Abstract:
Disclosed are zirconium-containing borosilicate glass compositions having a refractive index n e  between 1.55 and 1.72, and Abbe number ν e  between 50 and 34 and a negative anomalous partial dispersion value Δν e  between -3.0 and -7.7. Also disclosed is a process for the production of such glass compositions.

Description:
BACKGROUND OF THE INVENTION 
     The invention pertains to optical borosilicate glasses containing high zirconium concentrations and having intermediate to high refractive indices and relatively low Abbe numbers, as well as to a process for the production of such glass compositions. 
     The demand for optical glasses with special physical properties on the part of optical engineers has risen strongly in recent times. For the redesign of optical systems, particularly microscope and photographic lenses, optical grade glasses characterized by high refractive indices n e  but at the same time possessing the lowest possible Abbe numbers ν e  are increasingly required. By using such special glasses, optical engineers are able to eliminate optical abberations in optical systems and to improve the corrective quality of an objective to a much higher degree. Anomalous partial dispersion values play a decisive role in this respect. U.S. Pat. No. 3,451,829 recites additional details concerning the significance, the application and the nomenclature of optical glasses having anomalous partial dispersions. 
     In addition to these purely physical properties which are demanded of advanced optical glasses, those skilled in the glass art must also consider chemical-industrial requirements, i.e., the glasses must be capable of being produced on an industrial scale and they must not have any distinct crystallization tendencies. Economical production requires melting temperatures which are not excessive. Finally, technical processing factors play an important role in the selection and use of glasses of this type. 
     Even though glasses with compositions containing essentially silicon dioxide (SiO 2 ), boron trioxide (B 2  O 3 ), zirconium dioxide (ZrO 2 ) and alkaline earth metal oxides are known, such glasses have proven to be unsuited for industrial application, because, as a result of their distinct crystallization tendencies, they may be melted only in small and therefore uneconomical units. Were one to produce comparable glasses containing alkali metal oxides in place of alkaline earth metal oxides, the negative deviations from the so-called &#34;standard straight line,&#34; i.e., the (-)Δν e  values, would be significantly lower. 
     Furthermore, U.S. Pat. No. 2,150,694 describes a process for the manufacture of optical glasses having high refractive indices and low scatter, which contain, inter alia, one or more oxides of the elements titanium, yttrium, zirconium, niobium, lanthanum, tantalum and thorium. Further additives may consist of thorium and hafnium. However, such known glasses have different values in their n e  /ν e  pairs of parameters. Also, no data are given concerning the required anomaly of the partial dispersion. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to eliminate the disadvantages of known glasses and to provide optical glasses which, in addition to n e  /ν e  values which are correlated with one another, exhibit negative anomalous partial dispersions for the correction of the secondary spectrum of optically imaging systems. 
     It is another object of the invention to provide a process for the production of such optical glasses. 
     In accomplishing the foregoing objects, there has been provided in accordance with the present invention a zirconium-containing borosilicate glass composition having a refractive index n e  in the range of between about 1.55&lt;n e  &lt;1.72, an Abbe number ν e  in the range of between about 50&gt;ν e  &gt;34 and negative anomalous partial dispersion value Δν e  between about -3.0 and -7.7 comprising the following components: 
     
         ______________________________________(a)  from about 8.4            to 45.5% by weight SiO.sub.2, andfrom about 0.9            to 33.0% by weight B.sub.2 O.sub.3,            wherein the sum of (SiO.sub.2 + B.sub.2 O.sub.3)            amounts to between about 31.6 and            46.4% by weight(b)  from about 13.5            to 18.9% by weight ZrO.sub.2,            wherein the sum of (SiO.sub.2 + B.sub.2 O.sub.3 +            ZrO.sub.2) amounts to between about 46.1            and 64.4% by weight;(c)  0           to about 5.0% by weight Li.sub.2 O,0           to about 22.3% by weight Na.sub.2 O,0           to about 10.0% by weight K.sub.2 O,            and0           to about 24.0% by weight NaF,            wherein the sum of the alkali            metal oxides (Li.sub.2 O + Na.sub.2 O + K.sub.2 O)            amounts to between 0 and about            21.8% by weight and the sum of            (alkali metal oxides + NaF)            amounts to between about 8.0 and            24.0% by weight;(d)  0           to about 2.0% by weight BaO,0           to about 2.3% by weight ZnO,0           to about 0.5% by weight CdO,            and0           to about 5.0% by weight PbO,            wherein the sum of the bivalent            oxides (BaO + ZnO + CdO + PbO)            amounts to between 0 and about            5.5% by weight;(e)  0           to about 9.9% by weight Al.sub.2 O.sub.3,0           to about 24.8% by weight La.sub.2 O.sub.3,0           to about 23.1% by weight Sb.sub.2 O.sub.3,            and0           to about 4.0% by weight Y.sub.2 O.sub.3,            wherein the sum of the trivalent            oxides (Al.sub.2 O.sub.3 + La.sub.2 O.sub.3 + Sb.sub.2            O.sub.3 +            Y.sub.2 O.sub.3) amounts to between 0 and            about 26.9% by weight;(f)  0           to about 5.0% by weight GeO.sub.2, and0           to about 1.0% by weight TiO.sub.2,            wherein the sum of the tetravalent            oxides (ZrO.sub.2 + GeO.sub.2 + TiO.sub.2) amounts            to between about 13.5 and 20.6%            by weight;(g)  0           to about 5.0% by weight Nb.sub.2 O.sub.5, and0           to about 33.1% by weight Ta.sub.2 O.sub.5,            wherein the sum of the pentavalent            oxides (Nb.sub.2 O.sub.5 + Ta.sub.2 O.sub.5) amounts to            between 0 and about 36.1% by            weight; and(h)  0           to about 2.1% by weight WO.sub.3.______________________________________ 
    
     According to a further aspect of the invention, the glass compositions may further comprise up to about 3% by weight of a modifying additive selected from SnO 2 , P 2  O 5  and mixtures thereof. 
     There has also been provided according to the present invention a process for the production of a glass composition as defined above, comprising the steps of: (a) heating the mixture of the aforesaid components in a platinum crucible to a temperature of between about 1345° and 1400° C. to form a melt; (b) refining the melt at a temperature of between about 1385° and 1450° C. for a period of time of from about 7 to 15 minutes; (c) homogenizing the melt with agitation at a temperature of between about 1360° and 1400° C. for a period of time of from about 80 to 120 minutes; (d) lowering the temperature of the melt down under agitation to a pouring temperature of between about 1135° C. and 1185° C. within a period of time of from about 3 to 8 minutes; and (e) pouring the melt into at least one preheated mold. 
     Further objects, features and advantages of the invention will become readily apparent from the detailed description of preferred embodiments which follows. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The glasses of the present invention have the further advantages that they may be agitated down to relatively low pouring temperatures without crystallization. This alone makes it possible to obtain in larger units glasses free of striation and segregation. 
     Exemplary embodiments of the invention are presented in the tables which follow hereafter (in weight %). In addition to the individual components, intermediate sums of groups of components are also entered. Because of their similar effects in complex inorganic multi-component systems, these groups are suitably combined. The following physical parameters are given at the end of each vertical column: 
     n e  : refractive index 
     ν e  : Abbe number (reciprocal value of dispersion) 
     θ&#39; g  : anomalous partial dispersion, where: ##EQU1##  with the meaning of the subscripts as follows: g=blue mercury line (435.84 nm), 
     F&#39;=blue cadmium line (479.99 nm), 
     C&#39;=red cadmium line (643.85 nm); 
     -Δν e  : negative deviation from the so-called &#34;standard straight line&#34;, such as shown graphically for example in the drawing of U.S. Pat. No. 3,451,829. 
     In Table 1, in addition to the principal glass forming components such as silicon dioxide (SiO 2 ) and boron trioxide (B 2  O 3 ) and the essential component of zirconium dioxide (ZrO 2 ), sodium compounds and tantalum and tungsten oxides are also listed. In the process, sodium fluoride (NaF) is partially replaced by sodium oxide. The examples show that SiO 2  and B 2  O 3  are most extensively interchangeable. 
     Table 2 presents examples which, in addition to a variation of the proportion of alkali metals, also demonstrate the effect of antimony trioxide (Sb 2  O 3 ) on physical properties. 
     Tables 3 to 7 show examples with additional oxides of higher valence, such as germanium dioxide (GeO 2 ), titanium dioxide (TiO 2 ), lanthanum oxide (La 2  O 3 ), yttrium oxide (Y 2  O 3 ) and niobium pentoxide (Nb 2  O 5 ). Beginning with Example 57, bivalent metallic oxides such as barium oxide (BaO), zinc oxide (ZnO) and cadmium oxide (CdO) are also added. 
     The process for the production of the glasses of the invention shall be demonstrated in more detail by the following example of a melt: 
     A weighed charge of 5 kg. consisting of 
     20% by weight SiO 2   
     23.8 by weight B 2  O 3   
     17.7 by weight ZrO 2   
     17.9 by weight Na 2  O 
     20.6 by weight Ta 2  O 5   
     is mixed well and placed by portions in a platinum crucible preheated to 1380° C. After the charge mixture has melted down, the melt is refined at 1430° C. for 10 minutes and is subsequently homogenized at 1385° C. for approximately 100 minutes with constant stirring. The temperature of the melt is then lowered under agitation conditions within a few minutes down to the pouring temperature of 1150° C. and is poured into preheated molds. 
     Following precision cooling, the optical parameters of the glass melted by the process described hereabove are: 
     n e  =1.641323 
     ν e  =42.23 
     θ&#39; g  =0.4970 
     Δν e  =-7.02 
     Obviously, the melting conditions to be applied vary as a function of the charge composition chosen (temperature, time), but without exceeding or going below the temperature intervals or reaction times given above. 
     
                                           TABLE 1__________________________________________________________________________(Weight %)Example  1   2   3   4   5   6   7   8   9   10  11  12__________________________________________________________________________SiO.sub.2    45.5        40.5            34.4                28.4                    19.4                        16.4                            8.4 40.5                                    41.7                                        41.7                                            29.7                                                25.7B.sub.2 O.sub.3     0.9         0.9             7.0                13.0                    22.0                        25.0                            33.0                                 0.9                                     0.9                                         0.9                                            12.9                                                16.9Σ (SiO.sub.2 + B.sub.2 O.sub.3)    46.4        41.4            41.4                41.4                    41.4                        41.4                            41.4                                41.4                                    42.6                                        42.6                                            42.6                                                42.6ZrO.sub.2    18.0        18.0            18.0                18.0                    18.0                        18.0                            18.0                                18.0                                    18.5                                        18.5                                            18.5                                                18.5Σ (SiO.sub.2 + B.sub.2 O.sub.3    64.4        59.4            59.4                59.4                    59.4                        59.4                            59.4                                59.4                                    61.6                                        61.6                                            61.6                                                61.1  + ZrO.sub.2)Na.sub.2 O    --  --  --  --  --  --  --  12.0                                     9.4                                        17.4                                            17.4                                                17.4NaF      24.0        24.0            24.0                24.0                    24.0                        24.0                            24.0                                12.0                                    12.4                                         4.4                                             4.4                                                 4.4Ta.sub.2 O.sub.5     9.6        14.6            14.6                14.6                    14.6                        14.6                            14.6                                14.6                                    15.0                                        15.0                                            15.0                                                15.0WO.sub.3  2.0         2.0             2.0                 2.0                     2.0                         2.0                             2.0                                 2.0                                     2.1                                         2.1                                             2.1                                                 2.1n.sub.e  1.5587        1.5674            1.5714                1.5811                    1.5909                        1.5929                            1.5906                                1.5831                                    1.5962                                        1.6074                                            1.6210                                                1.6221ν.sub.e    49.7        48.3            47.9                47.0                    45.7                        45.5                            45.3                                46.7                                    46.0                                        44.9                                            44.3                                                44.2θ&#39; × 10.sup.-4    4862        4938            4896                4949                    4934                        4945                            4929                                4916                                    4929                                        4952                                            4964                                                4962-Δν.sub.e     7.2        --   6.6                 3.8                     6.1                         5.6                            6.9  6.6                                     6.2                                         5.7                                            5.4  5.7__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________(Weight %)Example  13  14  15  16  17  18  19  20  21  22  23  24__________________________________________________________________________SiO.sub.2    25.7        13.7            18.1                16.5                    17.3                        24.8                            18.9                                17.7                                    15.7                                        21.7                                            21.7                                                21.7B.sub.2 O.sub.3    16.9        28.9            25.4                23.2                    21.4                        15.8                            17.2                                20.9                                    20.9                                        20.9                                            20.9                                                20.9Σ (SiO.sub.2 + B.sub.2 O.sub.3)    42.6        42.6            43.5                39.7                    38.7                        40.6                            36.1                                38.6                                    36.6                                        42.6                                            42.6                                                42.6ZrO.sub.2    18.5        18.5            18.9                16.8                    16.4                        13.5                            15.6                                18.5                                    18.5                                        18.5                                            18.5                                                18.5Σ (SiO.sub.2 + B.sub.2 O.sub.3    61.1        61.1            62.4                56.5                    55.1                        54.1                            51.7                                57.1                                    55.1                                        61.1                                            61.1                                                61.1  + ZrO.sub.2)Li.sub.2 O    --  --  --  --  --  --  --  --  --   5.0                                            --  --Na.sub.2 O    21.8        21.8            22.3                20.4                    19.8                        15.4                            16.7                                21.8                                    21.8                                        16.8                                            16.8                                                11.8K.sub.2 O    --  --  --  --  --  --  --  --  --  --   5.0                                                10.0Sb.sub.2 O.sub.3    --  --  --  --  --  --  --  21.1                                    23.1                                        17.1                                            17.1                                                17.1Ta.sub.2 O.sub.5    15.0        15.0            15.3                23.1                    25.1                        30.5                            31.6                                --  --  --  --  --WO.sub.3  2.1         2.1            --  --  --  --  --  --  --  --  --  --n.sub.e  1.6274        1.6260            1.6312                1.6419                    1.6603                        1.6613                            1.6807                                1.6471                                    1.6577                                        1.6449                                            1.6415                                                1.6314ν.sub.e    43.6        43.4            43.3                41.7                    40.0                        39.9                            38.0                                39.0                                    37.2                                        41.4                                            40.0                                                41.3θ.sub.g &#39; × 10.sup.-4    4961        4948            4960                4983                    4994                        5019                            5033                                5064                                    5098                                        5000                                            5014                                                5002-Δν.sub.e     6.3         7.4             6.7                 6.7                     7.6                         5.9                             6.7                                 3.5                                     3.0                                         5.9                                             6.0                                                 5.7__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________(Weight %)Example: 25  26  27  28  29  30  31  32  33  34  35  36__________________________________________________________________________SiO.sub.2    12.4        21.7            21.7                17.7                    17.7                        21.7                            21.7                                21.7                                    21.7                                        21.7                                            17.7                                                17.7B.sub.2 O.sub.3    29.0        20.9            20.9                24.9                    24.9                        20.9                            20.9                                20.9                                    20.9                                        20.9                                            24.9                                                24.9Σ(SiO.sub.2 + B.sub.2 O.sub.3)    41.4        42.6            42.6                42.6                    42.6                        42.6                            42.6                                42.6                                    42.6                                        42.6                                            42.6                                                42.6ZrO .sub.2    18.0        16.5            14.5                18.5                    18.5                        18.5                            18.5                                18.5                                    18.5                                        18.5                                            18.5                                                18.5Σ(SiO.sub.2 + B.sub.2 O.sub.3    59.4        59.1            57.1                61.1                    61.1                        61.1                            61.1                                61.1                                    61.1                                        61.1                                            61.1                                                61.1+ ZrO.sub.2)Na.sub.2 O    --  16.8            16.8                21.8                    21.8                        21.8                            21.8                                21.8                                    21.8                                        21.8                                            21.8                                                21.8K.sub.2 O    --   5.0             5.0                --  --  --  --  --  --  --  --  --NaF      24.0        --  --  --  --  --  --  --  --  --  --  --La.sub.2 O.sub.3    --  --  --  --  --  --  --  --  --  --   4.0                                                8.0Sb.sub.2 O.sub.3     2.0        17.1            17.1                 2.1                     4.1                         6.1                            8.1 10.1                                    12.1                                        14.1                                             2.1                                                2.1Ta.sub.2 O.sub.5    12.6         2.0             4.0                15.0                    15.0                        11.0                            9.0  7.0                                     5.0                                         3.0                                            11.0                                                7.0WO.sub.3  2.0        --  --  --  --  --  --  --  --  --  --  --n.sub.e  1.5934        1.6359            1.6367                1.6286                    1.6303                        1.6347                            1.6347                                1.6363                                    1.6339                                        1.6334                                            1.6317                                                1.6336ν.sub.e    45.0        40.8            40.6                43.1                    42.8                        42.5                            42.0                                41.6                                    41.6                                        41.4                                            44.6                                                43.6θ&#39;.sub.g  × 10.sup.-4    4929        5023            5022                4958                    4964                        4981                            4979                                4992                                    5009                                        5008                                            4931                                                4948-Δν.sub.e     7.2         4.7             5.0                 6.9                     6.9                         6.0                             6.6                                 6.2                                     5.1                                         5.2                                             7.4                                                 7.2__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________(Weight %)Example: 37  38  39  40  41  42  43  44  45  46  47  48__________________________________________________________________________SiO.sub.2    17.7        17.7            20.7                19.7                    20.7                        20.7                            12.7                                12.7                                    12.7                                        10.7                                            12.7                                                11.0B.sub.2 O.sub.3    24.9        24.9            24.9                24.9                    24.9                        24.9                            24.9                                24.9                                    24.9                                        24.9                                            24.9                                                24.9Σ(SiO.sub.2 + B.sub.2 O.sub.3)    42.6        42.6            45.6                44.6                    45.6                        45.6                            37.6                                37.6                                    37.6                                        35.6                                            37.6                                                35.9ZrO.sub.2    18.5        18.5            18.5                16.5                    14.5                        14.5                            14.5                                14.5                                    14.5                                        14.5                                            14.5                                                14.5Σ(SiO.sub.2 + B.sub.2 O.sub.3    61.1        61.1            64.1                61.1                    60.1                        60.1                            52.1                                52.1                                    52.1                                        50.1                                            52.1                                                50.4  + ZrO.sub.2)Na.sub.2 O    21.8        19.8            16.0                14.0                    12.0                         8.0                            10.0                                11.0                                    12.0                                        12.0                                            12.0                                                12.0La.sub.2 O.sub.3    12.0        10.0            13.8                17.8                    20.8                        24.8                            12.8                                 6.8                                    16.8                                        12.8                                             6.8                                                 6.8Sb.sub.2 O.sub.3     2.1         2.1             2.1                 2.1                     2.1                         2.1                            10.1                                15.1                                     2.1                                        10.1                                            10.1                                                14.1Y.sub.2 O.sub. 3    --  --  --  --  --  --  --  --   4.0                                        --  --  --GeO.sub.2    --  --  --  --  --  --  --  --  --   2.0                                             4.0                                                 1.7Ta.sub.2 O.sub.5     3.0         7.0             4.0                 5.0                     5.0                         5.0                            15.0                                15.0                                    13.0                                        13.0                                            15.0                                                15.0n.sub.e  1.6292        1.6386            1.6376                1.6475                    1.6487                        1.6616                            1.6936                                1.6875                                    1.6863                                        1.6884                                            1.6750                                                1.6875ν.sub.e    45.5        44.7            46.4                46.5                    47.4                        47.9                            39.8                                37.8                                    43.5                                        39.7                                            39.5                                                37.9θ.sub.g &#39; × 10.sup.-4    4920        4928            4914                4930                    4904                        4904                            5022                                5051                                    4965                                        4997                                            5021                                                5047-Δν.sub.e     7.2         7.5             6.8                 5.5                     6.5                         6.0                             5.8                                 5.8                                     6.1                                         7.7                                             6.3                                                 6.0__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________(Weight %)Example: 49  50  51  52  53  54  55  56  57  58  59  60__________________________________________________________________________SiO.sub.2    12.7        12.7            19.1                18.3                    18.1                        19.1                            19.1                                19.1                                    19.0                                        18.1                                            19.0                                                19.1B.sub.2 O.sub.3    24.9        24.9            16.4                16.7                    16.5                        17.4                            17.4                                17.4                                    17.3                                        25.4                                            17.3                                                17.4Σ(SiO.sub.2 + B.sub.2 O.sub.3)    37.6        37.6            35.5                35.0                    34.6                        36.5                            36.5                                36.5                                    36.3                                        43.5                                            36.3                                                36.5ZrO.sub.2    14.5        15.0            16.5                15.8                    15.6                        16.5                            16.5                                16.5                                    16.4                                        18.9                                            16.4                                                16.5Σ(SiO.sub.2 + B.sub.2 O.sub.3    52.1        52.6            52.0                50.8                    50.2                        53.0                            53.0                                53.0                                    52.7                                        62.4                                            52.7                                                53.0+ ZrO.sub.2)Na.sub.2 O    12.0        12.0            16.9                16.3                    16.2                        16.9                            16.9                                16.9                                    16.8                                        20.0                                            16.8                                                15.9BaO      --  --  --  --  --  --  --  --  --  --  --   1.0ZnO      --  --  --  --  --  --  --  --   0.5                                         2.3                                            --  --CdO      --  --  --  --  --  --  --  --  --  --   0.5                                                --Al.sub.2 O.sub.3     2.0         9.9             1.0                --  --  --  --  --  --  --  --  --La.sub.2 O.sub.3     8.8        --  --  --  --  --  --  --  --  --  --  --Sb.sub.2 O.sub.3    10.1        --  --  --  --  --  --  --  --  --  --  --Y.sub.2 O.sub.3    --   0.2            --  --  --  --  --  --  --  --  --  --GeO.sub.2    --  --  --   4.0                     5.0                        --  --  --  --  --  --  --Nb.sub.2 O.sub.5    --  --   1.0                --  --   1.0                             3.0                                --   3.0                                        --   3.0                                                 1.0Ta.sub.2 O.sub.5    15.0        15.0            29.1                28.9                    28.6                        29.1                            27.1                                29.1                                    27.0                                        15.3                                            27.0                                                29.1TiO.sub.2    --  --  --  --  --  --  --   1.0                                    --  --  --  --n.sub.e  1.6705        1.6830            1.6778                1.6769                    1.6742                        1.6808                            1.6835                                1.6814                                    1.6817                                        1.6334                                            1.6806                                                1.6803ν.sub.e    40.0        39.4            38.1                38.6                    38.8                        38.0                            37.5                                37.7                                    37.7                                        43.2                                            37.8                                                38.2θ&#39;.sub.g  × 10.sup.-4    5017        5021            5035                5026                    5021                        5035                            5040                                5047                                    5046                                        4949                                            5049                                                5040-Δν.sub.e     5.8         6.3             6.6                 6.7                     6.9                         6.7                             6.8                                 6.2                                     6.3                                         7.5                                             5.9                                                 6.2__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________(Weight %)Example: 61  62  63  64  65  66  67__________________________________________________________________________SiO.sub.2    19.1        19.0            18.6                17.4                    17.7                        17.5                            17.5B.sub.2 O.sub.3    17.4        17.3            17.0                15.8                    16.1                        15.8                            15.8Σ(SiO.sub.2 + B.sub.2 O.sub.3)    36.5        36.3            35.6                33.2                    33.8                        33.3                            33.3ZrO.sub.2    16.5        16.4            16.0                15.2                    15.5                        15.6                            15.6Σ(SiO.sub.2 + B.sub.2 O.sub.3    53.0        52.7            51.6                48.4                    49.3                        48.9                            48.9+ ZrO.sub.2)Li.sub.2 O    --  --  --  --  1.0 2.5 1.0Na.sub.2 O    14.9        16.8            16.4                15.0                    14.4                        12.5                            14.0BaO       2.0         0.5             0.5                 0.5                     0.5                        0.5 0.5Nb.sub.2 O.sub.5     1.0         3.0             5.0                 3.0                     3.0                        3.5 3.5Ta.sub.2 O.sub.5    29.1        27.0            26.5                33.1                    31.8                        32.1                            32.1n.sub.e  1.6811        1.6793            1.6914                1.7052                    1.7050                        1.7144                            1.7054ν.sub.e    38.3        37.5            36.6                35.8                    35.7                        36.5                            36.3θ&#39;.sub.g  × 10.sup.-4    5026        5059            5061                5089                    5086                        5061                            5065-ν.sub.e     7.0         5.5             6.3                 5.2                     5.4                         6.4                             6.3__________________________________________________________________________ 
    
     
                                           TABLE 7__________________________________________________________________________(Weight %)Example: 68  69  70  71  72  73  74__________________________________________________________________________SiO.sub.2    17.4        17.0            16.8                16.8                    12.7                        16.8                            16.6B.sub.2 O.sub.3    15.7        15.3            15.2                15.2                    24.9                        15.2                            15.0Σ (SiO.sub.2 + B.sub.2 O.sub.3)    33.1        32.3            32.0                32.0                    37.6                        32.0                            31.6ZrO.sub.2    15.2        14.9            14.7                14.7                    14.5                        14.7                            14.5Σ (SiO.sub.2 + B.sub.2 O.sub.3    48.3        47.2            46.7                46.7                    52.1                        46.7                            46.1  + ZrO.sub.2)Na.sub.2 O    15.0        14.7            14.5                14.5                    12.0                        14.5                            14.4BaO       0.5         0.5             0.5                 0.5                    --   0.5                             0.5ZnO      --  --  --  --   2.0                        --  --PbO       2.0         4.0             5.0                 4.0                    --   4.0                             4.0La.sub.2 O.sub.3    --  --  --   1.0                     6.8                        --  --Sb.sub.2 O.sub.3    --  --  --  --  12.1                        --  --Nb.sub.2 O.sub.5     3.0         3.0             3.0                 3.0                    --   3.0                             3.0Ta.sub.2 O.sub.5    31.2        30.6            30.3                30.3                    15.0                        30.3                            30.0WO.sub.3 --  --  --  --  --   1.0                             2.0n.sub.e  1.7041        1.7173            1.7185                1.7183                    1.6866                        1.7131                            1.7181ν.sub.e    35.9        34.8            34.7                34.9                    38.4                        35.0                            34.3θ.sub.g &#39; × 10.sup.-4    5075        5102            5122                5109                    5042                        5097                            5105-Δν.sub.e     6.0         5.2             4.6                 4.6                     5.8                         5.2                             5.5__________________________________________________________________________