These, however, are the exceptions which prove the rule, and, broadly speaking, a stained-glass window must consist, to the eye, of flat patches of colour, large or small, worked on with dark monochromatic line work and shading. These patches of colour must each be separated from the next by a black line--the leading--varying from a quarter to half an inch in thickness, and crossed at intervals by still thicker black lines--the iron bars. [Sidenote: Limitations of the art.] It follows from this that anything like illusion is impossible in stained glass, and no artist with any sympathy for the medium would attempt it. Unwise persons in decadent times have wasted much ingenuity in the endeavour, but the result has always been disastrous and ridiculous. Apart from its higher mission,--the expression of ideas and emotions,--which it shares with every other branch of art, the mission of stained glass is to beautify buildings and nothing else. It is the handmaid of architecture, and can only justify itself by loyal service of its mistress. The ideal of the stained-glass artist must not be a picture made transparent, but a window made beautiful. Let no one suppose, however, that the artist is hampered by these limitations on the higher side of his work. On the contrary, they set him free to tell his story his own way. Ruskin--poor Ruskin, out of date, ridiculed, forgotten--pointed out long ago, in writing of Giotto's frescoes, the advantage which the pure colourist has over the chiaroscurist in his power of telling a story. In our times the fact has been rediscovered with a flourish of trumpets by the Post-Impressionists. I have no great enthusiasm, I confess, for the way in which they have carried out their principles, but I do know two perfect tellings of the story of the Creation. One is in mosaic on the ceiling of the narthex of St. Mark's at Venice, and the other is in the upper part of the east window of York Minster; and in each case the language used consists of flat forms and colour only. II THE BEGINNINGS OF STAINED GLASS I have said that stained-glass work is the product of the union of two crafts, the glazier's and the enameller's. The glazier's work being the groundwork of a window, I will take it first. [Sidenote: Glass-making.] What, to begin with, is glass? It is sand melted and run together. The best sand for the purpose is that which is most largely composed of the substance called silica, such as sand formed of powdered quartz, or flint. For some reason, the silica when melted does not recrystallize on cooling as might have been expected, but forms an even transparent substance, plastic while still hot. Think of the tremendous effect this one natural fact has had on the architecture, dress, and, probably, the physique of the nations of northern Europe.