2 q6 Dr. Brewster on the laws of polarisation, &c. 
all the bodies in Hauy’s table, except topaz, which he has ac- 
cidentally omitted.* 
The faculty of depolarisation, which I have explained in a 
former paper, -f has been considered as a sufficient indication 
of the existence of two separate images ; and upon this prin- 
ciple it has been stated, that all crystals have the property of 
double refraction, whose primitive form is neither the cube 
nor the regular octahedron. J This statement, however, is 
by no means correct, for the rhomboido-dodecahedral crys- 
tals ought also to have been excepted ; and I have besides 
shown, by numerous experiments, that almost all the crystals 
which have these forms, exhibit an imperfect doubly refrac- 
ting structure, which some of them possess to a very consi- 
derable degree. But admitting the statement to be unex- 
ceptionable, it could never have been used as a rule for 
determining whether a crystal refracts doubly or singly ; for 
it is much more difficult to detect the primitive form of a 
crystal, than to examine, by direct experiments, its optical 
properties. Tungstate of lime, for example, would have 
been reckoned a crystal without double refraction, when 
Hauy believed its primitive form to be the cube, although it 
possesses this property in a very high degree. 
In order, therefore, to determine whether crystals have the 
property of double or of single refraction, we must ascertain 
by direct observation, if they form two images, or possess 
the property of regular depolarisation. In this way I have 
obtained the results in the following table ; but as the exist- 
* Theorie de la Double Refraction, p. 96, Paris, 1810. f Pbil.Trans. 1815, p. 2 7. 
J Edin. Transactions, vol. viii. Part. I. Biot’s Trait'e de Physique, tom. iii. p, 325. 
