SECT, xxii.] CIKCULAK POLAEIZATION. 215 



which give sugar like that obtained from grapes. This 

 eminent philosopher is now engaged in a series of experi- 

 ments on the progressive changes in the sap of vegetables 

 at different distances from their roots, and on the products 

 that are formed at the various epochs of vegetation, from 

 their action on polarized light. 



It is a fact established by M. Biot, that in circular polar- 

 ization the laws of rotation followed by the different simple 

 rays of light are dissimilar in different substances. Whence 

 he infers that the deviation of the simple rays from one an- 

 other ought not to result from a special property of the 

 luminous principle only, but that the proper action of the 

 molecules must also concur in modifying the deviations of 

 the simple rays differently in different substances. 



One of the many brilliant discoveries of M. Fresnel is the 

 production of circular and elliptical polarization by the inter- 

 nal reflection of light from plate-glass. He has shown that, if 

 light polarized by any of the usual methods be twice reflected 

 within a glass rhomb (N. 166) of a given form, the vibrations 

 of the ether that are perpendicular to the plane of incidence 

 will be retarded a quarter of a vibration, which causes the 

 vibrating particles to describe circles, and the succession of 

 such vibrating particles throughout the extent of a wave to 

 form altogether a circular helix, or curve like a cork-screw. 

 However, that only happens when the plane of polarization is 

 inclined at an angle of 45 to the plane of incidence. When 

 these two planes form an angle either greater or less, the suc- 

 cession of vibrating particles forms an elliptical helix, which 

 curve may be represented by twisting a thread in a spiral 

 about an oval rod. These curves will turn to the right or 

 left, according to the position of the incident plane. 



The motion of the ethereal medium in elliptical and 

 circular polarization may be represented by the analogy of a 

 stretched cord ; for, if the extremity of such a cord be agitated 

 at equal and regular intervals by a vibratory motion entirely 

 confined to one plane, the cord will be thrown into an undu- 



