TO CHEMICAL RESEARCHES, IN REPLY TO M. CHKVREUL. 601 



In the second he specifies the use to w hich he thinks it may be 

 applied. 



I also shall follow this division of ideas. But in employing this ar- 

 rangement, it is necessaiy 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 i-esulting 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, alcohol, 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 oppoi'tunity 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 symmetrj' 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 i? 

 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 tiie 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 



