‘FLUATE OF LIME, AND THE DIAMOND. 161 
‘duced, but when the same fringes were held parallel to the 
fringe be, the sum of their -effects was produced. Hence it 
follows, that the structure which produces the fringes a d, cf, 
is the same as that of one class of doubly refracting crystals, 
and the structure which produces the fringe 4e, the same as 
that of the other class. An effect exactly similar to what is 
shewn in Fig. 5. is exhibited by the sclerotic coat in the eyes 
of fishes, and may be produced by crushing a piece of soft 
isinglass, or by pressing a mixture of rosin and bees-wax *, 
‘between two plates of glass. 
The preceding experiments entitle us to conclude, that mu- 
riate of soda, fluor-spar and the diamond, combine in the same 
specimen three different structures, and form a new class of 
doubly retracting crystals. In some parts they act upon light 
dike that class of crystals in which the deviation of the ex- 
traordinary ray is supposed to be produced by an attractive 
force. In other parts they act upon light like the other class 
of crystals in which the extraordinary ray deviates from the 
axis in virtue of a repulsive force; and in intermediate por- 
tions they exhibit that mean structure, in which the light is 
urged neither by attraction nor repulsion, and where there is 
neither a polarisation nor a division of the transmitted pen- 
cil. If the laws which regulate the crystallisation of these 
minerals had been allowed an undisturbed operation, it is pro- 
bable that the crystals would have had a perfect cube or an 
octahedron for their primitive form, and would have exhibited 
none of the phenomena of double refraction. 
The slightest irregularity, however, in the operation of these 
jaws, would produce a deviation from the perfect primitive 
form, and the crystal would therefore deviate from the inter- 
mediate class into the attractive and repulsive classes, and 
Vou. VIII. P. 1. x would 
* See Phil. Trans. 1815, p. 32, 33.; and 1816, p. 172, 173. 
