326 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



the crystal, so that by carefully grinding these off, the beauty 

 and regularity of the figure is greatly improved. 



In order to ascertain the order of the colours polarised by 

 the crystal, and observe in what manner they passed into one 

 another, I transmitted the polarised light in a direction pa- 

 rallel to one of the diagonals of the quadrangular prism, and 

 thus obtained, as it were, a section of the different orders of 

 colours, from the zero of their scale. The result of this expe- 

 riment, which is shewn in Fig. 14., was highly interesting, 

 as it displayed to the eye not only the law according to which 

 the intensity of the polarising forces varied in different parts of 

 the crystal ; but also the variation in the nature of the tints, 

 and the connection between these two classes of phenomena. 

 At the points in the diagonal m ra, opposite to a and b of the 

 crystal, the tints rose to the seventh order of colours ; at other 

 two places, opposite to c and d, they rose only to the sixth ; 

 while near the summits, at m and w, they descended so low 

 as the fourth order. Hence it follows, that the portions at 

 a, a, have the maximum polarising force ; that the four seg- 

 ments coloured green in Plate XXI. Fig. 1., are next to these 

 in intensity ; that the central portions of the red squares are 

 again inferior to these ; and that the weakest polarising force 

 is near the summits of the prism. At a and b the 4th, 5th and 

 6th fringes, have a singularly serrated outline, exhibiting in a 

 very interesting manner the sudden variation which takes 

 place in the polarising forces of the successive laminae. Al- 

 though the variation in the polarising power of the succession 

 of circumscribing plates, is clearly represented by the differ- 

 ences of tint shewn in Fig. 1. of the coloured engraving, yet 

 the exact amount of that variation may be rendered evident, 

 by making the isochromatic lines cross these plates, and ob- 

 serving the serrated outline which is thus produced. 



In 



