1815.) Royal Institute of France. .- 457 
subject toa similar proof have offered the same equality with respec 
to the polarizing forces. ‘This would demonstrate, if it were neces- 
sary, that the theory of the oscillations of light attenuates these 
phenomena at their origin, and brings them to the consideration of 
the true forces by which they are produced. 
In the work which the same author has published on the polari- 
zation of light, M. Biot was led to conclude that the luminous 
molecules, in traversing crystallized bodies, not only undergo 
geometrical deviations in the position of their axes, but that they 
acquire new physical properties which they retain, and which make 
themselves known in experiment by affections quite new. The 
proofs of this result depended upon a very delicate discussion, They 
required the aid of a great number of experiments. The author, in 
his memoir on the Physical Properties which the luminous Mole- 
cules acquire in traversing doubly refracting Crystals, read the 22d 
of May, 1514, has sought for simpler proofs to establish so extra- 
ordinary a consequence. ‘The theory which he had deduced from 
-it furnished him with the simplest means of establishing it directly. 
. He begins by polarizing a white ray by means of reflection from 
a mirror, He then transmits it perpendicularly across a natural 
plate of sulphate of lime of a thickness, e, which exceeds +45. ofa 
millimetre, and the axis of which forms an angle of 45° with the 
primitive plane of polarization, The ordinary and extraordinary 
pencils which are produced proceed both in the same direction. 
Further, from the theory already established, these two pencils 
proceed white; and if the thickness does not exceed a few centi- 
metres, they appear as if they were polarized at right angles, one 
in the direction of the primitive polarization, and the other in a 
rectangular direction. 
He excludes this second pencil by transmission across.a pile of 
plate glasses, disposed in such a way as to reflect totally without 
acting in the least upon the first pencil, which remains alone visible 
through the pile. 
Then if we compare this with a ray polarized in the same direc~ 
tion by simple reflection from a plate glass, we sce that they appear 
perfectly similar as to the geometrical arrangement of the particles 
and the direction of the polarization; for they exhibit exactly the 
same phenomena when examined by a prism of Iceland crystal, or 
by reflection from an inclined plate glass. In the first case they are 
resolved equally into two white images, which disappear and reap- 
pear at the same limits. In the second they are reflected in the 
same manner, and escape together from the reflection.: Further, if 
we make them traverse thin plates of sulphate of lime or of rock 
erystal, they give equally coloured images, and coloured with the 
same tints; and both cease to give them when the plates have 
acquired a certain thickness. But with so great a resemblance, they 
exhibit a striking difference. Beyond these limits, the thickness 
always increasing, the vay polarized by simple reflection never gives 
colours 5, while the pencil that has first passed the thickness ¢ of 
