The only way in which I can imagine that such an effect could be produced is as follows, but in offering any such explanation I desire to do so with all due deference to practical glass-workers. I believe that two metal-moulds were made, one with the ornament in relief, and the other with the same ornament sunk in intaglio. From each mould, glass reproductions having been made of about equal substances (so as to contract equally in cooling), and, with the exception of a black film in one case, of the same glass, the two reproductions were stuck together firmly by the calcareous cement. The black glass in "cameo" would then be encased within the white glass in "intaglio;" and the pattern would of course be invisible, the two reproductions being firmly stuck together face to face, making apparently one white glass tessera of double the requisite thickness. The back of the cameo side would then have to be ground away, probably at a lapidary's wheel, until the back of the black pattern in cameo should be reached. At the same moment the face of the white intaglio would be exposed; and the tessera, being reduced to its proper thickness for insertion with the rest of the adjoining glass mosaic, would be fit to permanently combine with it; showing an elaborate black pattern held in by calcareous cement, on a white face, exactly as it now appears. Any such resolution of a difficult technical problem exhibits the Moors to us as excelling in two of their favourite Arts, viz., inlaying and glass manufacture. For much of their knowledge of both of these arts there is no doubt that the Moors were indebted to the Arabians. The Arabians were in their turn inheritors from the Byzantine Greeks of many of the traditions of manufacturing excellence once practised by the Romans. Amongst these were, no doubt, almost every process of glass-working and mosaic.[43] Considerable doubts exist as to the inheritance by the Greek of the lower empire of the process of inlaying from the Romans, and to their originality in adapting the process to their architecture. The first building in which it appears to have been freely used by the Greeks was the Mosque of Santa Sofia, built by Justinian. For that building he is known to have invoked the assistance of Persian designers and artificers; and from the divergence in the patterns of those inlays from any patterns usual in Roman contemporary work, I am inclined to believe that they represent the foreign element to which I have alluded. A most interesting comparison may be made, by the student, of the patterns from the Aya Sofia given in Salzenburg's great work, with those of the principal of the Cairene Mosques drawn by Mr. James Wild and given in the "Grammar of Ornament." [Illustration: PLATE 72 GRANADA THE ALHAMBRA HALL OF THE AMBASSADORS MOSAIC FULL SIZE MDW 1869] PLATE LXXII. _GRANADA.--THE ALHAMBRA._ MOSAIC FROM THE HALL OF THE AMBASSADORS.