IN the description of the last sketch I alluded to the sources whence the Moors derived much of their knowledge of glass-making and mosaic-working. In the specimen now given, the full size of the original, on the opposite page, a considerable advance is shown upon what was usual in the contemporary, "Opus Grecanicum," as executed, either in Italy or in Greece itself. The advance is principally to be seen in this particular, that whereas in the last mentioned work, every complicated pattern is made up out of tesseræ, or glass strips cut into squares, oblongs, triangles, or other simple figures; in the Moorish work, arbitrary shapes of considerable geometrical complexity are given to each separate piece of mosaic. When these tesseræ, so shaped, are brought together, their combination immediately results in the formation of perfect patterns, such as the one now illustrated. Tesseræ of this description were no doubt formed by squeezing plastic clay into metal moulds, and almost perfect identity was obtained between the tesseræ obtained from the same mould. These, after firing, were then apparently covered with coloured vitreous glazes by a subsequent operation. In illustration of the advantages possessed by the Moors over the Greeks, in working such mosaics as the one I have sketched, it may be noted, that while a Greek would have required one hundred and nineteen separate pieces to make up what is shown, the Moor wanted only forty-nine. Moreover, instead of having to chip every one of the one hundred and nineteen pieces to a definite size and shape, and then to place them slowly so as to ensure the truth of his angles of forty-five and twenty-two and a half degrees, as the Greek or Italian had, the Moor had only to place one of his forty-nine pieces with precision; and, provided he never took any of the eleven patterns, of which his repeats are composed, out of their right turn, his mosaic would work itself with scarcely any other attention on his part. Another source of anxiety was saved to him; viz., constant heedfulness as to the working of the interlacement of his lines--_i. e._, their running, as it were, under and over one another. The result, in this particular, is far clearer and more effective in the Moorish, than according to the Greco-Italian method; since, while in the former there are no joints which do not help to define an interlacement, according to the latter, the joints occurring on the line of mitre of every angle become confused with the joints which express interlacement. A comparison of the Sicilian, with the Alhambrese, geometrical mosaics, would show in a moment the superiority of the last mentioned method. No people, except perhaps the Chinese, have ever equalled the Moors in devising patterns of most complicated appearance, in which colours were, as it were, counterchanged by combining tiles, or tesseræ, of similar geometrical forms, but made in different tints or tones.