This arrangement much reduced the price of reflectors and rendered their polishing extremely easy, and at the present time discs of glass up to four feet in diameter are being thus produced and formed into mirrors, though in the opinion of competent judges this size is likely to be the limit for some time. But there is this important difference, that although glass is now used both for reflectors and refractors, almost any glass, even common glass, will do, if we wish to use it for a speculum; but if we wish to grind it into lenses it is impossible to overrate the difficulty of manufacture and the skill and labour required in order to prepare it for use, first in the simple material, and then in the finished form in which it is used by the astronomer. In a former chapter we considered some _chefs-d’œuvre_ of the early opticians, some specimens of a quarter or half-an-inch in diameter, with extremely long focus; and as we went on we found object-glasses gradually increasing in diameter, but they were limited to the same material, namely, crown glass. Dollond, whose name we have already mentioned in connection with that of Hall, gave us the foundation of the manufacture of the precious flint glass, the connection of which with crown glass he had insisted upon as of critical importance. The existence of a piece of flint glass two inches in diameter was then a thing to be devoutly desired, that is to say, flint glass of sufficient purity for the purpose; it could not be made of a size larger than that, and not only was the material wanted, but the material in its pure state. In the year 1820 we hear of a piece of flint glass six inches in diameter, and in 1859 Mr. Simms reported that a piece of flint glass of seven and three-quarter inches was produced, six inches of which were good for astronomical purposes. But even at this time they did these things better in Germany and Switzerland, where M. Guinand made large discs at the beginning of the present century. He was engaged by Fraunhofer and Utzschneider at their establishment in Bavaria in 1805, and by his process achromatics of from six to nine inches in diameter were constructed. Afterwards Merz, the successor of Fraunhofer, succeeded in obtaining flint glass of the then unprecedented diameter of fifteen inches. Now we have in part turned the tables, and Mr. Chance, of Birmingham, owing to the introduction of foreign talent, has since constructed discs of glass of a workable diameter of twenty-five inches for Mr. Newall’s telescope, and for the American Government he has completed the large discs used in constructing the refractor of 26 inches’ diameter for the observatory at Washington (the Americans are never content till they go an inch beyond their rivals), while M. Feil of Paris, a descendant of the celebrated Guinand, has also made one of nearly 28 inches’ diameter for the Austrian Government.