ON THE OPTICAL PECULIARITIES OF THAT MINERAL. 319 



bit by the microscope, under favourable circumstances of illu- 

 mination, the beautiful figure represented in Fig. 1, and 2. The 

 outer case MONF, Fig. 1. which binds the interior parts toge- 

 ther, is composed of a great number ofparallel veins, which, from 

 their minuteness, display the colours of striated surfaces. This 

 external coating envelopes no fewer than nine separate crystals, 

 viz. the central lozenge abed, the four prisms A, B, C, D, 

 with trapezial bases, and the four triangular prisms e hi, Im ?i y 

 nkg, gfe y all of which are separated from one another by 

 distinct veins. The inflected lines eh l y Imn, nkg, gfe, are 

 most easily seen by the microscope. The central lozenge is 

 seen much less frequently, and the radial lines h a, ck, fd, 

 bm y require a particular mode of illumination to be distinctly 

 recognised. The colours displayed by one of these plates, 

 when crossed with an uniform plate of sulphate of lime, having 

 a polarised tint of a blue colour, is shewn in Plate XXL Fig. 2. 

 In some plates the whole of the triangular space efg y Plate XX. 

 Fig. 1. has the same colour and structure as A ; gkn the same 

 as C, and so on ; while, in other plates, part of the veined 

 border belonging to A, has the colour and structure of C ; but 

 the most common effect is that shewn in Fig. 2. of Plate XXI. 



In order to discover if there was any form intermediate be- 

 tween the summit plate and the tesselated laminae, I have cut 

 up various crystals, but have not been able to observe a gra- 

 dual transition from the one structure to the other. It must, 

 therefore, take place either per saltum *, or the one must pass 

 into the other by a curve; whose vertical branch is less than 

 the 200th or 300th of an inch. 



vol. ix. p. ii. s s Although 



* More recent observations have proved this to be the case, as will be seen in a 

 subsequent part of this paper. 



