Chapter II: , and little remains to be added here. Broadly speaking, a

high degree of hardness and a low refractive index are found together. This statement is certainly true where any considerable difference of hardness is considered, as, for example, in comparing a hard crown glass with a dense flint; but where the difference of refractive index or of density is small, it is not at all certain that the lighter glass will also be the harder.

The properties involved in the quality known as “hardness” also affect in a very marked manner the behaviour of glass when subjected to the grinding and polishing processes. The ease with which a good polish can be obtained varies very much in different kinds of glass, both the hardest and the softest glasses showing themselves difficult in this respect. The harder glasses are certainly less liable to accidental scratching during the polishing operations, and generally work in a cleaner manner; but the time required to produce a satisfactory polish is much greater owing to the resistance to displacement offered by the molecules. Both the speed of working and the pressure exerted during the polishing operation have, in fact, to be carefully adapted to the quality of the glass in this respect if the best possible results are to be obtained.

Another property which is essential in optical glass of the highest quality is that of freedom from internal strains. This subject will be again referred to later in connection with the annealing processes used in the manufacture of optical glass, and it need only be mentioned here that the presence of internal strain is readily recognised in glass, by the aid of the polariscope. Perfectly annealed glass, entirely free from internal strains, produces no effect upon a beam of polarised light passing through it, while even slightly strained glass becomes markedly doubly-refracting. For many purposes of optics this double refraction becomes undesirable or even inadmissible, especially as it is accompanied by small variations in the effective index of refraction of various portions of the mass of glass. Further, if the amount of double refraction observed is at all serious it indicates a state of strain which may easily lead to the fracture of the whole piece, particularly when undergoing the earlier stages of the grinding process or if exposed to shocks of any sort. As will be seen below, perfectly annealed glass is obtainable, but very special means are required for its production, and the optician should for that reason avoid making unnecessarily extreme demands in this direction. The very _small_ amount of double refraction frequently found in the better class of optical glass is entirely harmless for most purposes.