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. One would think this simple enough; easily tested on any illuminated manuscript, Arab, Persian, or Byzantine; verified by any Oriental rug, old or new; freely illustrated by any Chinese pattern on a Ming jar, or cloisonne vase; and offering a kind of alphabet for the shop-window of a Paris modiste. A strong red; a strong and a weak yellow; a strong and a weak purple; a strong and a weak green, are all to be tied together, given their values, and held in their places by blue. The thing seems simpler still when it appears that perspective is forbidden, and that these glass windows of the twelfth and thirteenth centuries, like Oriental rugs, imply a flat surface, a wall which must not be treated as open. The twelfth- century glassworker would sooner have worn a landscape on his back than have costumed his church with it; he would as soon have decorated his floors with painted holes as his walls. He wanted to keep the coloured window flat, like a rug hung on the wall. The radiation of translucent colours in windows cannot be modified by the artist; all his talent consists in profiting by it, according to a given harmonic scheme on a single plane, like a rug, but not according to an effect of aerial perspective. Do what you like, a glass window never does and never can represent anything but a plane surface; its real virtues even exist only on that condition. Every attempt to present several planes to the eye is fatal to the harmony of colour, without producing any illusion in the spectator ... Translucid painting can propose as its object only a design supporting as energetically as possible a harmony of colours.