When a plate of mica held as close to the eye as possible, at such an inclination as to transmit the polarized ray along one of its optic axes, is viewed through the tourmaline with its axis vertical, a most splendid appearance is presented. The cloudy spot, which is in the direction of the optic axis, is seen surrounded by a set of vividly colored rings of an oval form, divided into two unequal parts by a black curved band passing through the cloudy spot about which the rings are formed. The other optic axis of the mica exhibits a similar image.

When the two optic axes of a crystal make a small angle with one another, as in nitre, the two sets of rings touch externally; and if the plate of nitre be turned round in its own plane, the black transverse bands undergo a variety of changes, till at last the whole richly colored image assumes the form of the figure 8, traversed by a black cross. Substances having one optic axis have but one set of colored circular rings with a broad black cross passing through its centre and dividing the rings into four equal parts. When the analyzing plate revolves, this figure recurs at every quarter revolution, but in the intermediate positions, it assumes the complementary colors, the black cross becoming white.

It is in vain to attempt to describe the beautiful phenomena exhibited by innumerable bodies, all of which undergo periodic changes in form and color when the analyzing plate revolves, but not one of them shows a trace of color without the aid of tourmaline or something equivalent to analyze the light, and as it were to call these beautiful phantoms into existence. Tourmaline has the disadvantage of being itself a colored substance, but that inconvenience may be avoided by employing a reflecting surface

as an analyzing plate. When polarized light is reflected by a plate of glass at the polarizing angle, it will be separated into two colored pencils, and when the analyzing plate is turned round in its own plane, it will alternately reflect each ray at every quarter revolution, so that all the phenomena that have been described will be seen by reflection on its surface.

Colored rings are produced by analyzing polarized light transmitted through glass melted and suddenly or unequally cooled, also in thin plates of glass bent with the hand, in jelly indurated or compressed, &c. &c.; in short all the phenomena of colored rings may be produced, either permanently or transiently, in a variety of substances, by heat and cold, rapid cooling, compression, dilatation, and induration; and so little apparatus is necessary for performing the experiments, that, as Sir John Herschel observes, a piece of window-glass or a polished table to polarize the light, a sheet of clear ice to produce the rings, and a broken fragment of plate-glass placed near the eye to analyze the light, are alone requisite to produce one of the most splendid of optical exhibitions.

It has been observed that, when a ray of light, polarized by reflection from any surface not metallic, is analyzed by a doubly refracting substance, it exhibits properties which are symmetrical both to the right and left of the plane of reflection, and the ray is then said to be polarized according to that plane. This symmetry is not destroyed when the ray, before being analyzed, traverses the optic axis of a crystal having but one optic axis, as evidently appears from the circular form of the colored rings already described. Regularly crystallized quartz, or rock crystal, however, forms an exception. In it, even though the

rays should pass through the optic axis itself, where there is no double refraction, the primitive symmetry of the ray is destroyed, and the plane of primitive polarization deviates either to the right or left of the observer, by an angle proportional to the thickness of the plate of quartz. This angular motion, or true rotation of the plane of polarization, which is called circular polarization, is clearly proved by the phenomena. The colored rings produced by all crystals having but one optic axis are circular, and traversed by a black cross concentric with the rings; so that the light entirely vanishes throughout the space enclosed by the interior ring, because there is neither double refraction nor polarization along the optic axis; but in the system of rings produced by a plate of quartz, whose surfaces are perpendicular to the axis of the crystal, the part within the interior ring, instead of being void of light, is occupied by a uniform tint of red, green, or blue, according to the thickness of the plate. Suppose the plate of quartz to be of an inch thick, which will give the red tint to the space within the interior ring; when the analyzing plate is turned in its own plane through an angle of 1710, the red hue vanishes. If a plate of rock crystal,

25

of an inch thick, be used, the analyzing plate must revolve through 35° before the red tint vanishes, and so on; every additional 25th of an inch in thickness requiring an additional rotation of 170, whence it is manifest that the plane of polarization revolves in the direction of a spiral within the rock crystal. It is remarkable that, in some crystals of quartz, 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 phenom

ena, 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 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 circumstances are reversed.

state.

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 or 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,