ILLUSTRATION 



Plate I. Spectra of nickel-iron arc transmitted by various glasses. 



1. Rontgen. 9. No. 6, amber. 



2. Clear crown optical. 10. No. 1, London smoke. 



3. No. 1, blue. 11. No. 3, London smoke. 



4. No. 3, blue. 12. No. 4, London smoke. 



5. No. 5, blue. 13. No. 5, London smoke. 



6. No. 2, amber. 14. No. 6, London smoke. 



7. No. 3, amber. 15. No. 1, amethyst. 



8. No. 4, amber. 16. No. 3, amethyst. 



197 



