Dr Gustavus Hose on Epistilbilc. 285 



With these evidences in favour of the vast superiority of 

 Dr Blair's telescopes to all other achromatic instruments, we 

 earnestly hope that the Royal Society of London, or the 

 Board of Longitude, will take some measures for reviving 

 their manufacture. As night-glasses, they would be invaluable, 

 and, if generally used at sea, they might save to the country 

 many a vessel, and to society many a valuable life. 



The Society for the Encouragement of the Useful Arts in 

 Scotland, has offered an honorary medal for the best achroma- 

 tic telescope with fluid object-glasses, and we trust their ex- 

 ample will be followed by other public scientific institutions. 



Edinburgh, February %\st 182fi. D. B. 



Art. XX. — On Epistilbite, a New Mineral Species of the 

 Zeolite Family. By Dr Gustavus Rose, Berlin, F.R. S. 

 Edin. Communicated by the Author. 



As the fundamental form of the species, we may consider a 

 rhombic octahedron, the three axes of which, a : b : c, Fig. 

 20, Plate V, are in the ratio of ^ 2.022 : V 11.886 : 1. 



The crystals most generally observed, are very obtuse 

 rhombic prisms M, Fig. 21, terminated by two planes s, which 

 are set on the acute edges, and having the more obtuse solid 

 angles of combination replaced by the rhombic planes L 

 From this rhombic form of the planes it appears, that the 

 three prisms M, s, and t, may be obtained by laying planes 

 on the edges of a single scalene four-sided pyramid, the same 

 which has been assumed as the fundamental form of the spe- 

 cies, though its faces have not yet been observed in nature. 

 There are also crystals, in which the faces t are large enough 

 to intersect each other, and to produce an edge, which then 

 forms another termination of the crystals. In these, the faces 

 s replace the acute solid angles of combination, and likewise 

 possess the figure of rhombs, as in Fig. 22. Sometimes, also, 

 the edges between s and M are replaced by the narrow faces 

 marked m; two of them produce parallel edges of combina- 

 tion with /. 



