Abstract: The invention relates to a glass composition intended for the manufacture of thermally stable substrates or panes, which comprises the following constituents in the following proportions by weight:______________________________________ SiO.sub.2 55-70% Al.sub.2 O.sub.3 0-5% or 5-10% ZrO.sub.2 5-10% or 0-5% B.sub.2 O.sub.3 0-3% Na.sub.2 O 2-6% K.sub.2 O 5-11% MgO 0-6% CaO 2-11% SrO 4-12% BaO 0-2% ______________________________________with the following relationships:Na.sub.2 O+K.sub.2 O.gtoreq.8%MgO+CaO+SrO+BaO>10%the said composition having a strain point which is in excess of 600.degree. C.

Abstract: This invention is directed to photochromic glasses which darken to a brown coloration upon exposure to actinic radiation, which glasses have a refractive index of about 1.6, an Abbe number between 42-47, a density less than 2.82 g/cm.sup.3, and are preferably essentially free of TiO.sub.2. The glass compositions consist generally, in weight percent, of:______________________________________ SiO.sub.2 43-52 K.sub.2 O 2-9 Nb.sub.2 O.sub.5 6-16 B.sub.2 O.sub.3 12.5-18 MgO 0-5 Ag 0.100-0.175 Al.sub.2 O.sub.3 0-3 CaO 0-5 Cl 0.140-0.350 ZrO.sub.2 6-14 SrO 0-9 Br 0.093-0.195 Li.sub.2 O 1.5-3.5 BaO 0-9 CuO 0.008-0.030. Na.sub.