188 DR BREWSTER on a New Species of Double Refraction, 



Having lately received from the Reverend Dr FLEMING 

 of Flisk, some very transparent crystals of Analcime from the 

 Macdonalds' Cave in the Island of Eigg, and having also been 

 favoured with a very fine specimen from Montecchio Mag- 

 giore in the Vicentine, through the liberality of Mr HEULAND, 

 I have been enabled to resume the inquiry, and have obtained 

 the results which it is the object of this paper to describe. 



The most common form of the Analcime is the Icositetrahe- 

 dron, a solid contained by twenty-four equal and similar trape- 

 ziums, and formed by three truncations on the eight solid angles 

 of the circumscribing cube, inclined 144 44' 8" v to each of its 

 faces, and 146 26' 33" to one another, (See Fig. 1. Plate VII.). 

 If we suppose this cube to be dissected, as in Fig. 2., by planes 

 passing through all the twelve diagonals of its six faces, it will 

 be reduced into twenty-four irregular tetrahedrons. The same 

 planes divide the icositetrahedron into twenty-four similar pen- 

 tahedrons, two of whose planes are placed at right angles to each 

 other, having for their common section one of the axes of the 

 solid, while a third, equally inclined to these two, and forming an 

 angle of 45 with their common section, passes through the 

 centre of the icositetrahedron. The other two planes are halves 

 of two of the adjoining trapezia, which form the surface of the 

 general solid. 



If we transmit polarised light in a direction perpendicular 

 to any of the faces of the cube, we shall find that all the divi- 

 ding planes now mentioned, are planes of no double refraction 

 or polarisation, that is, that they consist of an infinite number of 

 axes parallel to the four axes of the solid *. 



When any of the axes of the cube are in the plane of primi- 

 tive polarisation, the tints will disappear, and continue invisible 



* These planes correspond \vitli the double set of cleavage-planes which, accord- 

 ing to HAUY, are found in Amphigene. 



