prepared through the conventional melting process is 535 u00b0 c and thus the crystallization temperature of the fluoride glass obtained by the process of the invention is higher than that of the glass prepared by the conventional process by 25 u00b0 c this shows that the thermal stability of the fluoride glass prepared by the present invention is improved over that of the fluoride glass prepared by the conventional melting process the improvement in thermal stability was attributed by removal of oxide impurities in the fluoride glass prepared according to the invention the oxide impurities being not removed in the fluoride glass prepared by the conventional fusing process the refractive index of the fluoride glass prepared by this example was n d 1 44 fluoride glasses were prepared by using sr dho 2 and ca dho 2 in place of mg dho 2 and using mgf 2 and srf 2 as the substrate in place of caf 2 about 7 mm thick glasses free of scatters were prepared a fluoride glass was deposited on the substrate while using the apparatus shown in fig2 zrbr 4 and ba pivaloyltrifluoromethylacetyacetone 2 hereinafter referred to ba pta 2 were introduced through the inlet 1a zrbr 4 was prepared by sublimating granular zirconium 5 from the heater 2a which was heated to 350 u00b0 c and reacting the sublimated zirconium with bromine gas the feed pipe 7 having the variable leak valve was connected to the container in which granular zirconium 5 was contained bromine gas and argon were fed to the container so that zrbr 4 was sublimated by the reaction between zr and br 2 and the sublimated zrbr 4 was supplied through the inlet 1a.