A consideration of the various factors that are involved in the production of a piece of perfect optical glass will make it apparent that the cost and difficulty of its production increases rapidly with the weight of the piece to be produced, so that it is not surprising to find that the price of very large discs of perfect optical glass such as those required for large astronomical telescopes, reaches figures which become prohibitive when very large sizes are considered. Thus, while it is quite possible to obtain say 100 pounds of good glass from a single melting if the glass is to be used in the form of pieces not weighing more than five or six pounds each, it is only rarely that a single block of perfect glass can be found weighing 100 pounds. In the former case the best pieces can be picked, the worst defects can be eliminated by chipping the rough fragments, and at a later stage other defective pieces can be cut off or ground away; not so where a large single block is required. A single fine vein, perhaps too small to be visible to the unaided eye, may be found to run through a whole block in such a way that it cannot be removed without breaking or cutting up the whole piece, and it will be seen that the frequency with which this is liable to occur increases with the volume of the piece required. The difficulties of re-heating and moulding are also increased enormously with the size of the individual pieces of glass that have to be dealt with, and where very large pieces have to be heated and cooled accidental breakage becomes a serious risk. In view of these difficulties it is not surprising to find that the dimensions of our astronomical refractors appear to have approached their limit, but rather are we led to admiration of the skill and enterprise that has pushed this limit so far as to produce discs of optical glass measuring as much as one metre in diameter. CHAPTER XIV. MISCELLANEOUS PRODUCTS. The field of glass-manufacture is so wide and the number and variety of its products so great, that in the limited compass of this volume it is impossible to fully enumerate them all; there are, however, a certain number of these products which, while of considerable importance in themselves, yet do not fall readily under any of the headings of the preceding chapters. A short space will therefore be devoted to some of these in this place.