211 
Dr. Brewster on the laws of polarisation, &c. 
the common light through two surfaces very much inclined, 
it will be seen that the separation of the two images is a 
maximum, when the transmitted ray is perpendicular to the 
axis of the prism, and that the two images gradually coalesce 
into one, as the transmitted ray becomes parallel to the axis. 
The axis of the prism, therefore, is the apparent axis of 
double refraction, and likewise the axis of the system of 
rings shown in fig. 1. Hence we conclude that beryl has 
one apparent axis of extraordinary refraction, and one appa- 
rent axis of polarisation which are coincident. The same 
property of producing the rings in fig. 1, I have discovered 
in the following crystals. 
A list of crystals with one apparent aocis of double refraction 
and polarisation . 
Beryl, emerald 
Calcareous spar, bitter spar 
Sapphire, ruby 
Rubellite, tourmaline 
5 Phosphate of lime 
Hydrate of magnesia 
strontites 
Idocrase 
Mellite 
10 Nepheline 
Arseniate of copper 
Potash 
Muriate of lime 
J 
Muriate of strontian 
15 Apophyllite 
Nitrate of soda 
Subphosphatc of potash 
Sulphate of nickel (certain 
specimens of) 
Zircon 
20 Quartz 
Ice 
Super acetate of copper and 
lime 
Sulphate of potash (certain 
specimens of) 
o 
&. 
As several of the crystals contained in the preceding list 
are not capable of being cut into transparent prisms with 
large refracting angles, it is impossible to ascertain the rela- 
tion between the force of double refraction, and the polarising 
