of the Brazilian Topaz. 5 
which have their principal section in the direction dd’ inclined 
from 10° + to 22° + to MN, the inclination differing in different 
crystals. The lines aa’, bb’, cc’, dd’ are also the principal sections 
of the corresponding laminz on the side NC. In like manner 
the principal sections aa’, 86’, v7, 60 of the lamine in BCFE 
are the principal sections of the corresponding laminz on the 
other side AN. As the laminze however are infinite in number, 
the principal sections have every possible direction between MN 
and dd’. 
III. On the Optical Structure and Properties of Brazilian 
Topaz. 
Having found that the Brazilian Topaz exercised a super- 
ficial action upon light different from the colourless Topaz ot 
New Holland, I was induced to compare the relative intensities 
of their polarizing axes. In the blue Topaz of Aberdeenshire, 
and the colourless Topaz of New Holland, the inclination of 
the resultant axes of double refraction is about 65°, and the 
system of coloured rings round each axis deviates from the tints 
of Newton’s scale, the red ends of the rings being outwards*. 
In the Topazes of Brazil, the inclination of the resultant axes 
varies in different specimens; I have found it in some crystals 
50° 5’, and even so low as 43°; and, what is very remakable, the 
one resultant axis is often more inclined than the other to the 
natural surfaces of the Jaminz, an effect which no doubt arises 
from the peculiarities of crystallization already described. In 
one specimen where the axes formed an angle of 50° 5’, the one 
axis was inclined only 22° 37’ to the axis of the prism, while 
* In the Phil. Trans. 1814, p. 204. and Plate VII. Fig. 1. I have described the 
order of the tints, from which it will be seen, that the rings are red at one end and 
blue at the other, and that the colours do not originate at the centre of the rings. 
