Ancient Roman glass is of great variety in colour, and many specimens show the highest technical skill combined with great artistic beauty. The lovely iridescent effect on Roman and other antique glass is due to the chemical changes of the surface decomposition, and in other instances to the minute flaking of the glass, which reflects the light at various angles, and thus producing the prismatic hues. At the time of the Roman occupation of Egypt, during the period of rule under the Ptolemies, glass making was a great industry in the latter country. It is said that the Romans learnt the art from the Egyptians, and it is known that many of the latter were brought to Rome to practise their art as glass-blowers. The making of the glass known as _mille-fiori_ was taught to the Romans by the Egyptians, and was extensively employed in vase and bottle making by the former; the Venetians from the fourteenth century have imitated the Romans in their uses of mille-fiori glass for bead making and other purposes, with great success. The method of making this variety of glass consists in arranging a number of thin rods or threads of glass of the required variety of colours, gold being sometimes used; these united rods were then fused together by heat, and drawn out or twisted, so that when transverse sections were cut the pattern would always be the same. Another way of mixing the colour in glass, which was employed by the Egyptians, Romans, and later by the Venetians, was in the making of regular patterns of mosaic-like designs of the various colours, and another was in imitating the precious stones and marbles, such as onyx, agate, serpentine, porphyry, and murrhine; the latter is supposed to be a variety of agate, with red and purple shades. The murrhine glass examples of Roman manufacture are very rare. The Romans used these glass imitations of the precious stones and marbles to line their walls and floors, as in mosaic work. Egyptian and Roman glass in their transparent varieties have such colours as yellow, purple, blue, green, and pink; opaque colours are generally found in shades of yellow, blue, green, and black. The most valuable kind of glass was that of the clearest white or crystal; this, it would appear, was the most difficult to make, as the commoner clear variety had usually a slight greenish or bluish hue. The Romans made a special variety of glass ware which consisted of interlaced bands of opaque white or coloured glass, ingeniously made to form a pattern by twisting them with clear or coloured transparent glass, like that of the elegant lace-glass variety known as the “vitro di trina” of the Venetians (Fig. 302). In the arts of glass cameo and intaglio engraving, and in the imitation of gems, the Romans were exceedingly skilful.