Refractive uniformity of a borosilicate glass after different annealing treatments

In order to inves tigate cla im s that on ly 10 \\' ho ld in g tempe ratu res a re adequate when annealing optical glass for a hi ghly hom oge neo us p roduct, in te l"ferometri c tests were made o n te n 2-in ch cubes of borosi li cate glass a fte r a n annealing at 515 0 C, a nd then t he tests we re repeated afte r t ha cubes were r eann ealed , fi ve at 490 0 and fi ve at 530 0 C.For ea ch of t hree presentations of t he cubes with res pect to li gh t paths, co n to urs of differences ill refract ive index were draw n at, in tervals of 5 X 10-7• It was fo und t ha t ind ex vari atio ns seldo m exceed ed ± 1 X 10-6 i n t hi s a nn ealed glass.From a nalyses of t he data, it was cO ll cluded t ha t t he re need be li ttle, if any , differe nce in degree of homogeneity, eve n if t he h olding te mperat ure du rin g a nn ealin g is 30 or 40 0 C above t he lowest feasibl e va lue.