A dioptric substance is a transparent material through which rays of light pass in straight lines. Although for certain purposes coloured materials may be used, yet in the majority of cases the glass, or whatever the substance may be, should be colourless, clear, free from bubbles or opaque pieces, and free from any defect. In the middle of the 18th century, flint glass, or "English crystal" as it was once called, was principally made in England. Crown glass has a dispersion considerably less than that of flint glass. Durability is a measure of the resistance of a polished surface of glass to atmospheric attack. The varying humidity of the atmosphere results in corresponding variations in the water content of the glass surface, accompanied by alternations of deposition and evaporation of moisture. The expansion of glass comes to the notice of the optician most palpably during the working of a single prism or the like.