Perhaps the most remarkable fact about the quality of modern plate-glass is its relatively high degree of homogeneity. Glass, as we have seen in Chapter I., is not a chemically homogeneous substance, but rather a mixture of a number of substances of different density and viscosity. Wherever this mixture is not sufficiently intimate, the presence of diverse constituents becomes apparent in the form of striæ, arising from the refraction or bending of light-rays as they pass from one medium into another of different density. Except in glass that has undergone elaborate stirring processes, such striæ are never absent, but the skill of the glass-maker consists in making them as few and as minute as possible, and causing them to assume directions and positions in which they shall be as inconspicuous as possible. In plate-glass this is generally secured in a very perfect manner, and to ordinary observation no striæ are visible when a piece of plate-glass is looked at in the ordinary way, _i.e._, through its smallest thickness; if the same piece of glass be looked at transversely, the edges having first been polished in such a way as to render this possible, the glass will be seen to be full of striæ, generally running in fine lines parallel with the polished surfaces of the glass. This uniform direction of the striæ is partly derived from the fact that the glass has been caused to flow in this direction by the action of the roller when first formed into a slab, but this process would not obliterate any serious inequalities of density which might exist in the glass as it leaves the pot, so that successful results are only attainable if great care is taken to secure the greatest possible homogeneity in the glass during the melting process.