Whether this law is absolute you can tell best by looking at modern glass which is mostly perspective; but, whether you like it or not, the matter of perspective does not enter into a twelfth-century window more than into a Japanese picture, and may be ignored. The decoration of the twelfth century, as far as concerns us, was intended only for one plane, and a window was another form of rug or embroidery or mosaic, hung on the wall for colour,--simple decoration to be seen as a whole. If the Tree of Jesse teaches anything at all, it is that the artist thought first of controlling his light, but he wanted to do it not in order to dim the colours; on the contrary, he toiled, like a jeweller setting diamonds and rubies, to increase their splendour. If his use of blue teaches this lesson, his use of green proves it. The outside border of the Tree of Jesse is a sort of sample which our schoolmaster Viollet-le-Duc sets, from which he requires us to study out the scheme, beginning with the treatment of light, and ending with the value of the emerald green ground in the corners. Complicated as the border of the Tree of Jesse is, it has its mates in the borders of the two other twelfth-century windows, and a few of the thirteenth-century in the side aisles; but the southern of the three lancets shows how the artists dealt with a difficulty that upset their rule. The border of the southern window does not count as it should; something is wrong with it and a little study shows that the builder, and not the glassworker, was to blame. Owing to his miscalculation--if it was really a miscalculation--in the width of the southern tower, the builder economized six or eight inches in the southern door and lancet, which was enough to destroy the balance between the colour-values, as masses, of the south and north windows. The artist was obliged to choose whether he would sacrifice the centre or the border of his southern window, and decided that the windows could not be made to balance if he narrowed the centre, but that he must balance them by enriching the centre, and sacrificing the border. He has filled the centre with medallions as rich as he could make them, and these he has surrounded with borders, which are also enriched to the utmost; but these medallions with their borders spread across the whole window, and when you search with the binocle for the outside border, you see its pattern clearly only at the top and bottom. On the sides, at intervals of about two feet, the medallions cover and interrupt it; but this is partly corrected by making the border, where it is seen, so rich as to surpass any other in the cathedral, even that of the Tree of Jesse. Whether the artist has succeeded or not is a question for other artists--or for you, if you please--to decide; but apparently he did succeed, since no one has ever noticed the difficulty or the device.