llustration: AMBO AT RAVELLO; SPECIMEN OF COSMATI MOSAIC]

The mosaic pavement in Westminster Abbey Presbytery is as fine
an example of Roman Cosmati mosaic as one can see north of the
Alps. An inscription, almost obliterated, is interpreted by Mr.
Lethaby as signifying, that in the year 1268 "Henry III. being
King, and Odericus the cementarius, Richard de Ware, Abbot, brought
the porphyry and divers jaspers and marbles of Thaso from Rome." In
another place a sort of enigma, drawn from an arbitrary combination
of animal forms and numbers, marks a chart for determining the end
of the world! There is also a beautiful mosaic tomb at Westminster,
inlaid with an interlacing pattern in a ground of marble, like the
work so usual in Rome, and in Palermo, and other Southern centres
of the art.

While the material used in mosaic wall decoration is sometimes a
natural product, like marble, porphyry, coral, or alabaster, the
picture is composed for the most part of artificially prepared
smalts--opaque glass of various colours, made in sheets and then
cut up into cubes. An infinite variety in gradations of colour
and texture is thus made possible.

The gold grounds which one sees in nearly all mosaics are constructed
in an interesting way. Each cube is composed of plain rather coarse
glass, of a greenish tinge, upon which is laid gold leaf. Over
this leaf is another film of glass, extremely thin, so that the
actual metal is isolated between two glasses, and is thus impervious
to such qualities in the air as would tarnish it or cause it to
deteriorate. To prevent an uninteresting evenness of surface on
which the sun's rays would glint in a trying manner, it was usual
to lay the gold cubes in a slightly irregular manner, so that
each facet, as it were, should reflect at a different angle,
and the texture, especially in the gold grounds, never became
monotonous. One does not realize the importance of this custom
until one sees a cheap modern mosaic laid absolutely flat, and then
it is evident how necessary this broken surface is to good effect.
Any one who has tried to analyze the reason for the superiority of
old French stained glass over any other, will be surprised, if
he goes close to the wall, under one of the marvellous windows
of Chartres, for instance, and looks up, to see that the whole
fabric is warped and bent at a thousand angles,--it is not only
the quality of the ancient glass, nor its colour, that gives this
unattai