table i records a group of glass compositions melted on a laboratory scale and reported in terms of parts by weight on the oxide basis illustrating the present invention . because the sum of the individual components totals or very closely approximates 100 , for all practical purposes the tabulated values may be deemed to represent weight percent . table ia recites the same group of glass compositions expressed in terms of cation percent . the actual batch ingredients may comprise any materials , either oxides or other compounds , which , when melted together , are converted into the desired oxides in the proper proportion . for example , either tlno 3 or tl 2 co 3 may be employed as the source of thallium . the batch ingredients were compounded , tumble mixed together to assist in securing a homogeneous melt , and then charged into gold , platinum , or vycorr ® brand crucibles . after placing lids thereon , the crucibles were introduced into a furnace operating at about 800 ° to 1100 ° c . and the batches were melted for about 15 to 60 minutes . the melts were subsequently poured into steel or graphite molds to yield glass discs having a diameter of about 111 and a thickness of about 5mm which were transferred immediately to an annealer operating at about 175 ° to 300 ° c . table i______________________________________ 1 2 3 4 5 6 7______________________________________tl . sub . 2 o 54 . 8 55 . 9 56 . 0 56 . 1 -- -- 17 . 6pbo -- -- -- -- 78 . 0 73 . 2 55 . 4bi . sub . 2 o . sub . 3 30 . 3 30 . 6 30 . 7 30 . 8 -- -- -- ga . sub . 2 o . sub . 3 -- 2 . 5 4 . 5 6 . 2 -- -- -- sio . sub . 2 -- -- -- -- 4 . 8 4 . 9 5 . 0geo . sub . 2 10 . 8 11 . 0 6 . 9 6 . 9 -- -- -- teo . sub . 2 4 . 1 -- -- -- 17 . 2 21 . 8 22 . 0______________________________________ 8 9 10 11 12 13 14______________________________________tl . sub . 2 o 46 . 9 57 . 9 44 . 0 54 . 4 32 . 7 30 . 9 18 . 4pbo -- -- -- -- -- 13 . 1 19 . 4sb . sub . 2 o . sub . 3 -- -- 56 . 0 45 . 6 67 . 3 56 . 0 62 . 0sio . sub . 2 0 . 3 0 . 3 -- -- -- -- teo . sub . 2 52 . 8 47 . 8 -- -- -- -- -- ______________________________________ 15 16______________________________________tl . sub . 2 o 48 . 6 50 . 9teo . sub . 2 24 . 3 41 . 1sio . sub . 2 0 . 3 0 . 4wo . sub . 3 26 . 9 -- zno -- 7 . 7______________________________________ table ia______________________________________ ( cation percent ) ______________________________________ 1 2 3 4 5 6 7______________________________________tlo . sub . 0 . 5 50 . 0 50 . 0 50 . 0 50 . 0 -- -- 15 . 0pbo -- -- -- -- 65 . 0 60 . 0 45 . 0bio . sub . 1 . 5 25 . 0 25 . 0 25 . 0 25 . 0 -- -- -- gao . sub . 1 . 5 -- 5 . 0 9 . 0 12 . 5 -- -- -- sio . sub . 2 -- -- -- -- 15 . 0 15 . 0 15 . 0geo . sub . 2 20 . 0 20 . 0 16 . 0 12 . 5 -- -- -- teo . sub . 2 5 . 0 -- -- -- 20 . 0 25 . 0 25 . 0______________________________________ 8 9 10 11 12 13 14______________________________________tlo . sub . 0 . 5 39 . 5 44 . 5 35 . 0 45 . 0 25 . 0 25 . 0 14 . 5pbo -- -- -- -- -- 10 . 0 14 . 5sbo . sub . 1 . 5 -- -- 65 . 0 55 . 0 75 . 0 65 . 0 71 . 0sio . sub . 2 1 . 0 1 . 0 -- -- -- -- -- teo . sub . 2 59 . 5 54 . 5 -- -- -- -- -- ______________________________________ 15 16______________________________________tlo . sub . 0 . 5 45 . 6 40 . 5teo . sub . 2 30 . 4 43 . 5sio . sub . 2 0 . 9 1 . 0wo . sub . 3 23 . 1 -- zno -- 16 . 0______________________________________ table ii records the density ( ρ ) in grams / cm 3 and linear index of refraction ( n d ) determined in accordance with measuring techniques conventional in the glass art , as well as the first diagonal term of the third order susceptibility tensor ( x . sup . ( 3 )) in units of 10 - 14 esu as measured by the aforementioned technique of degenerate four wave mixing . table ii______________________________________1 2 3 4 5 6 7______________________________________ρ7 . 891 7 . 783 7 . 913 7 . 904 7 . 402 7 . 210 7 . 186n . sub . d2 . 44 -- 2 . 35 -- -- 2 . 27 -- x . sup . ( 3 ) 71 ± 11 -- 88 ± 13 -- -- 40 ± 6 -- ______________________________________8 9 10 11 12 13 14______________________________________ρ6 . 693 6 . 882 6 . 445 6 . 654 6 . 132 5 . 931 5 . 813n . sub . d2 . 27 2 . 28 -- -- 2 . 22 -- -- x . sup . ( 3 )-- -- -- -- 32 ± 5 -- -- ______________________________________15 16______________________________________ρ7 . 449 6 . 570n . sub . d -- -- x . sup . ( 3 )-- -- ______________________________________ it is noteworthy that the value of x . sup . ( 3 ) for example 3 , 88 × 10 - 14 esu , is , to the best of our knowledge , the largest nonlinear susceptibility that has been measured for any oxide glass and is roughly 250x that of vitreous silica . based upon an overall review of physical properties and melting and forming characteristics , the preferred compositions of the present inventive glasses consist essentially , in cation percent on the oxide basis , of ( a ) 38 - 60 % tlo 0 . 5 , 10 - 45 % bio 1 . 5 , 5 - 30 % geo 2 , and 5 - 10 % gao 1 . 5 , the sum of tlo 0 . 5 + bio 1 . 5 + geo 2 + gao 1 . 5 constituting at least 90 % of the total composition ; ( b ) 20 - 40 % teo 2 , 5 - 20 % sio 2 , 0 - 50 % tlo 0 . 5 , and 10 - 65 % pbo , the sum of teo 2 + sio 2 + tlo 0 . 5 30 pbo constituting at least 70 % of the total composition ; and ( c ) 50 - 75 % sbo 1 . 5 , 5 - 50 % tlo 0 . 5 , and 0 - 20 % pbo . in the glasses defined in paragraph ( a ), up to 10 % total of at least one member of the group in the indicated proportion of up to 10 % sio 2 and up to 5 % teo 2 may desirably be included . in the glasses defined in paragraph ( b ), up to 30 % total of at least one member of the group in the indicated proportion of up to 30 % wo 3 and up to 20 % zno may desirably be included . application of the aforementioned glasses to devices may necessitate the synthesis of an optical waveguide structure . as is generally known to one skilled in the art , in order for such a structure to operate properly there must be a difference in refractive index existing between the core and cladding . this difference is dependent upon the type of waveguide , i . e ., a single mode or multimode , and upon the use of the waveguide , e . g ., whether the fiber is used in a straight configuration or whether bent . accordingly , whereas both the core and the cladding glasses can be prepared from compositions within the same inventive ranges , it will be appreciated that the compositions of the core and cladding glasses will be sufficiently dissimilar to achieve a desired difference in refractive index between the core and cladding . while the principles of the instant invention have been described above in connection with specific embodiments and particular modifications thereof , it is to be clearly understood that this description is made only by way of example , and not as a limitation on the scope of the invention . said principles may be otherwise embodied within the scope of the following claims .