17 



a strong spiral spring, the compression of which measures the 

 pressure employed, by means of a graduation on one of the bars 

 from to 50 kilograms. 



51. Apparatus for demonstrating double refraction by 

 flexure. 



E, 93. 1885. Made by B. Fuess, Berlin. 



This consists of two pieces of plate glass, screwed together at 

 the two ends. When it is to be used for the purpose named, a 

 metal rod is placed crosswise between the plates of glass at the 

 centre, and the two screws on either side are made to draw 

 the ends together. By this means the glass is bent, and the 

 double refraction is observed. 



52. Apparatus for demonstrating double refraction by 

 heating. 



E. 94.- 1S85. Made by B. Fuess, Berlin. 



This is merely a handle with a spring lever on one side, and at 

 the end, two rectangular holders of such a shape that when 

 closed they leave a cubical space within, by which cubes of glass 

 may be taken up, heated, and put down again on a stage for 

 examination ; with this holder are 12 glass cubes to be used for 

 the purpose. 



53. Crystals for heating. 



E. 79. 1877. Prepared by L. Laurent, Paris. 



These are sections of crystals with parallel faces fitted on to 

 tongues of brass, with perforated plates, between which the 

 crystal is placed and secured by screws, when it can be heated. 

 The specimens are, Felspar, Gypsum, Carbonate of Lead. 



54. Collection of plates of crystals (31) containing 

 uniaxial and biaxial species. Prepared for examination 

 with, the polariscope. 



E. 17 to 47. 1885. Prepared by B. Sturtz Bonn. 



These are selected from a larger list, with numbers attached, 

 corresponding to their position in that list. The selected types 

 are: 



Uniaxial Crystals. 



17. Quartz, left handed. 



18. Quartz, right handed, 



4 mm. thick. 



19. Dioptase, Siberia. 



20. Quartz, 8 mm. thick. 



21. Brucite, Texas. 



22. Apophyllite, Poonah. 



23. Zircon, Ceylon. 



24. Apatite, Tyrol. 



25. Beryl, Brazil. 



26. Corundum, Ceylon. 



27. Calcspar, | mm. thick. 



28. Calcspar, 1 mm. thick, 



29. Calcspar, 3 mm. thick. 



30. Calcspar, hemitrope. 



31. Meionite, Vesuvius. 



32. Tourmaline, Elba. 



