The side walls of the aisles for a height of some fourteen feet to the bottom of their vaulting ribs, the triforium, commencing but a foot above the arches which separate nave from side aisles, and immediately above the triforium, forty feet of clerestory,--all is glass, emerald, turquoise, and peacock, amber, straw, scarlet, and crimson, encased in a most delicate, strangely reckless, and bold-traceried framework of stained ivory. Indeed, the jeweled portals of Heaven are wide open when the sun throws all the colors from above across the otherwise colorless fields of the pavement. "The color of love's blood within them glows." There is glazing of many centuries and all styles. In some of the triforium windows are bits of glass, which, after the destruction or falling of the old windows, were carefully collected, put together, and used again in the reglazing. Some of it is of the earliest in Spain, probably set by French, Flemish, or German artisans who had immigrated to practise their art and set up their factories on Spanish soil adjacent to the stone-carvers' and masons' sheds under the rising walls of the great churches. Like all skilled artisans of their age, the secret of their trade, the proper fusing of the silica with the alkalies, was carefully guarded and handed down from father to son or master to apprentice. They were chemists, glaziers, artists, colorists, and glass manufacturers, all in one. The heritage was passed on in those days, when the great key of science which opens all portals had not yet become common property. Some of the oldest glass is merely a crude mosaic inlay of small bits and must date back to early thirteenth century. Coloring glass by partial fusion was then first practised and soon followed by the introduction of figures and themes in the glass, and the acquisition of a lovely, homogeneous opalescence in place of the purely geometrical patterns. Scriptural scenes or figures painted, as the Spanish say, "en caballete," became more and more general. The best of the Leon windows are from the fifteenth century, when the glaziers' shops in the city worked under the direction of Juan de Arge, Maestro Baldwin, and Rodrigo de Ferraras, and its master colorists were at work glazing the windows of the Capilla Mayor, the Capilla de Santiago, and a portion of those of the north transept. "Ces vitreaux hauts en couleur, qui faisaient hésiter l'œil émerveillé de nos pères entre la rose du grand portail et les ogives de l'abside." The glazing has gone on through centuries; even to-day the glaziers at Leon are busy in their shops, making the sheets of sunset glow for their own and other Spanish cathedrals.