flanking porticos:

Ft. In.
The space of its central archivolt is 31 8
" the two on each side, about[44] 19 8
" the two succeeding, about 20 4
" small arches at flanks, about 6 0

[Illustration: Fig. VI.]

I need not make any comment upon the subtle difference of eight inches
on twenty feet between the second and third dimensions. If the reader
will be at the pains to compare the whole evidence now laid before him,
with that deduced above from the apse of Murano, he cannot but confess
that it amounts to an irrefragable proof of an intense perception of
harmony in the relation of quantities, on the part of the Byzantine
architects; a perception which we have at present lost so utterly as
hardly to be able even to conceive it. And let it not be said, as it was
of the late discoveries of subtle curvature in the Parthenon,[45] that
what is not to be demonstrated without laborious measurement, cannot
have influence on the beauty of the design. The eye is continually
influenced by what it cannot detect; nay, it is not going too far to
say, that it is most influenced by what it detects least. Let the
painter define, if he can, the variations of lines on which depend the
changes of expression in the human countenance. The greater he is, the
more he will feel their subtlety, and the intense difficulty of
perceiving all their relations, or answering for the consequences of a
variation of a hair's breadth in a single curve. Indeed, there is
nothing truly noble either in color or in form, but its power depends on
circumstances infinitely too intricate to be explained, and almost too
subtle to be traced. And as for these Byzantine buildings, we only do
not feel them because we do not _watch_ them; otherwise we should as
much enjoy the variety of proportion in their arches, as we do at
present that of the natural architecture of flowers and leaves. Any of
us can feel in an instant the grace of the leaf group, _b_, in the
annexed figure; and yet that grace is simply owing to its being
proportioned like the facade of St. Mark's; each leaflet answering to an
arch,--the smallest at the root, to those of the porticos. I have tried
to give the proportion quite accurately in _b_; but as the difference
between the second and third leaflets is hardly discernible on so small
a scale, it is somewhat exaggerated in _a_.[46