The practical function of stained glass is comparable with that of mosaic. Mosaic is an enrichment of the shadow. Buildings def signed for mosaic usually have quite small windows, low down in the big domes or sparsely set in the side walls,and it is the mo saic enrichment of shadow, vaguely lit by re?ected light from these windows, which gives it its highest beauty. The peculiar charm of mosaic depends largely on the gold treatment of the background, which is infinitely more attractive when seen on curved surfaces and lit from below.