II. SELECTORS. 215 



854a. Collection of Prisms, for optical purposes. 



-Laurent, Paris. 



854b. Spectroscope. M. J. Duboscq, Paris. 



854c. Prisms, by Arago. Paris Observatory. 



854d. Prisms, by Borda. Paris Observatory. 



854e. Collection of various kinds of Glass for optical pur- 

 poses. Feil, Paris. 



1. Disc of Crown Glass, 4 inches. 



2. Disc of Flint Glass, 4 inches. 



3. Plate of Crown Glass (heavy English). 



4. Parallelepiped, of ordinary flint glass. 



5. Crown Glass Prism. 



6. Flint Glass Prism. 



7. Flint Glass Prism, (sp. gravity 4*4.) 



7a. Prism manufactured by Fraunhaufer Guinand. 



8. Flint Glass Prism (very heavy), sp. gr. 4 '4. 



9. Flint Glass Prism, sp. gravity 4. 



10. Flint Glass Prism, sp. gravity 5 '2. 



11. Flint Glass Prism, sp. gravity 5 '5. 



Series of Assays of the Metallic Earths. 



lla. Silicate of Potassium and Calcium with Titanium. 



12. Aluniinate of Silicium and Magnesium. 



13. Crystallisation of Alumina and Magnesia. 



14. Crystallisation of Fluosilicate of Magnesium and Calcium. 



15. Alumina and Magnesia Crystallised by Fluosilicate of 



Potassium. 



16. Crystallisation of Fluosilicate of Aluminium. 



17. Crystallisation of Barosilicate of Aluminium. 



18. Crystallised Aluminate of Magnesium and Silicium. 



19. Manufacture of Adamantine Boron. 



20. Plate of Crystals of Aluminium. 



21. Blue Obsidian. 



22. Obsidian coloured by Cobalt. 



23. Samples of Crown Glass, extra white. 



854f. Objective of Bock Crystal, 10 centimetres in dia- 

 meter. M. Lutz, Paris. 



854g. Astronomical Glasses, for cabinets of physics. 



M. Lutz, Paris. 



855. Micro-spectroscope, with prism for comparing two 

 spectra, and with Abbe's measuring apparatus for the direct esti- 

 mation of the wave lengths of dark or bright lines in a spectrum. 



C. Zeiss, Jena. 



