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el it i ts 
TO CHEMICAL RESEARCHES, IN REPLY TOM. CHEVREUL. 60] 
In the second he specifies the use to which he thinks it may be 
applied. 
I also shall follow this division of ideas. But in employing this ar- 
rangement, it is necessary here to recall with precision the nature of 
the character under consideration, as it was conceived and explained by 
me when either experimentally determining or applying its physical 
laws : for (a circumstance resulting probably from its novelty, and 
from the scarcity of the apparatus hitherto constructed for its appli- 
cation, ) I differ almost as much from M. Chevreul with regard to the 
appreciation of the advantages which he attributes to it, as I do with 
regard to the limitations to which he supposes it liable. Nor will a 
clear and precise explanation of this new method of studying bodies be 
misplaced in the annals of natural history, in which I have several times 
described the results which I have deduced from it relative to various 
particulars of vegetation. 
When a ray of homogeneous light is polarized by reflection in a 
certain plane, which I shall suppose to be vertical, both sides of this 
plane manifest symmetrical properties, when it is analysed immediately 
with a doubly refractive achromatic prism. This symmetry is still 
preserved when the ray thus prepared traverses certain transparent 
liquids, water, alechol, and the fat oils, for example, before it arrives at 
the prism ; at least such is the case within the limits of the thickness 
in which I have had opportunity of testing them. Other liquids on the 
contrary, such as solutions of sugar, camphor, and gum, and many of 
the essential oils, destroy this primitive symmetry even when the sur- 
faces of entrance and of emergence are perpendicular to the direction 
of the transmitted ray. If this ray be analysed after its emergence, it 
is again found polarized in one direction, but that different to its pri- 
mitive direction, with an angular deviation towards the right or the left 
of the observer, according to the quality of the substance interposed. 
The angle of deviation for each substance when in a similar state is 
exactly in proportion to the thickness that the simple ray has traversed, 
which assimilates the observable effect to a continuous and uniform ro- 
tation of the plane of polarization. But the arc of rotation described 
in each substance of equal thickness differs for the different simple 
rays, according to the fixed laws which I have experimentally deter- 
mined, and which up to the present time are sensibly identical for all 
substances, with the exception of tartaric acid, which alone offers an 
anomaly in this respect, whence it may, not without probability, be in- 
ferred that it is a combination of two atomic groups of contrary rota- 
tions having unequal dispersive powers. Whatever may be the fact, 
the general law of the deviation of different rays in all other cases, 
enables us to predict numerically the composition and the succession 
of the coloured images that the crystallized prism presents when the 
