322 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



and cdef. Fig. 10., the one of which has its central lozenge n 

 greater than the lozenge m of the other. Hence it follows, that 

 the compound plate will produce such an effect, that the parts 

 of the lozenge n surrounding m will appear to have two axes 

 of double refraction, while the effect is actually produced by 

 the biaxal structure round the lozenge m of the upper plate 

 abed. In the fourth slice, represented in Figs. 8 and 9., the 

 four veined angles now meet one another, and surround the 

 tesselae with only one central lozenge. The slice from below 

 this, or the Jifth> exhibited a very irregular structure. 



When all the four slices were together, they distinctly pro- 

 duced the tesselated structure shewn in Fig. 3. Their united 

 thickness was 0.12 of an inch. 



When the crystal was examined across two opposite surfaces 

 of the pyramid, it exhibited the fringes shewn in Fig. 7. of my 

 former paper, and in Fig. 11. of the present one; the fringes 

 being all bent as they approached the edges, where they be- 

 came serrated, which proves that the polarising force of the 

 outer coats is less than that of the interior ones, and that the 

 different coats near the edges have an alternation of different 

 polarising powers*. Near the base of one of these pyramids, 

 as shewn in Fig. 11., the curves which had been convex towards 

 the summit became concave in the middle, though they still 

 retained their convexity at their junction with the serrated 

 portions, so as to have the form of curves of contrary flexure. 

 Another crystal from Faroe produced the opposite effect, as 

 represented in Fig. 12. ; the isochromatic lines being now con- 

 cave towards the summit, and displaying a very rapid variation 



of 



* See Edinburgh Philosophical Journal, vol. i. p. A, and Plate I. Fig, 7. . 



