ON THE OPTICAL PECULIARITIES OF THAT MINERAL. 331 



they establish is not characteristic of any of the varieties of 

 the mineral, but has been found only in a detached portion of 

 a crystal. In order to ascertain how far it might be general, 

 I have examined, with the greatest care, numerous specimens 

 of all the varieties of Apophyllites, including those from Fas- 

 sa, Utoe, Faroe, and Iceland ; but having effected a distinct 

 separation of the ordinary and extraordinary images, I have 

 invariably found, that the doubly refracting force was Positive 

 or Attractive, like that of Quartz, whether the ray was Red, Blue, 

 or Yellow. We must, therefore, regard the property discover- 

 ed by Mr Herschel as an accidental anomaly, having its ori- 

 gin in some peculiar relation of the polarising forces of Apo- 

 phyllite. What this relation is, we shall now proceed to con- 

 sider. 



In the Biaxal Apophyllite, one of the polarising axes must be 

 in the plane of the laminae, and in both the Biaxal, and the 

 Uniaxal Apophyllite, there appears to be an axis perpendicular 

 to the laminae. As the form of the prism of Apophyllite is 

 perfectly symmetrical in relation to the axis, it is probable that 

 there are two equal and rectangular axes, of a Positive or At- 

 tractive character, in the plane of the laminae, each axis being 

 perpendicular to the parallel faces of the crystal, and we know 

 that there is a Positive or Attractive axis at right angles to the 

 laminae, and coincident with the axis of the prism *. The 

 two equal Positive axes, which we shall call the Horizontal axes, 

 on the supposition that the prism is placed upon its base, will 

 obviously produce a single Negative axis, coincident with 



the 



* I must refer the reader to my paper " On the Laws of Polarisation and 

 Double Refraction,'" in the Phil. Trans. Lond. for 1818, p. 231, and p. 245, — 

 254. for the grounds upon which this resolution of axes is made. In the case of 

 Apophyllite, there are reasons of a peculiar kind for supposing the existence of 

 three axes. 



