302 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
ray by material molecules independent of one another, I proceed 
to establish theoretically the observable results which this power 
should produce through masses of a sensible thickness. I shall 
afterwards compare these results with facts, both in those cases 
where the active molecules are individually free, as well as in 
those where, without being decomposed, they enter into chemical 
combinations with the active or inactive elements of an ambient 
medium. 
8. I consider, first, a liquid chemically homogeneous, all the 
constituent molecular groups of which are equally active. I at- 
tribute to this system a constant and uniform temperature, which 
I shall always do, unless I state the contrary. This being agreed 
on, in the pencil of rays of light supposed homogeneous, which 
reaches the pupil through the liquid, let us in thought isolate a 
mathematical, rectilinear ray. This ray will never enter the 
pupil alone; it will always form part of the luminous cylinder of 
sensible dimension which the aperture of this organ admits. 
Such a cylinder may therefore be imagined as composed of a mul- 
titude of others of very large lateral dimensions compared to the 
constituent molecules of the active medium, or at least of the 
same order, and yet, like them, imperceptible to the senses, with- 
out at the same time being wholly null, or even infinitely small in 
the geometrical acceptation of the word. Each of these luminous 
threads, in traversing the total mass of active matter which oc- 
curs in its course, will experience in its plane of polarization a 
total deviation «, which, supposing the medium to be of a finite 
and constant thickness, will be sensibly equal for all the rays of 
the same nature which enter the eye simultaneously. And 
for each finite thickness /, this deviation « will depend on the 
density 3 to which the active system is actually brought, since 
the total mass of active matter which enters into the same volume 
of one and the same elementary cylinder, varies proportionally 
to this density. Admitting then that these variations of ¢ are 
effected under such conditions that they influence only the in- 
tervals of the active groups without modifying their peculiar ac- 
tion on polarized light, which may be ascertained experimentally, 
as I shall observe presently, all the effects thus produced by one 
and the same active system with different thicknesses and den- 
sities, will be connected one with another by the individuality 
of the action, which will render them proportional to the total 
mass of the active groups contained in the finite length / of the 
