have lastly to note the conditions under which the glass is to be
attached to the bars; and the sections of the bars themselves.

[Illustration: Fig. XLVII.]

These bars we have seen, in the perfect form, are to become shafts; but,
supposing the object to be the admission of as much light as possible,
it is clear that the thickness of the bar ought to be chiefly in the
depth of the window, and that by increasing the depth of the bar we may
diminish its breadth: clearly, therefore, we should employ the double
group of shafts, _b_, of Fig. XIV., setting it edgeways in the window:
but as the glass would then come between the two shafts, we must add a
member into which it is to be fitted, as at _a_, Fig. XLVII., and
uniting these three members together in the simplest way, with a curved
instead of a sharp recess behind the shafts, we have the section _b_,
the perfect, but simplest type of the main tracery bars in good Gothic.
In triforium and cloister tracery, which has no glass to hold, the
central member is omitted, and we have either the pure double shaft,
always the most graceful, or a single and more massy shaft, which is the
simpler and more usual form.

Sec. XVIII. Finally: there is an intermediate arrangement between the
glazed and the open tracery, that of the domestic traceries of Venice.
Peculiar conditions, hereafter to be described, require the shafts of
these traceries to become the main vertical supports of the floors and
walls. Their thickness is therefore enormous; and yet free egress is
required between them (into balconies) which is obtained by doors in
their lattice glazing. To prevent the inconvenience and ugliness of
driving the hinges and fastenings of them into the shafts, and having
the play of the doors in the intervals, the entire glazing is thrown
behind the pillars, and attached to their abaci and bases with iron. It
is thus securely sustained by their massy bulk, and leaves their
symmetry and shade undisturbed.

Sec. XIX. The depth at which the glass should be placed, in windows
without traceries, will generally be fixed by the forms of their
bevelling, the glass occupying the narrowest interval; but when its
position is not thus fixed, as in many London houses, it is to be
remembered that the deeper the glass is set (the wall being of given
thickness), the more light will enter, and the clearer the prospect
will be to a person sitting quietly in the centre of the room; on the
contrary, the