(6) _The Material used._--At the beginning of the fifteenth century there is a very marked change in the material used. It becomes thinner and flatter--sometimes very thin indeed--and the colour is more even. Thirteenth century "ruby," seen edgeways, reveals itself as composed, for nearly, if not quite, half its thickness, of alternate minute layers of red and white, the rest of the thickness being white. It has been thought that to this is due the wonderful luminous quality of the early ruby. Gradually the number of these layers are reduced till at the beginning of the fifteenth century the red is all concentrated into one layer on the surface. This is the "flashed" glass referred to at the beginning of the book, and one soon begins to find instances of ornament chipped out of it. The lion on the red shield in Plate XXXVI. has, I think, been got in this way, and a later instance may be seen in Plate L. in the girdle of the prophet Hosea on the right. [Illustration: PLATE XLII HEAD, FROM ST. MICHAEL'S, SPURRIERGATE, YORK Fifteenth Century] The rich blues of earlier times are replaced by a more sober greyish blue, which, however, is a very effective colour in glass. The colours are not perfectly flat tints, for there are gradations in them, but the streaky, crumbly quality of the early glass is gone. The craftsman was beginning to rely for quality less on the glass itself than on what he put on it. XIV FIFTEENTH CENTURY GLASS AT YORK The very well-defined and distinctive style I have described, which became universal in English fifteenth century work, and which, from the architecture with which it is associated, we call Perpendicular, was not, I think, evolved by the Winchester School, although no doubt they influenced it. Where it began must always be something of a mystery, but some work in the east window of Exeter Cathedral is very suggestive in this connection. [Sidenote: Lyen's work at Exeter.]