| 摘要 |
629,426. Glass manufacture. KODAK, Ltd., Aug. 9, 1947, No. 22182. Convention date, April 6, 1946. Addition to 606,509. [Class 56] In the optical glass according to claim 2 of the parent Specification (containing at least 60 per cent W. fluorides including BeF 2 12-50 per cent W., aluminium fluoride, one or more insoluble divalent metal fluorides, and alkali metal fluoride 0-25 per cent W.) there is included a PbF 2 content of more than 30 per cent W. but not more than 75 per cent W., the limits then being modified in that either the BeF 2 content range is 3-20 per cent W., or the glass consists solely of BeF 2 , PbF 2 , AlF 3 and MgF 2 . The following is one of four examples given, the preferred ranges being indicated in brackets: BeF 2 3.1; PbF 2 73.8 (50-75 or 70-75); AlF 3 16.8 (10-20); MgF, 6.3 (2-15). Small amounts of other fluorides may be added according to the first modification, e.g. of Ca, Sr, Ba, La, Ce or Th. The n D value is over 1.4. The dry powdered materials, including about 10 per cent of NH 4 F or NH 4 HF 2 , are melted in a covered platinum crucible at 850-900‹C., the PbF 2 being added only after the other ingredients have fused whereby attack of the platinum by free metallic lead is avoided.
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