Some sort of wooden roof, therefore, had to be adopted. That roof could not be a flat wooden ceiling; no beams of the length of 77 ft. could be had; and if put up they would have been unsafe. Instead, then, of a flat ceiling, Alan determined on a conical vault of wood like that of York Chapter-house. But what he wanted was not merely a roof, but a lantern. A roof like the York one would have darkened the whole central area, besides being ugly in appearance and difficult to construct. Instead, therefore, of taking the whole of the York cone, he cut the top off, replacing its sharp spike with an octagonal lantern. In this, or in some such way, Alan got at his design: one of the most original and poetic conceptions of the middle ages; but arising, like all the best things in Gothic architecture, out of nothing but the exigencies of building construction. The problem was solved on paper, but it proved immensely difficult in execution. Alan finished the stone piers and arches in six years, but the timber-work occupied twelve years more. He had to search all over England before he could find oaks big and straight and sound enough for the eight vertical angle-posts of his lantern. They are 63 feet long, with a sapless scantling of 3 ft. 4 in. by 2 ft. 8 in. Oaks like those do not grow now in England. The eight angle-posts are tied together at top and bottom by collars of horizontal beams, and the whole skeleton lantern rests on the tips of inclined beams, whose lower ends rest on corbels behind the capitals of the great piers below. And as the inclined beams spread to right and left from the great piers below, it follows that the eight sides of the lantern are not placed above the eight arches below. This engineering necessity, also, is wrought into a new source of beauty. For advantage is taken of it to pierce the wall-space above the narrow low arches with four tall windows, so that the central area, so gloomy at Winchester, Lincoln, and Wells, is irradiated with a flood of light from twelve vast windows; four below, eight above. There is not such a lantern church in the world, and certainly there is no such interior as Ely in England. To the west, dark nave; to the east, darker choir; the centre all light and atmosphere. The views from the aisles across the octagon into the choir are veritable glimpses into fairyland. Externally, too, the octagonal lantern groups marvellously with the great western tower; in height, in bulk, in shape they are in perfect ratio.