24 6 Dr. Brewster gw the laws of polarisation, &c. 
may lead us to select one combination of forces in preference 
to others, as the means which nature has employed in the ac- 
complishment of her purposes. 
If we consider a material particle in motion as under the 
influence of forces, the nature and the source of which are 
unknown, we may ascribe any change of direction which it 
experiences, either to a single attractive, or a single repulsive 
force, emanating from different sources ; or we may regard 
it as the resultant of a variety of forces of the same, or of 
opposite characters. In the phenomena of the solar system, 
a repulsive force is necessarily excluded by the simplest con- 
siderations. In the reflection of light, the return of the ray 
cannot, without the most manifest absurdity, be ascribed to 
an attractive force residing without the reflecting surface; 
and for the same reason, the refraction of the transmitted 
light cannot be considered as the effect of a repulsive force ex- 
isting without the transparent medium. But the case is quite 
different in the phenomena of double refraction and polari- 
sation. There are here no prominent physical circumstances 
which can lead us to a general determination of the nature 
of the forces. The deviation of the extraordinary ray in 
beryl, may be the result'of a repulsive force emanating from 
the axis of the prism, or of an attractive force emanating from 
two equal rectangular axes lying in a plane perpendicular to 
the axis of the prism, or of various other combinations of 
forces, either of the same or of opposite names. 
We shall now proceed to demonstrate the first of these 
positions, namely, that the action of two equal rectangular 
axes of a positive character, as calculated by the law of 
polarisation already explained, is the same as the action of 
