1 6' Dr. Brewster on the absorption of polarised light 
The property which these crystals possess of absorbing the 
different tints in different positions of the axis, with regard to 
the plane of primitive polarisation, does not belong to every 
specimen. There are many crystals of ruby, sapphire, eme- 
rald, &c. which give an ordinary and an extraordinary image 
of the same colour ; and whenever this is the case, they are 
destitute of the property of absorbing polarised light. These 
two classes of phenomena are indeed invariably connected, 
and will ultimately be found to have the same origin. 
The extreme generality of this property is indicated by the 
number of crystals in the preceding table, which embraces all 
the coloured crystals which are at present known to have only 
one axis of double refraction, excepting titanite , molybdate of 
lead , carbonate of iron and lime, arseniate of copper, certain spe- 
cimens of sulphate of nickel, and super-acetate of copper and 
lime, in which I have not detected the property of absorbing 
polarised light.* 
The various coloured minerals which have the cube, the 
regular octohedron, and the rhomboidal dodecahedron for 
their primitive form, , f are, as might have been expected, 
destitute of the property of absorption ; and I have not been 
able to discover it in differently coloured glasses, that have 
received the polarising structure from rapid cooling, or from 
mechanical compression, or dilatation. 
Some of the preceding crystals, such as the Sapphire and the 
Idocrase exhibit different colours when common light is trans- 
mitted in directions parallel and perpendicular to their axis 
of double refraction. A specimen of sapphire had a deep 
blue colour in one direction, and a yellowish green in the 
opposite direction ; and several specimens of idocrase had an 
* See the Phil. Trans, for 1818, p. 21 1. f Id. p. 254. 
