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cules of ether vibrate in similar ellipses at right angles to each other, but in different directions; that their ellipticity varies with the angle which the incident ray makes with the axis; and that, by the composition of their motions, they produce all the phenomena of the polarized light observed in quartz.

It appears from what has been said, that the molecules of ether always perform their vibrations at right angles to the direction of the ray, but very differently in the various kinds of light. In natural light the vibrations are rectilinear, and in every plane; in ordinary polarized light they are rectilinear, but confined to one plane; in circular polarization the vibrations are circular; and in elliptical polarization the molecules vibrate in ellipses. These vibrations are communicated from molecule to molecule in straight lines when they are rectilinear, in a circular helix when they are circular, and in an oval or elliptical helix when elliptical.

Some fluids possess the property of circular polarization, as oil of turpentine; and elliptical polarization, or something similar, seems to be produced by reflection from metallic surfaces.

The coloured images from polarized light arise from the interference of the rays. MM. Fresnel and Arago proved by experiment that two rays of polarized light interfere and produce coloured

fringes if they be polarized in the same plane, but that they do not interfere when polarized in different planes. In all intermediate positions, fringes of intermediate brightness are produced. The analogy of a stretched cord will show how this happens. Suppose the cord to be moved backwards and forwards horizontally at equal intervals: it will be thrown into an undulating curve lying all in one plane. If to this motion there be superadded another, similar and equal, commencing exactly half an undulation later than the first, it is evident that the direct motion every molecule will assume, in consequence of the first system of waves, will at every instant be exactly neutralized by the retrogade motion it would take in virtue of the second; and the cord itself will be quiescent, in consequence of the interference. But if the second system of waves be in a plane perpendicular to the first, the effect would only be to twist the rope, so that no interference would take place. Rays polarized at right angles to each other may subsequently be brought into the same plane without acquiring the property of producing coloured fringes; but if they belong to a pencil, the whole of which was originally polarized in the same plane, they will interfere.

The manner in which the coloured rays are formed may be conceived by considering that, when polarized light passes through the optic axis

of a doubly refracting substance, as mica, for example, it is divided into two pencils by the analyzing tourmaline; and as one ray is absorbed, there can be no interference. But when the polarized light passes through the mica in any other direction, it is separated into two, white rays, and these are again divided into four pencils by the tourmaline, which absorbs two of them; and the other two, being transmitted in the same plane, with different velocities, interfere and produce the coloured phenomena. If the analysis be made with Iceland spar, the single ray passing through the optic axis of the mica will be refracted into two rays polarized in different planes, and no interference will happen: but when two rays are transmitted by the mica, they will be separated into four by the spar, two of which will interfere to form one image, and the other two, by their interference, will produce the complementary colours of the other image, when the spar has revolved through 90°; because, in such positions of the spar as produce the coloured images, only two rays are visible at a time, the other two being reflected. When the analysis is accomplished by reflection, if two rays are transmitted by the mica, they are polarized in planes at right angles to each other; and if the plane of reflection of either of these rays be at right angles to the plane of polar

ization, only one of them will be reflected, and therefore no interference can take place; but in all other positions of the analyzing plate, both rays will be reflected in the same plane, and consequently will produce coloured rings by their interference.

It is evident that a great deal of the light we see must be polarized, since most bodies which have the power of reflecting or refracting light also have the power of polarizing it. The blue light of the sky is completely polarized at an angle of 74° from the sun in a plane passing through his centre.

A constellation of talent, almost unrivalled at any period in the history of science, has contributed to the theory of polarization, though the original discovery of that property of light was accidental, and arose from an occurrence which, like thousands of others, would have passed unnoticed, had it not happened to one of those rare minds capable of drawing the most important inferences from circumstances apparently trifling. In 1808, while M. Malus was accidentally viewing, with a doubly refracting prism, a brilliant sunset reflected from the windows of the Luxembourg palais in Paris, on turning the prism slowly round, he was surprised to see a very great difference in the intensity of the two images, the most refracted alternately changing from brightness to

obscurity at each quadrant of revolution. A phenomenon so unlooked for induced him to investigate its cause, whence sprung one of the most elegant and refined branches of physical optics.

SECTION XXIV.

THE numerous phenomena of periodical colours arising from the interference of light, which do not admit of satisfactory explanation on any other principle than the undulatory theory, are the strongest arguments in favour of that hypothesis ; and even cases which at one time seemed unfavourable to that doctrine have proved, upon investigation, to proceed from it alone. Such is the erroneous objection which has been made in consequence of a difference in the mode of action of light and sound under the same circumstances in one particular instance. When a ray of light from a luminous point, and a diverging sound, are both transmitted through a very small hole into a dark room, the light goes straight forward, and illuminates a small spot on the opposite wall, leaving the rest in darkness; whereas the sound, on entering, diverges in all directions, and is heard in every part of the room. These phenomena, however, instead of being at variance with the undulatory theory, are direct consequences of

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