Once their attention had been drawn to these facts, modern glass-makers endeavoured, and with much success, to reproduce the desirable qualities of the ancient glass, while still availing themselves of modern methods to produce more stable glasses and a wider range of colours. The irregular surface of the old glass is imitated by using rolled or “muffed” instead of ordinary blown glass, while the internal texture is rendered non-homogeneous by the deliberate introduction of solid and gaseous impurities and by manipulations so arranged as to leave the glass in layers of different density, which appear in the finished glass as “striæ.” As a consequence, it is probably not too much to claim that the modern workers in coloured glass have materials at their disposal which are at least as suitable for the purpose as those that were available in the best days of the ancient art. Some reference has already been made to the technical uses of coloured glass, but one or two further points in that connection remain to be discussed. For such technical purposes as railway and marine signals, the consensus of practical experience has decided in favour of certain colours of glass, such as red and green of particular tints. On the other hand, for various purposes in connection with photography, the glass-maker does not appear to have been able to meet the new requirements, with the result that flimsy and otherwise unsatisfactory screens made of gelatine or celluloid stained with organic dyes are employed in place of coloured glass in such cases, for example, as the covering of lamps for use in photographers’ “dark” rooms, and for the light-filters used for orthochromatic and tri-chromatic photography. In all these cases it is necessary to use a transparent coloured medium which transmits only light of a certain very definite range of wave-lengths, and there is no doubt that for the glass-maker, who is confined to the use of a number of elementary bodies for his colouring media, it is by no means easy to comply with these requirements of exact transmission and absorption. On the other hand, the field of available coloured glasses has not been fully explored from this point of view, the only extensive work on the subject having been done in connection with the Jena firm of Schott, who have put upon the market a series of coloured glasses of accurately-known absorbing power. There is, however, little doubt that a much greater extension of this field is possible, and that it will be opened up by a glass-maker who undertakes the exhaustive study of coloured glasses from this point of view, although it must be admitted that there is considerable doubt whether the results obtainable by the aid of aniline and other dyes as applied to gelatine can ever be equalled by coloured glasses. CHAPTER XII. OPTICAL GLASS.