nable expression to these windows, but the accidental warping
and wear of centuries have laid each bit of glass at a different
angle, so that the refraction of the light is quite different from
any possible reflection on the smooth surface of a modern window.

The dangers of a clear gold ground were, felt more fully by the
workers at Ravenna and Rome, than in Venice. Architectural schemes
were introduced to break up the surface: clouds and backgrounds,
fields of flowers, and trees, and such devices, were used to prevent
the monotony of the unbroken glint. But in Venice the decorators
were brave; their faith in their material was unbounded, and they
not only frankly laid gold in enormous masses on flat wall and
cupola, but they even moulded the edges and archivolts without
separate ribs or strips to relieve them; the gold is carried all
over the edges, which are rounded into curves to receive the mosaic,
so that the effect is that of the entire upper part of the church
having been _pressed_ into shape out of solid gold. The lights on
these rounded edges are incomparably rich.

It is equally important to vary the plain values of the colour,
and this was accomplished by means of dilution and contrast in
tints instead of by unevenness of surface, although in many of the
most satisfactory mosaics, both means have been employed. Plain
tints in mosaic can be relieved in a most delightful way by the
introduction of little separate cubes of unrelated colour, and
the artist who best understands this use of mass and dot is the
best maker of mosaic. The actual craft of construction is similar
everywhere, but the use of what we may regard as the pigment has
possibilities similar to the colours of a painter. The manipulation
being of necessity slow, it is more difficult to convey the idea
of spontaneity in design than it is in a fresco painting.

To follow briefly the history of mosaic as used in the Dark Ages,
the Middle Ages, and the period of the Renaissance, it is interesting
to note that by the fourth century mosaic was the principal decoration
in ecclesiastical buildings. Contantine employed this art very
extensively. Of his period, however, few examples remain. The most
notable is the little church of Sta. Constanza, the vaults of which
are ornamented in this way, with a fine running pattern of vines,
interspersed with figures on a small scale. The Libel Pontificalis
tells how Constantine built the Basilica of St. Agnese at