Optical glass differs so widely from all other varieties of glass that its manufacture may almost be regarded as a separate industry, to which, indeed, a separate volume could well be devoted. In the present chapter we propose to give an outline of the most important properties of optical glass, and in the next chapter to describe the more important features of the processes used in its production. The properties which affect the value of optical glass may roughly be divided into two groups. The first group comprises the specifically “optical” properties--_i.e._, those directly influencing the behaviour of light in its passage through the glass, while the second group covers those properties of a more general nature, which are of special importance in glass that is to be used for optical purposes. _Optical Properties of Glass._--The most essential property of glass in this respect is homogeneity. We have already indicated that glass can never be regarded as a definite chemical substance or compound, but that it usually consists of mutual solutions of various complex silicates, borates, etc. Solutions being of the very nature of mixtures of two or more different substances, it follows that they can only become homogeneous when _complete_ mixing has taken place. We have a familiar example of the formation of such a solution when sugar is dissolved in water. The water near the sugar becomes saturated with sugar and of different density from the remaining water; if the liquid is _slightly_ stirred a very characteristic phenomenon makes its appearance--the pure water and the dense sugar solution do not at once mix completely, the denser liquid remaining for a time disseminated throughout the whole fluid mass in the form of more or less fine lines, sheets, or eddies, and these are visible because the imperfectly mixed liquids have different effects on the light passing through them. In the case of sugar-water we are, however, dealing with a very mobile liquid, and a few turns of a tea-spoon suffice to render the mixture complete, and the liquid, which for a few moments had appeared turbid, becomes homogeneous and transparent. In the case of glass, when the raw materials are melted together, a mixture is formed of liquids of differing densities similar to that which was temporarily formed in the sugar-water solution. Molten glass, however, is never so mobile a liquid as ordinary water, nor is it in the ordinary course of manufacture subjected to any such thorough mixing action as that which is produced by a spoon in a glass of water. In glass as ordinarily manufactured, therefore, it is not surprising to find that the lack of homogeneity which originates during the melting persists to the end. Its effects can be traced whenever a thick piece of ordinary glass is carefully examined, when the threads or layers of differing densities can be recognised in the form of minute internal irregularities in the glass.