still to be regarded as a product whose evolution is not yet complete, and there can be no doubt that there are great possibilities open to the material when its manufacture has been more fully developed. CHAPTER X. SHEET AND CROWN GLASS. In the preceding chapter we have dealt with the processes of manufacture employed in the production of both the crudest and the most perfect forms of flat glass as used for such purposes as the glazing of window openings. The products now to be dealt with are of an intermediate character, sheet-glass possessing many of the properties of polished plate, but lacking some very important ones; thus sheet-glass is sufficiently transparent to allow an observer to see through it with little or no disturbance--in the best varieties of sheet-glass the optical distortion caused by its irregularities is so small that the glass appears nearly as perfect as polished plate--but in the cheap glass that is used for the glazing of ordinary windows, sheets are often employed which produce the most disturbing, and sometimes the most ludicrous, distortions of objects seen through them. It is a curious fact that even in good houses the use of such inferior glass is tolerated without comment, the general public being, apparently, remarkably nonobservant in this respect. In another direction sheet-glass has the great advantage over plate-glass that it is very much lighter, or can at least be produced of much smaller weight and thickness, although this advantage entails the consequent disadvantage that sheet-glass is usually much weaker than plate, and can only be used in much smaller sizes. In recent times the production of relatively thin plate-glass has, however, made such strides that it is now possible to obtain polished plate-glass thin enough and light enough for almost every architectural purpose. Finally, the most important advantage of sheet-glass, and the one which alone secures its use in a great number of cases in preference to plate-glass, is its cheapness, the price of ordinary sheet-glass being about one-fourth that of plate-glass of the same size.