t is executing the work _in situ_ by floating on a
limited expanse of cement, and sticking on the tesserae at once. It has
the advantage of enabling the artist or architect to see the effect of
his efforts under the fixed conditions of light and height.

I shall confine myself to vitreous or glass mosaic, which for
durability, extended scales of primary colors and their numerous
semi-transparent gradations is unequaled by any substance yet used for
wall or floor decoration. I am surprised, having all these fine
qualities, it is not more used by architects. If you require proofs of
its triumphs, go to St. Mark's, of Venice, and stand under its mellow
golden roof. There you will find its domes and vaulted aisles, nave and
transepts entirely overlaid with gold mosaic, into which ground is
worked--in the deepest and richest colors and their gradations that
contemporary manufacturers could produce--subjects selected from the
creation down to the life of Christ, in addition containing a complete
alphabet of early Christian symbolism. The roof surfaces being one
succession of over-arching curves become receptive of innumerable waves
of light and broad unities of soft shadows, giving the whole an
incomparable quality of tone and low juicy color.

Never use your gold but on curved or undulating surfaces. Flat planes of
gold only give the effect of a monotonous metallic yellow, and can never
be beautiful, owing to the absence of the variations that come with
waves of shadow. By letting out the reins of imagination we might feel
that in this a tenth century Giorgione has given off the mental
impressions of all the golden autumn of his life. His material gave him
an advantage over his great followers of the fifteenth and sixteenth
centuries, insomuch that glass has a living and glowing quality of light
not existing in the somewhat clouded purity of oil or fresco.

In St. Mark's we have an example of the superb treatment in deepest and
most Titianesque scales applied to curved forms, but to find a similarly
complete example of the use of lighter tones and on flat surfaces, we
must turn to Ravenna. I can give you no adequate description of the wall
mosaics of Ravenna. In the sense of delicate color they remind me of
some of the subtile harmonies of many of the finest works of the modern
French school--of the Impressionists and others who combine that quality
with a true instinct for design. In standing before them you feel that
the