ON THE OPTICAL PECULIARITIES OF THAT MINEKAL. 325 



is particularly remarkable. The existence of the curvilineal 

 solid in the centre ; — the gradual diminution in the length of 

 the circumscribing plates, in consequence of which they taper, 

 as it were, from the angles of the central square to the trun- 

 cated angles at the summits ; but, above all, the reproduc- 

 tion of similar tints on each side of the central figure, and at 

 equal distances from it, cannot fail to strike the observer 

 with surprise and admiration. 



The tints exhibited by each crystal vary of course, according 

 to its thickness *, but the range of tint in the same plate, and 

 at the same thickness, generally amounts, in the largest crys- 

 tals, to three of the orders of colours in Newton's scale. 

 The central portion, and the two red squares, have, in general, 

 the same intensity, while the four green segments round the 

 central portion, and some of the parts beyond each of the 

 red squares, are also isochromatic f . In the central part, the 

 colours have a decided termination ; but towards the summits 

 of the prism their outline is less regular, and less distinct- 

 ly marked ; though this irregularity has also its counter part 

 at the other termination. A part of these irregularities is some- 

 times owing to the longitudinal striae J on the natural faces of 



the 



* In comparing the polarising forces of different crystals of this variety of Apo- 

 phyllite, I have been surprised to observe the great diversity which exists among 

 them ; some of the prisms, which are only j^th of an inch thick, having a greater 

 polarising force than others which are ^Vth of an inch thick. 



f In some crystals the two smaller green segments have a different tint from 

 the two larger segments, and the same tint as the central square. 



j Owing to the feeble polarising power of the apophyllite, these superficial irre- 

 gularities produce a less degree of derangement in the tints than might have been 

 expected. 



