/ 
as exhibited in its propagation along plates of glass. 57 
set below OP, were converted into black; but when the axis 
of the sulphate of lime was parallel to CD, the blue of the 
same fringes was converted into a yellowish green. Hence it 
follows, that the axis of the parts of the glass which form the 
exterior sets of fringes, is at right angles to the axis of the parts 
which form the interior sets. The same result is deducible 
from the second experiment in Prop. VII. Since, therefore, 
the same effects as those which we have described, are pro- 
duced by combining crystallized plates taken from the two 
classes of doubly refracting crystals, as has been ably proved 
by M. Biot,* we may consider the truths stated in the Pro- 
position as completely established. 
Cor. It follows from this Proposition, that a single plate of 
glass, crystallized by the propagation of heat, and exposed to 
a polarised ray, exhibits the same variety of phenomena as 
all the crystals in the mineral kingdom. We have already 
seen, that it possesses the structure of all the three classes of 
doubly refracting crystals. But the individual crystals which 
compose these classes, are distinguished from each other by 
the magnitude of their polarising forces, and the same variety 
is exhibited in the polarising forces of the glass, the parts 
which are adjacent to CD, ab, and FE, having the structure 
which gives the greatest polarising force, and the parts adja- 
cent to OP, MN, the structure which gives the least polarising 
force. 
* See M. Biot’s Memoire sur la decouverte d’une propriete nouvelle dont jouissenf 
les forces polar isantes de certains cristaux. Mem. de 1’Institut, 1814. 
I 
MDCCCXVI, 
4 
