320 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



Although I have examined some hundred crystals of this 

 variety of Tesselite, I have never yet found one which was tran- 

 sparent across the faces of the prism, or which could be de- 

 tached from its bed in a complete state. The second or third 

 plate is often very transparent; but the incipient separation of 

 the laminas wliich produces the pearly lustre, renders it im- 

 practicable to examine the structure of the crystal across the 

 faces of the prism. In one specimen, however I succeeded 

 so far as to determine *, that in it, " and m several of the Py- 

 " ramidal crystals, the maximum tint decidedly varied in dif- 

 " ferent parts of the length of the prism, so as to produce a 

 " succession of coloured bands at the same thickness f." 



A 



♦ See Edinburgh Phibsophical Journal, vol. i. p. 5. 



•f- In an ingenious memoir, " On certain remarkable instances of deviation from 

 " Newtom's scale, in the tints developed by crystals with one axis of double refrac- 

 " tion, on exposure to polarised light," published in the Transactions of the Cam- 

 bridge Philosophical Society, vol. i. ; and reprinted in the Edinburgh Philosophical 

 Journal, vol. iv. p. 334 ; and vol. v. p. 334, Mr Hejjschel has, I think, mis- 

 conceived the full import of this experiment. " This alternation," says he, " or 

 superposition of laminae of different polarising powers, is no hypothetical case. I 

 have observed its occurrence not only in the instance before us, but in other crys- 

 tals of perfect regularity in their external forms. Dr Bbewsteb has also observed 

 phenomena referable to this principle in his paper on the Apophyllite." Upon re- 

 examining the passage in my paper, Mr Herschel will, I am sure, agree with 

 me in thinking, that the phenomena which I observed are not merely referable 

 to that principle, but are a complete demonstration of it, and distinctly prove, 

 that in several ci-ystals there were as many " laminae of different polarising 

 " powers," as there were " successions of coloured bands at the same thick- 

 " ness." The crystals, indeed, of the specimen which I employed, were the 

 upper portions of those represented in Plate XXI. Fig. 1. The bending and in- 

 equalities of the fringes, too, which I have described, and represented in Fig. 7. 

 of my former paper, establish in the clearest manner the alternation, as well as the 

 existence, of different polarising powers in the planes parallel to the faces of the 

 pyramid. 



