Transparency of Certain Coloured Media. 451 



nation was fixed with wax over a small hole in a scieen 

 of sheet ebonite, in such a way that no radiation could 

 reach the slit of the spectrograph save after transmission 

 through the specimen. In the case of a cut stone care was 

 taken that it was orientated so that the light had the best 

 chance of transmission, and in minerals where the distri- 

 bution o£ colour was irregular the convergent radiation was 

 in general directed so as to traverse the part of deepest 

 colour. Several exposures of varying duration were made 

 with each specimen, the maxima ranging from ten to thirty 

 minutes according to the nature of the specimen. 



In the following table the wave-lengths denote the point 

 at which complete absorption commences. 



\ 



1. Natural blue rock-salt Beyond 2250 



2. Natural rock-salt coloured by cathode rays. . Beyond 2250 



3. Sylvite, white Beyond 2250 



4. Ditto, coloured blue by cathode rays Beyond 2250 



5. Chili saltpetre, ordinary white variety 3512 



6. Ditto, violet 3248 



7. Fluorspar, coloured deep violet bv cathode J 15 , -.»_._ 



rays .'. } Be J oud ' 225Q 



8. Diamond, yellow 3200 



9. Ditto, blue 3150 



10. Kunzite < 3050 



11. Garnet, red 4023 



12. Zircon (hyacinth) red-brown 2618 



13. Ditto, decolorized bv heat 2442 



14. Ditto, green * 4023 



15. Ditto, yellow 4023 



16. Topaz,' pale yellow 2618 



17. Ditto, dark yellow 2294 



18. Ditto, pale pink-brow n 2618 



19. Ditto, blue 2961 



20. Emerald 3200 



21. Ruby 3000 



22. Tourmaline, green 3512 



23. Ditto, green-yellow 3000 



24. Ditto, pink 3064 



25. Spinel, blue . , 4023 



26. Ditto, purple . . 3248 



27. Ditto, pink 3000 



28. Kvanite, blue 3200 



29. Beryl, blue 3274 



30. Cordierite, blue-purple 3248 



31. Cairngorm 3248 



Notes on the above : — 



1. This is the natural blue rock-salt of Stassfurt. It is 

 said to be found at the boundary of the salt with some of the 

 potash minerals. 



2. Ordinary white , rock-salt coloured a deep blue by 

 cathode-ray bombardment. This has always been considered 

 kindred to the natural blue salt, and is now seen to behave 



