and a remarkable Structure in Analcime. 191 



ach sector, as COB, is, on the contrary, perpendicular to the 

 line CB, or parallel to one of the rectangular axes of the icosi- 

 tetrahedron, which is perpendicular to the axis passing through 

 O. The tint of any point p, for example, does not depend upon 

 its distance pO from O, but upon its distance p q from the nearest 

 plane of no polarisation, taken in a direction perpendicular to CB. 

 Calling T, then the tint, as determined by experiment, of any 

 point P, whose distance Pr, taken in the manner now men- 

 tioned, is D, we shall have the tint t at any other point p whose 

 distance p q is d, 



_ 

 D 2 



the thickness of the crystal being supposed equal at both these 

 points. The polarising structure, therefore, of any two opposite 

 sectors, is the same as if it were produced by compression, the 

 axis of pressure coinciding with the axis of the icositetrahe- 

 dron perpendicular to CB, and to the axis passing through O. 



This remarkable structure produces a distinct separation of 

 the ordinary and extraordinary images of a minute luminous ob- 

 ject, when the incident ray passes through any pair of the four 

 planes which are adjacent to any of the three axes of the solid. 

 The least refracted image is the extraordinary one, and conse- 

 quently the doubly refracting force is negative, like that of Cal- 

 careous Spar, in relation to the axis to which the refracted ray 

 is perpendicular. 



In order to convey some idea of the remarkable structure of 

 Analcime, I have represented the Planes of no Double Refraction 

 and Polarisation, and the tints of the intermediate solids, in 

 Figures 9, 10, 11, and 12. The dark shaded lines are the planes 

 of no double Refraction, and the faint shaded lines represent 

 the tints. The appearances, however, shewn in these figures, 

 can never be seen by the observer at once, but they will assist 

 the reader in following the experimental details, and in forming 



