The gallery before the single light clerestory windows gave once an open passage all round, but it is now blocked at the end of the south transept. In front of each window it has a triple screen of which the general form is shown in our illustration of a window of the choir proper and in our view of the east end. It is owing to the existence over the transept aisles of two rooms, known as the Treasury and the Indulgence Chamber, that no clerestory windows are to be seen there, but only blind arcading and blank wall. In the inner, wider bays of the transepts we notice that the usual triple screens are extended by two additional arches of the lower height towards the centre of the church. The clerestory gallery is, on each side of the choir proper, quite in the thickness of the wall. The core of the latter is Norman, but its facing, including the blind arcade at the triforium level, belongs to the Early English period. On either side of the presbytery the clerestory gallery springs from wall-piers with clustered Purbeck shafts. The tracery of the windows, thus ornamented here, is later than the windows themselves, and is an insertion of the Decorated period. So is also that of the windows on the east side, and at the end, of the south transept aisle. The latter is unique in this cathedral, and we have thought it worthy of illustration. Remains of clustered columns, to be seen in the east wall of the north transept aisle, remind us of the numerous changes that so many parts of the fabric have undergone. The east end has only taken its present form since Sir G. Scott's work, between 1870 and 1875. In 1825 Cottingham removed a huge altar screen thence, and opened and renewed the lower range of windows, of which the central had been quite, and the other two partly, blocked with brickwork. He, however, still left the communion table against the wall, and, instead of doing away with the great upper window then existing, only repaired it. This great window, occupying the whole space from the gallery to the vaulting, was divided into nine lights, of which the inner seven were cut by a transom or horizontal mullion. Photographs of three drawings by Mr. Gunning, made in 1842, are preserved in the chapter room, and show this east end, and the two sides of the organ screen, as they were before Scott's alterations. The north transept end is very like the east end in its general design, but has, low down, the two windows lighting the Merton tomb, and the tiny one over the same bishop's Elizabethan effigy. The south transept end is again much the same, but has the spaces between the wall-piers and under its outer windows filled in with masonry, in which are the openings to two passages, now blocked, which led respectively up to the Indulgence Chamber and down to the crypt.