which supplied a rough modelling and allowed the forms expressed to make their fullest effect from a distance. When, in the thirteenth century, the extreme austerity of religious buildings began to relax, the splendour of the painted windows increased proportionately; but the coloration, though it increased in glow and vigour, still preserved its complete harmony with its surroundings. An additional richness is perceptible in work of the fourteenth century, at which period red glass began to be used with a certain prodigality. The system of execution remains unchanged so far; but the black outline is considerably attenuated, and the half-tone which emphasises it loses much of its importance. The figures, in place of the hieratic repose of an earlier period, affect a certain grace and animation which herald a tendency towards realistic imitation. These germs of naturalism soon bore fruit. At the close of the fourteenth century the discovery of how to obtain yellow from salts of silver, and the facility with which it could be used to warm the grayer tones of glass by the help of the muffle, caused a revolution in the art of glass-painting, and prepared the way for polychromatic enamelling. This discovery, eminently useful when discreetly applied, was to lead to regrettable exaggerations. [Illustration: 123. PAINTED WINDOW OF THE THIRTEENTH CENTURY. CHARTRES CATHEDRAL] [Illustration: 124. PAINTED WINDOW OF THE THIRTEENTH CENTURY. CHURCH OF ST. GERMER, TROYES] In the fifteenth century the figures of saints were usually drawn upon glass so tinted as to be of a soft white tone; the hair, beards, head-dresses, jewels, trimmings, and embroideries were painted in yellow. The figures stood out in bold relief against a background of blue or red, and were divided by a damasked drapery of green or purple. Vast architectural motives were introduced enframing the figures and filling up the immense window spaces of the latest period of mediæval art. The transformation was radical. It is of interest to note that the final development of the Gothic style ought logically to have brought about a recrudescence of vigour in the coloration of stained glass; but the exact reverse was the case; and a marked modification took place in the glowing effects won by a diversity of strong tints. The sort of _camaïeu_ which was the result obliged the painter to insist more strongly on the modelling of the figures, and to give less importance to the black outline, which was eventually suppressed altogether.