Abstract: A highly refractive, low-density phototropic glass in which the carriers of phototropy are precipitates containing silver, halogens and copper oxide, and which has a density equal to or less than 3.0 g/cm.sup.3, a refractive index equal to or greater than 1.60 and an Abbe number equal to or greater than 40, consisting essentially of, in % by weight on an oxide basis:______________________________________ SiO.sub.2 35-50 B.sub.2 O.sub.3 13.5-21 .SIGMA.SiO.sub.2 and B.sub.2 O.sub.3 50-70 Li.sub.2 O 0.0-2.5 Na.sub.2 O 0.0-4.0 K.sub.2 O 0.0-10.0 .SIGMA.Alkali metal oxides 4-14 BaO.sub.2 5.0-10.5 TiO.sub.2 3.5-6.5 ZrO.sub.2 4.5-9.5 Nb.sub.2 O.sub.3 5.0-11.5, ______________________________________the total of the foregoing oxides being 100% by weight, and in addition to the basic glass composition, carriers of phototropy consisting essentially of, in % by weight: ______________________________________ Ag.sub.2 O 0.05-0.25 Br 0.07-0.25 Cl 0.06-0.35 .SIGMA.Br and Cl .ltoreq.0.5 CuO 0.003-0.015.

Abstract: A highly refractive, low-density phototropic glass in which the carriers of phototropy are precipitates containing silver, halogens and copper oxide, and which has a density equal to or less than 3.0 g/cm.sup.3, a refractive index equal to or greater than 1.60 and an Abbe number equal to or greater than 40, consisting essentially of, in % by weight on an oxide basis:SiO.sub.2 : 35-50B.sub.2 O.sub.3 : 13.5-21.SIGMA.SiO.sub.2 and B.sub.2 O.sub.3 : 50-70Li.sub.2 O: 0.0-2.5Na.sub.2 O: 0.0-4.0K.sub.2 O: 0.0-10.0.SIGMA.Alkali metal oxides: 4-14BaO: 5.0-10.5TiO.sub.2 : 3.5-6.5ZrO.sub.2 : 4.5-9.5Nb.sub.2 O.sub.3 : 5.0-11.5,and in addition to the basic glass composition, carriers of phototropy consisting essentially of, in % by weight:Ag.sub.2 O; 0.05-0.25Br: 0.07-0.25Cl: 0.06-0.35.SIGMA.Br and Cl: .ltoreq.0.5CuO: 0.003-0.015.