ON THE OPTICAL PECULIARITIES OF THAT MINERAL. 327 



In some crystals of this variety of Apophyllite,- particularly 

 in the smaller ones, the central portion is only |th of the whole 

 length of the prism, and the maximum polarising power re- 

 sides in that central part. This last effect is shewn in Fig. 17. 

 where the first fringe is completed in the form of a rectangle 

 enclosing the central portion, while, in the other parts of the 

 prism, the tint has not risen above the yellow of the first order. 



Having thus ascertained the properties of the prism, when 

 examined through its parallel faces, I proceeded to determine its 

 structure at right angles to the axis, by observing the pheno- 

 mena which the separate laminae exhibited, in relation to the 

 part of the prism from which they were extracted. 



The plates near the summit of the crystals are exactly the 

 same as the uppermost slice A, Fig. 2. of the Apophyllite al- 

 ready described. The subjacent plates are also similar, with 

 this difference only, that the veined border increases in 

 breadth, and is often beautifully divided into groups of veins, 

 like the frame of a picture ; and they retain nearly the same 

 character till we reach the central figure. At this point the in- 

 flected lines ehly &c. shewn in Fig. 1. appear within each of the 

 four inner sides of the border; and when seen by polarised 

 light, the slices of the central figure exhibit the tesselated 

 structure shewn in Fig. 16., with some slight modifications. 

 It is very remarkable, however, that when these same slices 

 are examined with a microscope, the figure which they display 

 is different from Fig. 16., and resembles the structure shewn 

 in Fig. 15. In Fig. 16. some of the quadrilateral outlines 

 which form the border, have the planes passing through the 

 two axes at right angles to those of others ; from which it 

 follows, that some parts all round the border are luminous, 

 and others dark, when the diagonal is in the plane of polarisa- 

 tion, and vice versA when the diagonal is inclined 45° to this 

 plane. 



VOL. IX. p. II. T t There 



