200 Dr. Brewster on the laws of polarisation , &c. 
Dr. Thomas Young was the first person who directed the 
attention of philosophers to the perfect agreement between 
the beautiful theory of Huygens, and the double refraction 
of light as exhibited in calcareous spar, (see Phil. Trans. 
1802, p. 45). The experiments of Dr. Wollaston afforded 
additional evidence of its exactness, and the more numerous 
and diversified observations of Malus raised it to the rank 
of a general principle, which represented all the phenomena 
in the most accurate manner. About the same time, M. 
Laplace attempted to refer the deviation of the extraordinary 
ray to the action of those attractive and repulsive forces by 
which the ordinary refraction and reflection of light are 
produced. In this theory the aberration of the extraor- 
dinary ray is explained by a repulsive force emanating 
from the short diagonal of the rhomb of calcareous spar, 
or the axis of extraordinary refraction ; and it is shown 
that the difference of the squares of the velocities of the 
ordinary and extraordinary ray is proportional to the square 
of the sine of the angle which this last ray forms with the 
axis ; and that this difference represents that of the action of 
the crystal upon the two kinds of rays. Laplace then 
demonstrates that the principle of Fermat, and the principle 
of least action lead to the law of Huygens, provided that in 
the principle of Fermat the radius of the ellipsoid is taken to 
represent the velocity, while in the principle of least action it 
is made to represent the time in which light describes a 
certain space taken for unity. 
This fine theory will no doubt be received by philosophers 
as a new proof of the high genius of its author ; but w'hile it is, 
