84 
DR. BREWSTER ON PARTIAL POLARIZATION. 
fraction, and shows that the analysing crystal is not sufficient to distinguish 
light completely polarized from light in a state of approach to polarization. 
The difference, however, between these two kinds of light is marked by most 
distinctive characters, and will be found to show itself in some of the more 
complex phenomena of interference. 
In my paper of 1815, already referred to, I was led by a distant view of the 
phenomena which I have now developed, to consider common light as composed 
of rays in every state of positive and negative polarization* ; and upon this 
principle the whole of the phenomena described in this paper may be calcu- 
lated with the same exactness as upon the supposition of two oppositely po- 
larized pencils. Nothing indeed can be simpler than such a principle. The 
particles of light have planes, which are acted upon by the attractive and 
repulsive forces residing in solid bodies ; and as these planes must have every 
possible inclination to a plane passing through the direction of their motion, 
one half of them will be inclined — to this plane, and the other half + • When 
light in such a state falls upon a reflecting surface, the — and the -f- particles 
have each their planes of polarization brought more or less into a state of 
parallelism with the plane of reflexion, in consequence of the action of the 
repulsive force upon one side or pole of the particle through which the plane 
passes ; while in the particles which suffer refraction, the same sides or poles 
are by the action of the attractive force drawn downwards, so as to increase 
the inclination of their planes relative to the plane of incidence, and bring 
them more or less into a state of parallelism with a plane perpendicular to 
that of refraction. 
The formulae already given, and those for refracted light which are contained 
in another paper, represent the laws according to which the repulsive and at- 
tractive forces change the position of the planes of polarization ; and as we 
have proved that the polarization is the necessary consequence of these planes 
being brought into certain positions, we may regard all the various phenomena 
of the polarization of light by reflexion and refraction, as brought under the 
dominion of laws as well determined as those which regulate the motions of 
the planets. 
sillerly , December 25, 1829. 
* M. Biot has followed me in this opinion. See Traite dc Physique, tom. iv. p. 304. 
