glass  1. a hard, brittle, and often transparent material, usually consisting of the fused amorphous silicates of potassium or sodium, and of calcium, with silica in excess.  2. a container, usually cylindrical, made from glass. cover glass, a thin glass plate used to cover an object for microscopic examination. Written also coverglass. crown glass, a glass of low refractive index (achieved by incorporating a considerable percentage of phosphorus pentoxide); used in combination with flint glass in multielement lenses. cupping glass, a small vessel from which the air has been or can be exhausted, applied to the body in the practice of cupping (q.v.). flint glass, a highly refractive glass in which calcium has been replaced in large part by lead; used for lenses and prisms and in the manufacture of cut glass. object glass, see objective. optical glass, glass of high quality and controlled composition, used for lenses. quartz glass, pure fused silica, used for prisms, lenses, and chemical vessels because its index of thermal expansion is so small that it does not crack when heated or cooled and because it transmits more ultraviolet radiation than does ordinary glass. test glass, a small glass vessel, resembling a beaker, used in a chemical laboratory. Wood glass, see under filter.