Dr. Brewster on the laws of polarisation, &c. 271 
tractive and repulsive forces. If a ray of light is exposed 
to the ordinary action of a repulsive or an attractive force 
emanating from two equal and rectangular axes, there is no 
point of incidence at which the resultant of these forces is 
nothing, or there is no resultant axis along which the ray 
ceases to be divided. In like manner, if the ray is exposed 
to the action of two equal and rectangular axes, one of 
which is repulsive and the other attractive, there are innu- 
merable points of incidence in which the resultant of the 
forces is nothing, and these points are situated in the cir- 
cumference of two great circles perpendicular to the plane 
passing through the axes, and bisecting the right angle 
which they form with each other. These, however, are 
results entirely incompatible with the actual phenomena of 
double refraction. 
In the preceding sections of this letter, I have included 
only the more general results of my researches, and have 
reserved, for another paper, an account of my experiments 
on the absolute polarising forces of crystals ; on the form of 
the ellipsoids by which their double refraction is regulated, 
and on the position of their axes with respect to their natural 
faces, or with respect to certain fixed lines in their primitive 
forms. 
In treating of the coloured rings produced by polarised 
light, I have likewise omitted the phenomena which are pe- 
culiar to individual crystals ; and I have not ventured to 
adopt any theory of their formation. 
I cannot, however, conclude this paper without noticing 
the happy application which Dr. Thomas Young has made 
of his beautiful law of interference to the explanation of 
