and a fernarlcahle Structure in Analcime^ "^'57 
iav axes of the icositetrahedron, which is perpendicular to the 
axis passing through O. The tint of any point for example, 
does not depend upon its distance O from O, but upon its dis- 
tance p q from the nearest plane of no polarisation, taken in a 
direction perpendicular to CB. Calling T, then, the tint, as de- 
termined by experiment, of any point P, whose distance P r, 
taken in the manner now mentioned, is D, we shall have the tint 
^ at any other point p whose distance p q is 
.Td^ 
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 icositetrahedron 
perpendicular to CB, and to the axis passing through G. 
This remarkable structure produces a distinct separation of 
the ordinary and extraordinary images of a minute luminous 
object, when the incident ray passes through any- pair of die 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, m 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 
Plate VII. Fig. 10. The dark shaded lines are the planes of no 
double refraction -and polarisation, and the faint shaded lines 
represent the tints. The appearances, however, shewn in this 
figure, can never be seen by the observer at once, but they will 
assist the reader in following the experimental' details, and in 
forming a correct notion of the phenomena. 
One of the most important results of these experiments, is the 
singular distribution of the doubly refracting force, not merely 
in the crystal considered as a whole, but in each of the separate 
pentahedrons which compose it. In all other crystals in which 
the laws of double refraction have been studied, the axis to which 
the doubly refracting force is related has no fixed locality in the 
^mineral. Jt is a line paraliel to a given line "in the primitive 
