The next stage is perhaps the most important in the whole process. After numerous small samples of the molten glass have been taken, on the end of iron rods, to see if the air bubbles have been driven off, the mixture is stirred to render it homogeneous and to eradicate striæ. The stirring is carried out by means of a cylinder of fireclay which is first of all heated to the temperature of the molten glass before it is introduced. To the end of the fireclay cylinder a long, detachable iron handle is fixed so that the man who undertakes the stirring may stand at a distance from the hot furnace. The heat is great and the work of stirring is laborious and for this reason the stirrers are constantly changed. The iron handles must be watched carefully for, owing to the heat they rust rapidly and should any of the rust fall into the molten glass it would impart to it a colour and render it useless for optical purposes. When stirring begins the glass is liquid as water, but the stirring is continued during cooling and all the while the glass is gradually becoming more and more solid. During the final stages the operation is hard labour indeed and, finally, it is not possible to stir any more, then the fireclay cylinder is either removed or left in the glass. When the glass has solidified and, whilst it is still hot the furnace is sealed and allowed to cool very gradually till it has reached the ordinary temperature of its surroundings; this may take several weeks. When quite cool, the crucible is removed from the furnace and carefully broken; then the glass, which may be in one mass weighing as much as 1000 lbs., is freed from particles of fireclay and examined for defects. The next stage consists of moulding and annealing. Large pieces of glass are heated till they are just soft, then they are passed into iron or fireclay moulds designed so that the glass is formed into discs or slabs suitable for grinding by opticians. They are allowed to cool very slowly in the moulds. When the glass is taken from the moulds it is not yet ready to be made into lenses. It is subjected to another and very careful examination, when all defective parts are cut out. Should there be many defects the glass is again heated, moulded and annealed. From a crucible containing 1000 lbs. of molten glass it is unusual to obtain much more than 200 lbs. of optically perfect glass, it is obvious therefore that lenses cannot be cheap.