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.

The blue tone of the principal subject [that is to say, the ground of the Tree of Jesse] has commanded the tonality of all the rest.  This medium was necessary to enable the luminous splendour to display its energy.  This primary condition had dictated the red ground for the prophets, and the return to the blue on reaching the outside semicircular band.  To give full value both to the vigour of the red, and to the radiating transparency of the blue, the ground of the corners is put in emerald green; but then, in the corners themselves, the blue is recalled and is given an additional solidity of value by the delicate ornamentation of the squares.

This translation is very free, but one who wants to know these windows must read the whole article, and read it here in the church, the Dictionary in one hand, and binocle in the other, for the binocle is more important than the Dictionary when it reaches the complicated border which repeats in detail the colour-scheme of the centre:—­

The border repeats all the tones allotted to the principal subjects, but by small fragments, so that this border, with an effect both solid and powerful, shall not enter into rivalry with the large arrangements of the central parts.