a back and forth motion at a rate of 100 cycles per min the glass was melted at 950 u00b0 c for 2 hrs refined at 850 u00b0 c for 1 hr annealed at around the glass transition temperature of 375 u00b0 c for 30 min and then cooled to room temperature orbital mixing and reciprocal mixing were stopped when the temperature reached 375 u00b0 c at which temperature the melt had already solidified the resulting glass window 2 0 in diam by 0 75 in thick was examined under a high magnification microscope no bubbles micro crystals or inclusions were observed the index homogeneity was also investigated using a zygo interferometer distortion due to index inhomogeneity was measured to be as low as 100 nm peak to valley throughout the window about 1 300 g of glass of similar composition as in example 1 were melted in a round platinum crucible sealed with a tight platinum cap the crucible was placed inside a furnace positioned on top of a model g10 new brunswick scientific orbital mixer having an orbital diameter of 1 0 in nitrogen was introduced inside the furnace chamber the melt was heated to 950 u00b0 c at which temperature the mixer was turned on.