158 ON THE OPTICAL PROPERTIES OF MURIATE OF SODA, 
divisible into two classes ; one of which is represented by cal- 
careous-spar, and the other by rock-crystal. M. Dr La Pracz * 
had already shewn, in his fine theory of Double Refraction, 
that all the phenomena of calcareous-spar could be explained 
by supposing the deviation of the extraordinary ray to be pro- 
duced by a repulsive force, directed from the axis, and propor- 
tional to the square of the sine of the angle which the extraor- 
dinarily refracted ray forms with the axis of the crystal. In 
like manner, M. Bror perceived that the phenomena of double 
refraction in rock crystal were explicable by an attractive force 
directed to the axis of the crystal, and following the same 
law; and he was led to suppose, that muriate of soda, and 
fluate of lime, &c. composed an intermediate class of crystals, 
in which there was neither an attractive nor a repulsive force, 
and consequently neither a division nor a polarisation of the 
transmitted pencil. 
In this state of the subject, Philosophers will no doubt be 
surprised to learn, that muriate of soda, fluate of lime, the 
Diamond, Alum, and probably all other crystals of the same 
class, have actually the property of Double Refraction, but 
under circumstances of such a singular kind, as to entitle 
them to be regarded as a new class of doubly refracting crys- 
tals. ; 
The specimens of fluor-spar in which I first recognised this 
property, are represented in Plate IV. Figs. 1, and 2. and had 
the following dimensions : 
Fig. 1. Fig. 2. 
AB = 1.03 inches, AB = 0.80 
AC = 0.6 ACir= 0.3, _ 
AD = 0.6 AD = 1.0 
Both 
* Sur la lot de la Refraction extraordinaire dans les cristaux diaphanes. Mém, 
de l'Institut, 1609. 
