Two of them, the central one, which is the largest, being some 30 feet high and 10 wide, and that on the south, are medallion windows, containing scenes from the life of Christ. The bars of the iron-work, which are about 3 feet apart, divide them into a series of regular squares, which are filled with square and circular medallions; in the central window the figures in the medallions are relieved against alternate backgrounds of ruby and blue. The ruby of the twelfth and thirteenth centuries is a wonderful colour. It is never of a perfectly even tint, each piece having, as it were, its own character, and the colour seems to have a slightly granulated or "crumbly" texture, which gives it a quality unknown in later glass. So beautiful was it evidently considered that the artist seldom or never attempted to enrich it with painting. [Sidenote: The Jesse tree.] The third window, that on the north, is filled with a "Tree of Jesse." The subjects of stained-glass windows in this First Period were chosen with one object--the exposition of the Christian Doctrine, and of this the human descent of Christ was an essential part. Accordingly, at all periods in the history of mediæval stained glass we find windows devoted to the representation of "the Tree of Jesse." The arrangement is always the same as in this window at Chartres. The figure of Jesse lies recumbent at the foot of the window, and from his loins rises the "Tree"--a mass of branching scroll work with conventional foliage, spreading over the whole window, carrying on its branches David and other human ancestors of Christ, and culminating in the Virgin and Christ Himself at the top of the window. On either side are ranged the Prophets who foretold His coming, and the whole, surrounded by a rich border, forms, at Chartres, a mass of jewelled colour some 9 feet wide and 25 feet high. This window and the one of which a part, identical in design, remains at St. Denis, are the oldest examples I know of a Jesse tree in stained glass, and whether or not they were the first to be made, their design formed a model for others for long after. [Sidenote: La Belle Verrière.] The remainder of the windows in the Cathedral, including the western rose, are of the thirteenth century with one exception--the one in the south choir aisle, which contains the great figure of the Virgin known as "Notre Dame de la Belle Verrière": the only window, as far as I know, to which in former times people knelt by hundreds in adoration, and before which they still occasionally burn a candle.