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.

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.