white arsenic. Glass coloured throughout by fusion with the colouring matter in the main pot is called " pot metal "; but when plated with a thin layer of coloured glass (the flashing) it is called " flashed." Ordinary copper ruby glass cannot easily be made as a " pot metal," and is therefore flashed. In 1881 Drs. Abbe and Schott instituted a series of experiments at Jena, in Germany, on the. improvement of the manufacture of glass for optical purposes, and, after some successful trial meltings, in 1884 they received a handsome subsidy from the Prussian Bureau of Education, which enabled them to commence operations on a manufacturing scale. The old optical glasses were limited to practically few varieties, but the researches of these gentlemen have furnished opticians with new glasses of constant or standard quality ; hence the manufacture of lenses has been much simplified, as it is no longer necessary to determine the refractive and dispersive properties of each mass of glass. About one hundred kinds of optical glass are now made at the Jena Factory, ranging from 1.5063 to 1.9626 as regards refractive index, and of various dispersive power. In most cases the factory supplies masses roughly moulded to the shape required by the optician, and at a cost of from about 4.?. to 30.?. per lb. Most important among the glasses made at Jena is the silicate of barium glass - equivalent to a heavy flint glass - introduced by Baudrimont and Pelouze about 1850 as a commercial substitute for ordinary flint glass. Mr. Alfred Dawson about 1880 demon-strated the value of this glass for optical purposes, and prepared pure specimens, which he used in constructing objectives; and to him is due most of the credit for that modern development of optical glass-making which has put such new and consider-able powers into the hands of opticians.
Glass, Black (mirror). Used sometimes as a mirror by which the weakened reflections of clouds are photographed ; also by artists, as by viewing the reflection a better judgment can some-times be arrived at of the effect in a picture. Photographers especially would find a small square of optically worked black glass very convenient for this purpose, as the effect of colour is eliminated. The commercial " black" glass is usually a very deep amethyst colour, and contains about 10 per cent, of oxide of manganese. (See Claude Lorraine Glass.)