The claim may not be modest, but it is none of ours. Viollet-le-Duc must answer for his own sins, and he chose the lancet window of the Tree of Jesse for the subject of his lecture on glass in general, as the most complete and perfect example of this greatest decorative art. Once more, in following him, one is dragged, in spite of one's self, into technique, and, what is worse, into a colour world whose technique was forgotten five hundred years ago. Viollet-le-Duc tried to recover it. "After studying our best French windows," he cautiously suggests that "one might maintain," as their secret of harmony, that "the first condition for an artist in glass is to know how to manage blue. The blue is the light in windows, and light has value only by opposition." The radiating power of blue is, therefore, the starting-point, and on this matter Viollet-le-Duc has much to say which a student would need to master; but a tourist never should study, or he ceases to be a tourist; and it is enough for us if we know that, to get the value they wanted, the artists hatched their blues with lines, covered their surface with figures as though with screens, and tied their blue within its own field with narrow circlets of white or yellow, which, in their turn, were beaded to fasten the blue still more firmly in its place. We have chiefly to remember the law that blue is light:-- But also it is that luminous colour which gives value to all others. If you compose a window in which there shall be no blue, you will get a dirty or dull (blafard) or crude surface which the eye will instantly avoid; but if you put a few touches of blue among all these tones, you will immediately get striking effects if not skilfully conceived harmony. So the composition of blue glass singularly preoccupied the glassworkers of the twelfth and thirteenth centuries. If there is only one red, two yellows, two or three purples, and two or three greens at the most, there are infinite shades of blue, ... and these blues are placed with a very delicate observation of the effects they should produce on other tones, and other tones on them. Viollet-le-Duc took the window of the Tree of Jesse as his first illustration of the rule, for the reason that its blue ground is one continuous strip from top to bottom, with the subordinate red on either side, and a border uniting the whole so plainly that no one can fail to see its object or its method.