1869.] Mr. W. Crookes on some Optical Phenomena of Opals. 451 



No. 1. 



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No. 2. 



D F 



No. 5. 



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No. 9. 



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 I 111 



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Fig. 11. 



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No. 6. 



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No. 8. 



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No. 10. 



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VOL. XVII. 



