212 Dr. Brewster on the laws of polarisation , &c. 
force, by the action of which the coloured rings are produced ; 
but, in all those crystals upon which this experiment can be 
made, I have invariably found that the two axes from which 
these forces emanate, are coincident, and that the force which 
produces the deviation of the extraordinary ray increases 
and diminishes with the polarising force which produces the 
coloured rings.* 
In proceeding to examine the nature and properties of the 
coloured rings produced by the different crystals in the table, 
we shall first consider the effect produced by plates of 
different thicknesses. If we take a rhomboid, of calcareous 
spar, whose principal section is A B C D, fig. 2. and 
cement upon its two surfaces AB,CD, tw r o prisms of flint 
glass, having their sides perpendicular to the principal sec- 
tion, and their refracting angles EBF, GDH, a little greater 
than FB6 which is 45 0 23' 26"; and if polarised light is inci- 
dent, perpendicularly upon the surfaces BE, DG, it will be 
transmitted parallel to B b,f and will exhibit in the most 
beautiful manner the system of coloured rings shown in 
* M. Biot was the first who deduced this conclusion for crystals with one axis 
from experiments on calcareous spar and rock crystal ; but it could not be considered 
as a general fact when drawn from such a small number of crystals. It will be seen 
in the sequel of this paper, that I have extended the result to crystals with two axes, 
and have thus established it as a general principle. 
f This mode of exhibiting the coloured rings, is greatly superior to the mode 
employed by Biot, of grinding down the solid angles at B and D. Beside the 
advantage of procuring the greatest possible thickness from a given rhomboid, we 
preserve the polish of its natural faces, and the thickness of crystal through which 
the polarised ray passes can be calculated with the utmost accuracy from the thick- 
ness B& of the plate. This method is also peculiarly favourable for showing the 
influence of pressure upon the polarising structure of the crystal, and for various 
other experiments on the coloured rings. 
