In the earliest form of medallion window, such as those in west windows at Chartres and some of the earliest ones at Canterbury, the window is divided by the iron-work into a series of regular squares, each of which alternately is filled with a square and a circular figure-subject. Later, however, in the thirteenth century, the iron-work itself was bent into geometric patterns which the medallions were shaped to fit, producing the elaborate designs shown in the insets of the whole windows in Plates IV. and VIII. from Canterbury.
Even when in the latter part of the thirteenth century there was a return, prompted no doubt by motives of economy, to iron-work composed of straight bars, the influence of these elaborate lattices is still seen in the shapes of the medallions, though these are no longer outlined by the iron-work which now passes across or between them. An example of this is shown in Plate XIV. from Rouen Cathedral.
(4) _The Method of Painting._--This consists of vigorous line work in the brown enamel, laid on with a brush in beautiful, firm, expressive strokes on a ground of clear glass. Lettering and patterns are formed by being scratched out clear from a solid coat of enamel. There is no attempt at modelling in planes or at light and shade, and half-tone is only used, as I shall presently explain, to soften the edges of the line work.
Now the optical law which most affects the technique of stained glass is that of which the effect is known as "irradiation." In an unscientific work it is enough to say that it is the law which causes the filament of an electric light, in reality thin as a hair, to appear when incandescent as thick as a piece of worsted. In the same way it makes the clear spaces of glass appear larger than they really are in proportion to the obscured parts, and also tends to make them look rounded.
From the fact that the glass between the line work was left nearly clear, the work of this period is more affected by irradiation than any other, and the artist had to make his line work very black and thick in order to tell at all, especially in work far from the eye. For instance, if he wished to distinguish the fingers of a hand he separated them with solid black spaces as thick as the fingers themselves.