On Light. 15 
we vary never so little the inclination of the second, the intensity 
of the reflected light is no longer null in any azimuth, but it be~ 
comes the feeblest possible in the prime vertical, in which it was 
formerly null. 
Similar phenomena may be produced by substituting for the 
mirror glasses, polished plates, formed for the greater part of trans- 
parent bodies. The two planes of reflection must always remain ree- 
tangular, but they must be presented to the luminous ray, at dif- 
ferent angles, according to their nature. Generally, all polished 
surfaces have the property of thus polarizing, more or less com- 
pletely, the light which they reflect under certain incidences ; but 
there is for each of them a particular incidence, in which the po- 
larization it impresses is most complete, and for a great inany, 
it amounts to the whole of the reflected light. 
When a ray of light has received polarization in a certain di- 
rection, by the processes now described, it carries with it this 
property into space, preserving it without perceptible alteration, 
when we make it traverse perpendicularly a considerable mass of 
air, water, or any substance possessed of single refraction. But 
the substances which exercise double refraction, in general alter 
the polarization of the ray, and apparently in a sudden manner, 
and communicate to it a new polarization of the same nature, 
but in another direction. It is only in certain directions of the 
principal section, that the ray can escape this disturbing force. 
The following may be regarded as the most general view of this 
subject. 
When the particles of light pass through a crystallized body, 
_ endowed with double refraction, they experience different move- 
ments round their centre of gravity, which depend on the nature 
of the forces which the particles of the erystal exercise on them. 
Sometimes the effect of these forces is limited to the above po- 
farization, or to the arranging all the particles of one ray, parallel 
to each other, so that their homologous faces are turned towards 
the same parts of space. When this disposition occurs, the-lu- 
minous molecules preserve it, in the whole extent of the crystal, 
and experience no more movement around their centre of gravity. 
But there exist other cases, in which the molecules that traverse 
the crystal are uot fixed in any constant position. During the 
time of their passage, they oscillate round their centre of gravity, 
with velocities, and according to periods, which may be calcu- 
lated. Lastly, they sometimes revolve round their own axes, 
with an uninterrupted movement of rotation. ‘The former is 
called fixed polarization, the latter moveable. 
In the Phil. Trans. for 1813, we have the first of a scries of 
very interesting papers on polarized light by Dr. Brewster. This 
relates 
