[Ill.u.s.tration: Fig. 315.]
In order to show how two waves may interfere so as to exalt or destroy each other, two sets of waves may be propagated on the surface of a still tank or bath of water, from the two points A A (Fig. 315), the black lines or circles representing the tops of the waves. It will be seen that along the lines B B the waves interfere just half way between each other, so that in all these directions there will be a smooth surface, provided each set of waves is produced by precisely the same degree of disturbing force, so as to be perfectly equal and alike in every respect, and the first wave of one set exactly half a wave in advance of the first wave of the other, while at the curve in the direction of all the line C C, the waves coincide, and produce elevations or undulations of double extent; in the intermediate s.p.a.ces, intermediate effects will, of course, be produced.
Professor Wheatstone has invented some very simple and beautiful acoustic apparatus for the purpose of proving that the same laws of interference exist also in sound, which, as already stated, consists in the vibrations or undulation of the particles of air.
[Page 331]
The nature and effects of interference are also admirably ill.u.s.trated by the following models of Mr. Charles Woodward, President of the Islington Scientific Inst.i.tution, and to whom we have already alluded.
[Ill.u.s.tration: Fig. 316.--No. 1. A model of waves with moveable rods.--No. 2. A model of fixed waves.--No. 3. Intensity of waves doubled by the superposition and coincidence of two equal systems.--No. 4. Waves neutralized by the superposition and interference of two equal systems, the raised part of one wave accurately fitting into and making smooth the hollow of the other, ill.u.s.trating the fact that two waves of light or sound may destroy each other.]
[Ill.u.s.tration: Fig. 317. Appearance of Newton"s rings when produced in yellow light, 1, 3, 5, 7, being the yellow rings, and 2, 4, 6, 8, the dark rings. Light by the odd numbers; darkness by the even numbers. The central spot, where the two surfaces are in contact, is dark.]
Returning again to the coloured rings, we find that Newton discovered that at whatever thickness of the film of air the coloured ring first appeared, there would be found at twice that thickness the dark ring, at three times the coloured, at four times the dark, and so on, _the coloured rings_ regularly occurring at the _odd numbers_, and the _dark ones_ at the _even numbers_. This discovery is well ill.u.s.trated by the models (Fig. 316); and it maybe noticed at No. 3 that the highest and the lowest parts of the waves [Page 332] interfere, but coincide and produce a wave of double intensity; the little crosses of the upper model are in a straight line with the numbers 1, 3, 5, 7, and are supposed to represent the coloured rings, whilst in No. 4 the upper series of waves is half an undulation in advance of the lower; and if the eye is again directed from the little crosses downward, the figures 2, 4, 6, 8, even numbers, are apparent, and represent the dark rings, when the waves of light destroy each other. The phenomena of thin plates, such as colours from soap bubbles, and the films of varnish, are well explained by the law of interference. The light reflected from the second surface of the film of air (which must of course, however thin, have two surfaces, viz., a upper and a lower one) interferes with the light reflected from the first, and as they come from different points of s.p.a.ce, one set of waves is in advance of the other, No. 4, Fig. 316; they reach the eye with different lengths of paths, and by their _interference_ form alternately the luminous and dark fringes, bands, or circles. Bridge"s diffraction apparatus, manufactured only by Elliott Brothers, offers itself specially as a most beautiful drawing-room optical instrument. The purpose of this apparatus is to ill.u.s.trate in great variety, and in the most convenient and compact form, the phenomena of the diffraction or interference of light. This is attained by the a.s.sistance of photography. Transparent apertures in an opaque collodion film are produced on gla.s.s, and a point of light is viewed through the apertures. [Page 333] The forms of the apertures are exceedingly various,--triangles, squares, circles, ellipses, parabolas, hyperbolas, and combinations of them, besides many figures of fanciful forms, are included in the set. When an image of the sun is viewed through these apertures, figures of extraordinary beauty, both of form and colour, are produced; and of each of these many variations may be obtained by placing the eye-gla.s.s of the telescope at different distances from the object gla.s.s. Many of the figures produced, especially when the telescope is out of focus, might suggest very useful hints to those concerned in designing patterns. Although the phenomena are chiefly of interest to the student of science, in consequence of their bearing on theories of light, yet their beauty and variety render them amusing to all. A few words on the mode of using the apparatus may be of service. (Fig. 318.)
[Ill.u.s.tration: Fig. 318. Elliott Brothers" diffraction apparatus.]
Choose a very bright day, for then only can the apparatus be used. Place the mirror in the sun, and let the light be reflected on the back of the blackened screen. The lens which is inserted into this screen will then form an exceedingly bright image of the sun. Then at the distance of not less than twelve feet, clamp the telescope to a table in such a position as to view the image thus formed. Put the eccentric cap on the end of the telescope, clean the gla.s.s objects carefully, and attach them to the cap so that they may be turned each in order before the telescope. In this manner, all those which consist of a series of figures may be viewed. Then detach the eccentric cap, and replace it by the other. Into it place any of the single objects. In viewing some of the figures, brightness is advantageous--in others, delicacy; in the former case, let the lens of long focus be inserted in the screen--in the latter case, that of shorter focus. In every case, let the phenomena be observed not only when the telescope is in focus, but also when the eye-gla.s.s is pushed in to various distances.
Mr. Warren de la Rue has ingeniously taken advantage of the colours produced by thin films of varnish, and actually _fixed_ the lovely iridescent colour produced in that manner on highly polished paper, which is termed "iridescent paper." A tank of warm water at 80 Fahr., about [Page 334] six inches deep, and two feet six inches square, is provided, and a highly glazed sheet of white or black paper being first wetted on a perforated metallic plate, is then sunk with the plate below its surface, care being taken to avoid air bubbles. A peculiar varnish is then allowed to trickle slowly down a sort of tongue of metal placed in the middle of one of the sides of the tank, and directly the varnish touches the surface of the water it begins to spread out in exquisitely thin films, and by watching the operation close to a window and skimming away all the imperfect films, a perfect one is at last obtained, and at that moment the paper lying on the metal plate is raised from the bottom of the tank, and the delicate film of varnish secured. When dry, the iridescent colours are apparent, and the paper is employed for many ornamental purposes. An extremely simple and pretty method of producing Newton"s rings has been invented by Reade, and is called "Reade"s iriscope." A plate of gla.s.s of any shape (perhaps circular is the best) is painted on one side with some quickly drying black paint or varnish, and after the other side has been cleaned, it is then rubbed over with a piece of wet soap, and this is rubbed off with a clean soft duster. A tube of about half an inch in diameter, and twelve inches long, is provided, and is held about one inch above the centre of the soaped side of the gla.s.s plate, and directly the breath is directed down the tube on the gla.s.s, an immense number of minute particles of moisture are deposited on the gla.s.s, and these by inflection decompose the light, and all the colours of the rainbow are produced. (Fig. 319.)
[Ill.u.s.tration: Fig. 319. Reade"s iriscope.]
The iridescent colours seen upon the surface of _mother-of-pearl_, which Mr. Simonds" excellent commercial dictionary tells us is "the name for the iridescent sh.e.l.l of the pearl oyster, and other molluscs," are referrible to fine parallel lines formed by its texture, and are reproducible, according to Brewster"s experiments, by taking impressions of them in soft wax. The gorgeous colours of certain sh.e.l.ls and fish, the feathers of birds, Barton"s steel b.u.t.tons, are not due to any inherent _pigment_ or colouring matter that could be extracted from them, but are owing either to the peculiar fibrous, or parallel-lined, or laminated (plate-like) surfaces upon which the light falls, and being reflected in paths of different lengths, interference occurs, and coloured light is produced.
[Page 335]
CHAPTER XXVI.
THE POLARIZATION OF LIGHT.
This branch of the phenomena of light includes some of the most remarkable and gorgeous chromatic effects; at the same time, regarded philosophically, it is certainly a most difficult subject to place in a purely elementary manner before the youthful minds of juvenile philosophers, and unless the previous chapter on the diffraction of light is carefully examined, the rationale of the ill.u.s.trations of polarized light will hardly be appreciated. We have first to ask, "What is polarized light?" The answer requires us again to carry our thoughts back to the consideration of the undulatory theory of light, already ill.u.s.trated and partly explained at pages 262, 330.
After perusing this portion of the subject, it might be considered that waves of light were const.i.tuted of one motion only, and that an undulation might be either perpendicular or horizontal, according to circ.u.mstances. (Fig. 320.)
[Ill.u.s.tration: Fig. 320.--No. 1. A wire bent to represent a perpendicular vibration, which if kept in the latter position, will only pa.s.s through a perpendicular aperture.--No. 2. A wire bent to represent a horizontal wave which will only pa.s.s through a horizontal aperture.]
This simple condition of the waves of light could not, however, be reconciled theoretically with the actual facts, and it is necessary in regarding a ray of light, to consider it as a combination of two vibrating motions, one of which, for the sake of simplicity, may be considered as perpendicular, and the other horizontal; and this idea of the nature of [Page 336] an undulation of light originated with the late Dr. Young, who while considering the results of Sir D. Brewster"s researches on the laws of double refraction, first proposed the theory of transversal (cross-wise) vibration. Dr. Young ill.u.s.trated his theory with a stretched cord, which if agitated or violently shaken perpendicularly, produces a wave that runs along the cord to the other end, and may be often seen ill.u.s.trated on the banks of a river overhung with high bushes; the bargemen who drive the horses pulling the vessel by a rope, would be continually stopped by the stunted thick bushes, but directly they approach them, they give the horse a lash, and then violently agitate the rope vertically, which is thrown into waves that pa.s.s along the rope, and clear the bushes in the most perfect manner.
(Fig. 321.)
[Ill.u.s.tration: Fig. 321. Bargeman throwing his tow-rope into waves to get it over the thick bushes.]
[Ill.u.s.tration: Fig. 322. A section of a wave of common light made up of the transversal vibration, A B and C D.]
Now if a similar movement is made with the stretched rope from right to left, another wave will be produced, which will run along the cord in an horizontal position, and if the latter is compared with the perpendicular undulation, it will be evident that each set of waves will be in planes at right angles to and independent of each other. This is supposed to be the mechanism of a wave of common light, so that if a section is taken of such an undulation, it will be represented by a circle A B C D (Fig. 322), with two diameters A B, and C D; or a better mechanical notion of a wave of common [Page 337] light is acquired from the inspection of another of Mr. Woodward"s cardboard models. (Fig.
323.)
[Ill.u.s.tration: Fig. 323. Model of a wave of common light.]
The existence of an _alternating motion of some kind_ at minute intervals along a ray is, says Professor Baden Powell, "as real as the motion of translation by which light is propagated through s.p.a.ce. _Both_ must essentially be _combined_ in any correct conception we form of light. That this alternating motion must have reference to certain directions _transverse_ to that of the ray is equally established as a consequence of the phenomena; and these _two_ principles must form the basis of any explanation which can be attempted." A beam of common light is therefore to be regarded as a rapid succession of systems of waves in which the vibrations take place in different planes.
If the two systems of waves are separated the one from the other, viz., the horizontal from the perpendicular, they each form separately a ray of polarized light, and as Fresnel has remarked, _common light_ is merely _polarized light_, having _two planes_ of polarization at _right angles_ to each other. To follow up the mechanical notion of the nature of polarized light, it is necessary to refer again to Woodward"s card wave model (Fig. 323), and by separating the two cards one from the other it may be demonstrated how a wave of common light reduced to its skeleton or primary form is reducible into two waves of polarized light, or how the two cards placed together again in a transversal position form a ray of common light. (Fig. 324.)
[Ill.u.s.tration: Fig. 324.--No. 1. Common light, made up of the two waves of polarized light, Nos. 2 and 3. ]
The query with respect to the nature of polarized light being answered, it is necessary, in the next place, to consider how the separation of these transversal vibrations may be effected, and in fact to ask what optical arrangements are necessary to procure a beam of polarized light?
Light may be polarized in four different ways--viz., by reflection, single refraction, double refraction, and by the tourmaline--viz., by absorption.
[Page 338]
_Polarization by Reflection, and by Single Refraction._
[Ill.u.s.tration: Fig. 325.--No. 1. A is the lime light. B. The condenser lenses. C. The beam of _common_ light. Here the gla.s.s plates are removed.--No. 2. A. Lime light. B. The condenser lenses. C C. The bundle of plates of gla.s.s at an angle of 56 45". D is the ray of light polarized by reflection from the gla.s.s plates, C C, and E is the beam of polarized light by single refraction, having pa.s.sed through the bundle of plates of gla.s.s, C C.]
In the year 1810, the celebrated French philosopher, Mons. Malus, while looking through a prism of Iceland spar, at the light of the setting sun, reflected from the windows of the Luxemburg palace in Paris, discovered that a beam of light reflected from a plate of gla.s.s at an angle of 56 degrees, presented precisely the same properties as one of the rays formed by a rhomb of Iceland spar, and that it was in fact polarized. _One_ of the transversal waves of polarized light of the common light, being reflected or thrown off from the surface of the gla.s.s, whilst the other and second transversal vibration pa.s.sed _through_ the plate of gla.s.s, and was likewise polarized in another plane, but by _single refraction_, so that the experiment ill.u.s.trates two of the modes of polarizing light-?viz., by reflection, and by single refraction. This important elementary truth is beautifully ill.u.s.trated by Mr. J. T. G.o.ddard"s new form of the oxy-hydrogen polariscope, by which a beam of common light traverses a long square tin box without change; but directly a bundle of plates of gla.s.s composed of ten plates of thin flattened crown gla.s.s, or sixteen plates of thin parallel gla.s.s plates used for microscopes, are slid into the box at an angle of 56 45", then the beam of common [Page 339] light is split into two beams of polarized light, which pursue their respective paths, one pa.s.sing by single refraction through the gla.s.s, and the other being reflected, and rendered apparent by opening an aperture over the gla.s.s plates, and then again by using a little smoke from brown paper, the course of the rays becomes more apparent.
[Ill.u.s.tration: Fig. 326. A A. Model in wood of a bundle of plates of gla.s.s at an angle of 56 45". B. Beam of common light, with transversal vibration. C. Light polarized by reflection. D. Light polarized by refraction.]
The same truth is well ill.u.s.trated by the cardboard model wave and a wooden plane with horizontal and perpendicular slits, placed at an angle of 56 45", as at Fig. 326.
POLARIZATION BY DOUBLE REFRACTION.
The name of _Double_-refracting or Iceland Spar is given to a very clear, limpid, and perfectly transparent mineral, composed of carbonate of lime, and found on the eastern coast of Iceland. Its crystallographic features are well described by the Rev. Walter Mitch.e.l.l in his learned work on mineralogy and crystallography, and it is sufficient for the object of this article to state that it crystallizes in rhombs, and modifications of the rhomboidal system. It must not be confounded with rock or mountain crystal, which, under the name of quartz, crystallizes in six-sided prisms with six-sided pyramidal tops; quartz being composed of silica, or silicic acid and calcareous spar of carbonate of lime.
Very large specimens of the latter mineral are rare and valuable, and the _lion_ of specimens of calcareous, or double-refracting spar, is now in the possession of Professor Tennant, the eminent mineralogist of the Strand. It is nine inches high, seven and three-quarters inches broad, and five and a half inches thick; its estimated value being 100_l._ This beautiful specimen has been photographed, and its stereograph ill.u.s.trates in a very striking manner the double refracting properties of the spar.
If a printed slip of paper is placed behind a rhomb of Iceland spar, two images of the former are apparent, and the stereograph already alluded to shows this fact very perfectly, at the same time ill.u.s.trates the value of the stereoscope. Out of the stereoscope the words "Stereoscopic Magazine" appear doubled, but seem to lie in the same plane; but directly the picture is placed in the instrument, then it is clearly seen that one image is evidently in a very different plane from the other. The double-refracting power of this mineral is ill.u.s.trated by holding a small rhomb of Iceland spar, placed in a proper bra.s.s tube before the orifice as at Fig. 327, from which the rays of common light are [Page 340] pa.s.sing; if an opaque screen of bra.s.s perforated with a small hole is introduced behind the rhomb, then, instead of one circle of light being apparent on the screen, two are produced, and both the rays issuing in this manner are polarized, one being termed the ordinary and the other the extraordinary ray. (Fig. 327.)
[Ill.u.s.tration: Fig. 327. A. The condensers. B. The hole in the bra.s.s screen or stop. C. The rhomb of Iceland spar. O. The ordinary, and E the extraordinary, ray, both of which are polarized light.]
The polarizing property of the rhomb is perhaps better shown by the next diagram, where A B represents the obtuse angles of the Iceland spar, and a line drawn from A to B, would be the axis of the crystal. The incidental ray of common light is shown at C, and the oppositely polarized transmitted rays called the ordinary ray O, and extraordinary ray E, emerge from the opposite face of the rhomboid. If a black line is ruled on a sheet of paper as at K K, and examined by the eye at C, it appears double as at K K and J J. (Fig. 328.)
[Ill.u.s.tration: Fig. 328. Rhomb of Iceland spar.]
The cardboard model is again useful in demonstrating the polarization of light by double refraction, and if a model of a rhomb of Iceland [Page 341] spar is made of gla.s.s plates, one face of which has an aperture like a cross, and the other a horizontal and perpendicular slit, as at Nos. 1 and 2 (Fig. 329), the production of the ordinary and extraordinary rays is demonstrated in a familiar manner, and is easily comprehended.
[Ill.u.s.tration: Fig. 329.--No. 1. One face of the model rhomb to admit the transversal vibration, represented by the cardboard model.--No. 2.
The opposite face of the rhomb, from which issue the polarized, ordinary, and extraordinary rays.--No. 3. Side view of the model.]
In Newton"s "Optics" we find the following description of Iceland spar:--"This crystal is a pellucid fissile stone, clear as water or crystal of the rock (quartz), and without colour.... Being rubbed on cloth it attracts pieces of straw and other light things like amber or gla.s.s, and with aquafortis it makes an ebullition.... If a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double by means of a double refraction."
POLARIZATION BY THE TOURMALINE.