Polarization by Reflection.

685

sists of wave-motions of the ethereal medium, across the direction of the beam; that is to say, analogous to the vibrations of a sounding string. It is only on the supposition that the vibrations are transversal that the manifold phenomena of polarization can be accounted for; although, indeed, it is difficult to conceive of such a transverse wave-motion without the existence of a cohesive force among the ethereal particles, as there is between the particles of a vibrating string. Further, the vibrations of the ethereal particles resemble those of the parts of a sounding string in describing circular orbits round the axis of motion; each particle vibrating, not in one plane or line, but successively in all planes round the axis or direction of the beam.

Polarization of a beam of ordinary light, then, is the splitting up or resolving of the circular or helical wave-motion into two sets of plane vibrations in planes across each other.

922. We may picture a beam of common light before falling on the first reflecting plate, B, of the polariscope (fig. 241) as an exceedingly rapid helical or circular vibration of the ethereal medium, which on meeting the reflecting surface is split up into a plane wave of ethereal motion which rebounds up the tube, T, and another plane wave which passes through the plate, B, undergoing refraction, and which is quenched by the varnish on the back of the plate.

These plane or sheet waves are composed of ethereal vibrations parallel to the face of the reflecting plate, B; and, as may readily be conceived, when they fall on the plate, A, similarly disposed with B, they are simply thrown off or reflected again in their entirety, without any farther splitting up. If, however, they fall on A, disposed crossways, then they present, so to speak, their edge to the reflecting surface, and are thrown off proportionately enfeebled.

When a beam or ray of light falls on the glass plate at any other angle than that of about 56°, then the decomposition of the ethereal motion is but partial; or the ray is only partially polarized, and a second cross reflection in such case does not cause the same complete extinction of the beam.

A beam of light may be thus decomposed or polarized in other ways than by reflection. We may have polarization by refraction as well as by reflection. If a beam of light be made to pass through a pile of thin glass plates inclined to the direction of the beam at an angle of 56°, the polarizing angle, the components of the luminous rays which are transmitted, emerge in a state of

686

Polarization by Double Refraction.

polarization more or less perfect according to the number of plates employed.

The components of the waves perpendicular to those which emerge, are sifted out by reflection at the successive surfaces, and form the complementary polarized beam. Such a pile of plates may very conveniently be used to replace the upper or analysing plate in the polariscope (fig. 241).

Another remarkable method by which light may be thus analysed or resolved is by means of what are called double refracting crystals, of which the principal are Iceland spar and Tourmaline.

923. Iceland spar, or calc-spar, as it is frequently termed, is a crystalline form of carbonate of lime, which is found in considerable quantity in Iceland.

It cleaves naturally with faces shaped as in the figure, such that

X

P

D

Fig. 242.

B

the angle, A B C, is about 102°, and the angle, B C D, about 78°. Fig. 242 represents a rhomb or natural block of the crystal; the line, A F, which connects the two opposite obtuse angles of the crystal is called the optic axis of the rhomb; it is equally inclined (at 45° 23′) to the three edges, which meet at A, if the rhomb is an equilateral one.

If we lay such a block of spar on a line of writing, or a page of a book, we shall see each line and letter double; if we place it over a dot, we see, not one dot apparently raised by refraction, as we should do with a similarly shaped piece of glass, but two dots, one seemingly nearer than the other; and if we turn the crystal round, we shall see the upper image remain stationary while the lower one moves round it as a centre.

On examination it is found that the higher image undergoes refraction according to the ordinary law; it is therefore called the ordinary ray, or image; while the other is bent or refracted according to a different or extraordinary law, and is called the extraordinary ray or image. When the dot is viewed through the spar in a direction parallel to its optic axis, the two images fuse into one, and the extraordinary ray becomes coincident with the ordinary

one.

If we examine with the analyser of the polariscope (fig. 241) the two images, or two beams of light, thus produced by a rhomb of

Use of Nicol's Prism.

687

Iceland spar, we find that the two rays are polarized very completely, and are complementary to each other; that is to say, when the one image is getting extinguished as we turn the analyser found, the other one is getting bright, and conversely.

If we have a pretty thick crystal, so that the two images are well separated, we may cover the face of the crystal with varnish or black paper, and leave an opening for the one image only emerging; and in this way we obtain a very valuable means of polarizing light.

The crystal known as Tourmaline has a similar property of double refraction, with this peculiarity, that the ordinary beam is more rapidly quenched than the extraordinary; and with a certain thickness of plate, the extraordinary beam alone emerges; a piece of this crystal, therefore, fornis a most valuable means of polarizing light.

D

924. Nicol's prism.—Another and most convenient means of obtaining polarized light, or of examining it when polarized, is what is known as Nicol's prism. It consists of two halves of a crystal of Iceland spar, cut through the two oblique-angled corners and reunited by means of Canada balsam. Owing to the refractive index of

Fig. 243.

B

the balsam, the ordinary image meets it at such an angle that it is reflected away to one side of the crystal, while the extraordinary ray traverses the prism uninterruptedly.

Besides the important bearing which the phenomena of polarization have upon the undulatory ethereal hypotheses of light, for the exposition of which mathematical and technical details, unsuited to this work, would require to be introduced, it has some practically useful applications.

If a piece of selenite, or of mica, be introduced between the polarizing and the analysing plates of a polariscope, it will be found that the different thicknesses of the plate are revealed by differences of the colour, the purity of the colours being sometimes of surpassing beauty. If the piece of mica or selenite be fixed while the analyser, or polarizer, is slowly turned round, the colours are all

688

Polarization of Heat.

changed into their complemer.tary tints by a quarter of a turn, while another quarter turn restores them again.

Applied to the microscope a polarizing apparatus renders distinctly and vividly visible, transparent bodies, different thicknesses corresponding to or producing different colours; and minute objects which, being transparent and colourless in ordinary light, would otherwise escape detection, are thus revealed.

925. Polarization of Heat.-Finally, it may be here observed that exactly similar modifications of dark heat-rays are produced by reflection, or by double refraction; and this furnishes one of the most convincing arguments for the theory that heat and light are mere differences of rapidity of similar ethereal undulations. By transmitting dark heat-rays through a plate of Tourmaline, and passing them next through a similar analyser, it is found that the effect of the finally emerging rays on a delicate thermometer increases and decreases, as the analyser is rotated exactly like a beam of light.

The researches of Fresnel, Forbes, Tyndall, and other investigators appear quite conclusive on this point; and to the works of these scientific leaders the reader is referred for full details of the experimental arguments in support of this theory.

"Light reflected from, or traversing, bodies of irregular surface and structure, or which have other peculiarities, is so modified as to produce all the phenomena of colour and varied brightness seen among natural bodies, giving them their distinctive characters and beauty.”

926. General remarks on this part of our subject were made in the beginning of the section, in the explanations of how objects not selfluminous become visible by reflecting the light issuing from other bodies, and of the manner in which the prism separates a ray of white light into rays of the several colours which are seen in the rainbow. It was also shown that these rays, on being again mixed by convergence through another lens, became white light as before. To give an account of all that has been plausibly conjectured and written on this subject would occupy the pages of a whole volume. It would be to pass in review the various opinions which have existed respecting the nature of light, the numerous facts connected with the relation of light in its double or multiple refraction, or to the ultimate structure of material masses.

The investigations Eitherto made respecting the phenomena of

Old and New Theories of Light.

689

light, have furnished new proofs of the marvellous simplicity of nature, amidst the boundless extent and most curious variety. When men thought of the sense of touch chiefly as produced by pressure on the tips of the fingers, or elsewhere on the skin, they were far from suspecting that the sense of hearing had the near relation to it, which subsequent discoveries have proved, namely, that it is only a more soft or delicate pressure, made by undulations of the air or other substances on nerves protected within the cavity of the car; and still less did they suspect that the sense of sight was but a yet finer touch than hearing, produced by still more subtle vibrations of a medium of light on the interior nerves of the eye. But step by step they have ascertained the facts mentioned. It is a curious resemblance that, while in sound different tones or notes depend on the number of vibrations in a given time, so in light do different colours seem to depend on the number and extent of the vibrations of the more subtle medium, on which the phenomena of light depend. The human imagination cannot picture to itself a simplicity more fruitful of marvellous beauty and utility than all this; yet farther, as air answers in the universc innumerable important purposes besides that of conveying sounds, so also does the medium of light minister in numerous ways, as in connection with the phenomena of heat, electricity, and magnetism.

927. The truths now positively ascertained with respect to the nature of light and vision, are among those in the wide field of scientific inquiry, which, acting on ordinary mental susceptibility, place the student in the very midst of the work of creation, awakening the most elevated thoughts of which the human mind is capable. Had there been no light in the universe, everything else in it had been, in regard to man, utterly valueless. In a word, he could not have existed. But the material of light does exist, pervading all space; and impressions made on it in one place extend rapidly over the universe, in the progressive movement called rays or beams of light. These beams from all parts coming to every individual may be regarded as millions of supplementary arms or feclers belonging to the individual, and making him almost everywhere present; then these members or feelers have no weight, they are never in the way, they impede nothing, and they are only known to exist when they can render service!

But, again, this miracle of LIGHT would have been totally useless had there not been an organ of corresponding delicacy to perceive it. In the Eye is to be considered the round window called