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the plane of polarization revolves from right to left, and in others from left to right, although the crystals themselves differ apparently only by a very slight, almost imperceptible, variety in form. In these phenomena, the rotation to the right is accomplished according to the same laws, and with the same energy, as that to the left. But if two plates of quartz be interposed which possess different affections, the second plate undoes, either wholly or partly, the rotatory motion which the first had produced, according as the plates are of equal or unequal thickness. When the plates are of unequal thickness, the deviation is in the direction of the strongest, and exactly the same with that which a third plate would produce equal in thickness to the difference of the two.
M. Biot has discovered the same properties in a variety of liquids. Oil of turpentine and an essential oil of laurel cause the plane of polarization to turn to the left, whereas the syrup of the sugar-cane and a solution of natural camphor by alcohol turn it to the right. A compensation is effected by the superposition or mixture of two liquids which possess these opposite properties, provided no chemical action takes place. A remarkable difference was also observed by M. Biot between the action of the particles of the same substances when in a liquid or solid state. The syrup of grapes, for
example, turns the plane of polarization to the left as long as it remains liquid, but as soon as it acquires the solid form of sugar, it causes the plane of polarization to revolve towards the right, a property which it retains even when again dissolved. Instances occur also in which these cir
cumstances are reversed.
A ray of light passing through a liquid possessing the power of circular polarization is not affected by mixing other fluids with the liquid,such as water, ether, alcohol, &c., which do not possess circular polarization themselves, the angle of deviation remaining exactly the same as before the mixture; whence M. Biot infers that the action exercised by the liquids in question does not depend upon their mass, but that it is a molecular action, exercised by the ultimate particles of matter, which only depends upon their individual constitution, and is entirely independent of the positions and mutual distances of the particles with regard to each other. This peculiar action of matter on light affords the means of detecting varieties in the nature of substances which have eluded chemical research. For example, no chemical difference has been discovered between syrup from the sugar-cane and syrup from grapes; yet the first causes the plane of polarization to revolve to the right, and the other to the left, therefore some
essential difference must exist in the nature of their ultimate molecules. The same difference is to be traced between the juices of such plants as give sugar similar to that from the cane and those which give sugar like that obtained from grapes. M. Biot has shown, by these important discoveries, that circular polarization surpasses the power of chemical analysis in giving certain and direct evidence of the similarity or difference existing in the molecular constitution of bodies, as well as of the permanency of that constitution, or of the fluctuations to which it may be liable. This eminent philosopher is now engaged in a series of experiments 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.
One of the many brilliant discoveries of M. Fresnel is the production of circular and elliptical polarization by the internal 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 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 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 vibrating particles move in 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 undulating curve lying wholly in that plane. If to this motion there be superadded another, similar and equal, but perpendicular to the first, the cord will assume the form of an elliptical helix; its extremity will describe an ellipse, and every molecule throughout its length will successively do the But if the second system of vibrations commence exactly a quarter of an undulation later than the first, the cord will take the form of a circular helix, or corkscrew; the extremity of it will move uniformly in a circle, and every molecule throughout the cord will do the same in succession. It appears, therefore, that both circular and elliptical polarization may be produced by the
composition of the motions of two rays in which the particles of ether vibrate in planes at right angles to one another.
Professor Airy, in a very profound and able paper lately published in the Cambridge Transactions, has proved that all the different kinds of polarized light are obtained from rock crystal. When polarized light is transmitted through the axis of a crystal of quartz in the emergent ray, the particles of ether move in a circular helix ; and when it is transmitted obliquely, so as to form an angle with the axis of the prism, the particles of ether move in an elliptical helix, the ellipticity increasing with the obliquity of the incident ray; so that, when the incident ray falls perpendicularly to the axis, the particles of ether move in a straight line. Thus quartz exhibits every variety of elliptical polarization, even including the extreme cases where the excentricity is zero, or equal to the greater axis of the ellipse. In many crystals the two rays are so little separated, that it is only from the nature of the transmitted light that they are known to have the property of double refraction. M. Fresnel discovered, by experiments on the properties of light passing through the axis of quartz, that it consists of two superposed rays moving with different velocities; and Professor Airy has proved that, in these two rays, the mole