On the other hand, outside lenses are also exposed to direct mechanical injury from handling and “cleaning.” As far as the latter operation is concerned, it frequently happens, particularly in glasses containing soda, that a slight surface dimming is formed on the glass when it has been left in a more or less damp place for a long time. This dimming is chiefly due to the formation on the surface of a great number of very minute crystals of carbonate of soda, which are hard and sharp enough to scratch the glass itself if rubbed about over it. If such a lens be wiped with a dry cloth, however clean and soft, the effect is a permanent injury to the polished surface, which could readily be avoided by first washing the lens with clean water, or even by using a wet cloth instead of a dry one for the first wiping. The mechanical hardness of the glass is an important factor in determining its resistance to such injurious treatment or to the effects of accidental contact with hard, sharp bodies. The subject of the hardness of glass has already been discussed in a general way in 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.